[PATCH 0/3] indirectly reclaimable memory

[PATCH 0/3] indirectly reclaimable memory

[Roman Gushchin]

This patch set introduces the concept of indirectly reclaimable
memory and applies it to fix the issue, when a big number
of dentries with external names can significantly affect
the MemAvailable value.

v2:
1) removed comments specific to unreclaimable slabs
2) splitted into 3 patches

v1:
https://lkml.org/lkml/2018/3/1/961

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: kernel-team@fb.com

Roman Gushchin (3):
  mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES
  mm: treat indirectly reclaimable memory as available in MemAvailable
  dcache: account external names as indirectly reclaimable memory

 fs/dcache.c            | 29 ++++++++++++++++++++++++-----
 include/linux/mmzone.h |  1 +
 mm/page_alloc.c        |  7 +++++++
 mm/vmstat.c            |  1 +
 4 files changed, 33 insertions(+), 5 deletions(-)

-- 
2.14.3

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[PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Roman Gushchin]

This patch introduces a concept of indirectly reclaimable memory
and adds the corresponding memory counter and /proc/vmstat item.

Indirectly reclaimable memory is any sort of memory, used by
the kernel (except of reclaimable slabs), which is actually
reclaimable, i.e. will be released under memory pressure.

The counter is in bytes, as it's not always possible to
count such objects in pages. The name contains BYTES
by analogy to NR_KERNEL_STACK_KB.

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: kernel-team@fb.com
---
 include/linux/mmzone.h | 1 +
 mm/vmstat.c            | 1 +
 2 files changed, 2 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index e09fe563d5dc..15e783f29e21 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -180,6 +180,7 @@ enum node_stat_item {
 	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
 	NR_DIRTIED,		/* page dirtyings since bootup */
 	NR_WRITTEN,		/* page writings since bootup */
+	NR_INDIRECTLY_RECLAIMABLE_BYTES, /* measured in bytes */
 	NR_VM_NODE_STAT_ITEMS
 };
 
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 40b2db6db6b1..b6b5684f31fe 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1161,6 +1161,7 @@ const char * const vmstat_text[] = {
 	"nr_vmscan_immediate_reclaim",
 	"nr_dirtied",
 	"nr_written",
+	"nr_indirectly_reclaimable",
 
 	/* enum writeback_stat_item counters */
 	"nr_dirty_threshold",
-- 
2.14.3

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[PATCH 2/3] mm: add indirectly reclaimable memory to MemAvailable

[Roman Gushchin]

This patch adjusts /proc/meminfo MemAvailable calculation
by adding the amount of indirectly reclaimable memory
(rounded to the PAGE_SIZE).

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: kernel-team@fb.com
---
 mm/page_alloc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2836bc9e0999..2247cda9e94e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4695,6 +4695,13 @@ long si_mem_available(void)
 		     min(global_node_page_state(NR_SLAB_RECLAIMABLE) / 2,
 			 wmark_low);
 
+	/*
+	 * Part of the kernel memory, which can be released under memory
+	 * pressure.
+	 */
+	available += global_node_page_state(NR_INDIRECTLY_RECLAIMABLE_BYTES) >>
+		PAGE_SHIFT;
+
 	if (available < 0)
 		available = 0;
 	return available;
-- 
2.14.3

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[PATCH 2/3] mm: treat indirectly reclaimable memory as available in MemAvailable

[Roman Gushchin]

This patch adjusts /proc/meminfo MemAvailable calculation
by adding the amount of indirectly reclaimable memory
(rounded to the PAGE_SIZE).

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: kernel-team@fb.com
---
 mm/page_alloc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2836bc9e0999..2247cda9e94e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4695,6 +4695,13 @@ long si_mem_available(void)
 		     min(global_node_page_state(NR_SLAB_RECLAIMABLE) / 2,
 			 wmark_low);
 
+	/*
+	 * Part of the kernel memory, which can be released under memory
+	 * pressure.
+	 */
+	available += global_node_page_state(NR_INDIRECTLY_RECLAIMABLE_BYTES) >>
+		PAGE_SHIFT;
+
 	if (available < 0)
 		available = 0;
 	return available;
-- 
2.14.3

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[PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Roman Gushchin]

I was reported about suspicious growth of unreclaimable slabs
on some machines. I've found that it happens on machines
with low memory pressure, and these unreclaimable slabs
are external names attached to dentries.

External names are allocated using generic kmalloc() function,
so they are accounted as unreclaimable. But they are held
by dentries, which are reclaimable, and they will be reclaimed
under the memory pressure.

In particular, this breaks MemAvailable calculation, as it
doesn't take unreclaimable slabs into account.
This leads to a silly situation, when a machine is almost idle,
has no memory pressure and therefore has a big dentry cache.
And the resulting MemAvailable is too low to start a new workload.

To address the issue, the NR_INDIRECTLY_RECLAIMABLE_BYTES counter
is used to track the amount of memory, consumed by external names.
The counter is increased in the dentry allocation path, if an external
name structure is allocated; and it's decreased in the dentry freeing
path.

To reproduce the problem I've used the following Python script:
  import os

  for iter in range (0, 10000000):
      try:
          name = ("/some_long_name_%d" % iter) + "_" * 220
          os.stat(name)
      except Exception:
          pass

Without this patch:
  $ cat /proc/meminfo | grep MemAvailable
  MemAvailable:    7811688 kB
  $ python indirect.py
  $ cat /proc/meminfo | grep MemAvailable
  MemAvailable:    2753052 kB

With the patch:
  $ cat /proc/meminfo | grep MemAvailable
  MemAvailable:    7809516 kB
  $ python indirect.py
  $ cat /proc/meminfo | grep MemAvailable
  MemAvailable:    7749144 kB

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: kernel-team@fb.com
---
 fs/dcache.c | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 5c7df1df81ff..a0312d73f575 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -273,8 +273,16 @@ static void __d_free(struct rcu_head *head)
 static void __d_free_external(struct rcu_head *head)
 {
 	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
-	kfree(external_name(dentry));
-	kmem_cache_free(dentry_cache, dentry); 
+	struct external_name *name = external_name(dentry);
+	unsigned long bytes;
+
+	bytes = dentry->d_name.len + offsetof(struct external_name, name[1]);
+	mod_node_page_state(page_pgdat(virt_to_page(name)),
+			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
+			    -kmalloc_size(kmalloc_index(bytes)));
+
+	kfree(name);
+	kmem_cache_free(dentry_cache, dentry);
 }
 
 static inline int dname_external(const struct dentry *dentry)
@@ -1598,6 +1606,7 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 	struct dentry *dentry;
 	char *dname;
 	int err;
+	size_t reclaimable = 0;
 
 	dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);
 	if (!dentry)
@@ -1614,9 +1623,11 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 		name = &slash_name;
 		dname = dentry->d_iname;
 	} else if (name->len > DNAME_INLINE_LEN-1) {
-		size_t size = offsetof(struct external_name, name[1]);
-		struct external_name *p = kmalloc(size + name->len,
-						  GFP_KERNEL_ACCOUNT);
+		struct external_name *p;
+
+		reclaimable = offsetof(struct external_name, name[1]) +
+			name->len;
+		p = kmalloc(reclaimable, GFP_KERNEL_ACCOUNT);
 		if (!p) {
 			kmem_cache_free(dentry_cache, dentry); 
 			return NULL;
@@ -1665,6 +1676,14 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 		}
 	}
 
+	if (unlikely(reclaimable)) {
+		pg_data_t *pgdat;
+
+		pgdat = page_pgdat(virt_to_page(external_name(dentry)));
+		mod_node_page_state(pgdat, NR_INDIRECTLY_RECLAIMABLE_BYTES,
+				    kmalloc_size(kmalloc_index(reclaimable)));
+	}
+
 	this_cpu_inc(nr_dentry);
 
 	return dentry;
-- 
2.14.3

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Re: [PATCH 2/3] mm: add indirectly reclaimable memory to MemAvailable

[Roman Gushchin]

Please, ignore this particular patch, it was sent by mistake.

On Mon, Mar 05, 2018 at 01:37:41PM +0000, Roman Gushchin wrote:
> This patch adjusts /proc/meminfo MemAvailable calculation
> by adding the amount of indirectly reclaimable memory
> (rounded to the PAGE_SIZE).
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: kernel-team@fb.com
> ---
>  mm/page_alloc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2836bc9e0999..2247cda9e94e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4695,6 +4695,13 @@ long si_mem_available(void)
>  		     min(global_node_page_state(NR_SLAB_RECLAIMABLE) / 2,
>  			 wmark_low);
>  
> +	/*
> +	 * Part of the kernel memory, which can be released under memory
> +	 * pressure.
> +	 */
> +	available += global_node_page_state(NR_INDIRECTLY_RECLAIMABLE_BYTES) >>
> +		PAGE_SHIFT;
> +
>  	if (available < 0)
>  		available = 0;
>  	return available;
> -- 
> 2.14.3
> 

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Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Al Viro]

On Mon, Mar 05, 2018 at 01:37:43PM +0000, Roman Gushchin wrote:
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 5c7df1df81ff..a0312d73f575 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -273,8 +273,16 @@ static void __d_free(struct rcu_head *head)
>  static void __d_free_external(struct rcu_head *head)
>  {
>  	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
> -	kfree(external_name(dentry));
> -	kmem_cache_free(dentry_cache, dentry); 
> +	struct external_name *name = external_name(dentry);
> +	unsigned long bytes;
> +
> +	bytes = dentry->d_name.len + offsetof(struct external_name, name[1]);
> +	mod_node_page_state(page_pgdat(virt_to_page(name)),
> +			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
> +			    -kmalloc_size(kmalloc_index(bytes)));
> +
> +	kfree(name);
> +	kmem_cache_free(dentry_cache, dentry);
>  }

That can't be right - external names can be freed in release_dentry_name_snapshot()
and copy_name() as well.  When do you want kfree_rcu() paths accounted for, BTW?
At the point where we are freeing them, or where we are scheduling their freeing?

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Roman Gushchin]

On Mon, Mar 12, 2018 at 09:17:42PM +0000, Al Viro wrote:
> On Mon, Mar 05, 2018 at 01:37:43PM +0000, Roman Gushchin wrote:
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 5c7df1df81ff..a0312d73f575 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -273,8 +273,16 @@ static void __d_free(struct rcu_head *head)
> >  static void __d_free_external(struct rcu_head *head)
> >  {
> >  	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
> > -	kfree(external_name(dentry));
> > -	kmem_cache_free(dentry_cache, dentry); 
> > +	struct external_name *name = external_name(dentry);
> > +	unsigned long bytes;
> > +
> > +	bytes = dentry->d_name.len + offsetof(struct external_name, name[1]);
> > +	mod_node_page_state(page_pgdat(virt_to_page(name)),
> > +			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
> > +			    -kmalloc_size(kmalloc_index(bytes)));
> > +
> > +	kfree(name);
> > +	kmem_cache_free(dentry_cache, dentry);
> >  }
> 
> That can't be right - external names can be freed in release_dentry_name_snapshot()
> and copy_name() as well.  When do you want kfree_rcu() paths accounted for, BTW?
> At the point where we are freeing them, or where we are scheduling their freeing?

Ah, I see...

I think, it's better to account them when we're actually freeing,
otherwise we will have strange path:
(indirectly) reclaimable -> unreclaimable -> free

Do you agree?

Although it shouldn't be that important in practice.

Thank you!

--

>From ad9d6c627c2b9315de1967c40a1f4fa68705cf9e Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@fb.com>
Date: Mon, 12 Mar 2018 22:24:28 +0000
Subject: [PATCH] dcache: fix indirectly reclaimable memory accounting

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 fs/dcache.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 98826efe22a0..19bc7495a6c4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -266,6 +266,19 @@ static void __d_free(struct rcu_head *head)
 	kmem_cache_free(dentry_cache, dentry); 
 }
 
+static void __d_free_external_name(struct rcu_head *head)
+{
+	struct external_name *name;
+
+	name = container_of(head, struct external_name, u.head);
+
+	mod_node_page_state(page_pgdat(virt_to_page(name)),
+			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
+			    -ksize(name));
+
+	kfree(name);
+}
+
 static void __d_free_external(struct rcu_head *head)
 {
 	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
@@ -307,7 +320,7 @@ void release_dentry_name_snapshot(struct name_snapshot *name)
 		struct external_name *p;
 		p = container_of(name->name, struct external_name, name[0]);
 		if (unlikely(atomic_dec_and_test(&p->u.count)))
-			kfree_rcu(p, u.head);
+			call_rcu(&p->u.head, __d_free_external_name);
 	}
 }
 EXPORT_SYMBOL(release_dentry_name_snapshot);
@@ -2769,7 +2782,7 @@ static void copy_name(struct dentry *dentry, struct dentry *target)
 		dentry->d_name.hash_len = target->d_name.hash_len;
 	}
 	if (old_name && likely(atomic_dec_and_test(&old_name->u.count)))
-		kfree_rcu(old_name, u.head);
+		call_rcu(&old_name->u.head, __d_free_external_name);
 }
 
 static void dentry_lock_for_move(struct dentry *dentry, struct dentry *target)
-- 
2.14.3

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Al Viro]

On Mon, Mar 12, 2018 at 10:36:38PM +0000, Roman Gushchin wrote:

> Ah, I see...
> 
> I think, it's better to account them when we're actually freeing,
> otherwise we will have strange path:
> (indirectly) reclaimable -> unreclaimable -> free
> 
> Do you agree?

> +static void __d_free_external_name(struct rcu_head *head)
> +{
> +	struct external_name *name;
> +
> +	name = container_of(head, struct external_name, u.head);
> +
> +	mod_node_page_state(page_pgdat(virt_to_page(name)),
> +			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
> +			    -ksize(name));
> +
> +	kfree(name);
> +}

Maybe, but then you want to call that from __d_free_external() and from
failure path in __d_alloc() as well.  Duplicating something that convoluted
and easy to get out of sync is just asking for trouble.

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Andrew Morton]

On Tue, 13 Mar 2018 00:45:32 +0000 Al Viro <viro@ZenIV.linux.org.uk> wrote:

> On Mon, Mar 12, 2018 at 10:36:38PM +0000, Roman Gushchin wrote:
> 
> > Ah, I see...
> > 
> > I think, it's better to account them when we're actually freeing,
> > otherwise we will have strange path:
> > (indirectly) reclaimable -> unreclaimable -> free
> > 
> > Do you agree?
> 
> > +static void __d_free_external_name(struct rcu_head *head)
> > +{
> > +	struct external_name *name;
> > +
> > +	name = container_of(head, struct external_name, u.head);
> > +
> > +	mod_node_page_state(page_pgdat(virt_to_page(name)),
> > +			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
> > +			    -ksize(name));
> > +
> > +	kfree(name);
> > +}
> 
> Maybe, but then you want to call that from __d_free_external() and from
> failure path in __d_alloc() as well.  Duplicating something that convoluted
> and easy to get out of sync is just asking for trouble.

So.. where are we at with this issue?

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Roman Gushchin]

On Thu, Apr 05, 2018 at 03:11:23PM -0700, Andrew Morton wrote:
> On Tue, 13 Mar 2018 00:45:32 +0000 Al Viro <viro@ZenIV.linux.org.uk> wrote:
> 
> > On Mon, Mar 12, 2018 at 10:36:38PM +0000, Roman Gushchin wrote:
> > 
> > > Ah, I see...
> > > 
> > > I think, it's better to account them when we're actually freeing,
> > > otherwise we will have strange path:
> > > (indirectly) reclaimable -> unreclaimable -> free
> > > 
> > > Do you agree?
> > 
> > > +static void __d_free_external_name(struct rcu_head *head)
> > > +{
> > > +	struct external_name *name;
> > > +
> > > +	name = container_of(head, struct external_name, u.head);
> > > +
> > > +	mod_node_page_state(page_pgdat(virt_to_page(name)),
> > > +			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
> > > +			    -ksize(name));
> > > +
> > > +	kfree(name);
> > > +}
> > 
> > Maybe, but then you want to call that from __d_free_external() and from
> > failure path in __d_alloc() as well.  Duplicating something that convoluted
> > and easy to get out of sync is just asking for trouble.
> 
> So.. where are we at with this issue?

I assume that commit 0babe6fe1da3 ("dcache: fix indirectly reclaimable memory accounting")
address the issue.

__d_free_external_name() is now called from all release paths (including __d_free_external())
and is the only place where NR_INDIRECTLY_RECLAIMABLE_BYTES is decremented.

__d_alloc()'s error path is slightly different, because I bump NR_INDIRECTLY_RECLAIMABLE_BYTES
in a very last moment, when it's already clear, that no errors did occur.
So we don't need to increase and decrease the counter back and forth.

Thank you!

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Vlastimil Babka]

[+CC linux-api]

On 03/05/2018 02:37 PM, Roman Gushchin wrote:
> This patch introduces a concept of indirectly reclaimable memory
> and adds the corresponding memory counter and /proc/vmstat item.
> 
> Indirectly reclaimable memory is any sort of memory, used by
> the kernel (except of reclaimable slabs), which is actually
> reclaimable, i.e. will be released under memory pressure.
> 
> The counter is in bytes, as it's not always possible to
> count such objects in pages. The name contains BYTES
> by analogy to NR_KERNEL_STACK_KB.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: kernel-team@fb.com

Hmm, looks like I'm late and this user-visible API change was just
merged. But it's for rc1, so we can still change it, hopefully?

One problem I see with the counter is that it's in bytes, but among
counters that use pages, and the name doesn't indicate it. Then, I don't
see why users should care about the "indirectly" part, as that's just an
implementation detail. It is reclaimable and that's what matters, right?
(I also wanted to complain about lack of Documentation/... update, but
looks like there's no general file about vmstat, ugh)

I also kind of liked the idea from v1 rfc posting that there would be a
separate set of reclaimable kmalloc-X caches for these kind of
allocations. Besides accounting, it should also help reduce memory
fragmentation. The right variant of cache would be detected via
__GFP_RECLAIMABLE.

With that in mind, can we at least for now put the (manually maintained)
byte counter in a variable that's not directly exposed via /proc/vmstat,
and then when printing nr_slab_reclaimable, simply add the value
(divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
subtract the same value. This way we would be simply making the existing
counters more precise, in line with their semantics.

Thoughts?
Vlastimil

> ---
>  include/linux/mmzone.h | 1 +
>  mm/vmstat.c            | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index e09fe563d5dc..15e783f29e21 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -180,6 +180,7 @@ enum node_stat_item {
>  	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
>  	NR_DIRTIED,		/* page dirtyings since bootup */
>  	NR_WRITTEN,		/* page writings since bootup */
> +	NR_INDIRECTLY_RECLAIMABLE_BYTES, /* measured in bytes */
>  	NR_VM_NODE_STAT_ITEMS
>  };
>  
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 40b2db6db6b1..b6b5684f31fe 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1161,6 +1161,7 @@ const char * const vmstat_text[] = {
>  	"nr_vmscan_immediate_reclaim",
>  	"nr_dirtied",
>  	"nr_written",
> +	"nr_indirectly_reclaimable",
>  
>  	/* enum writeback_stat_item counters */
>  	"nr_dirty_threshold",
> 

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Roman Gushchin]

On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:
> [+CC linux-api]
> 
> On 03/05/2018 02:37 PM, Roman Gushchin wrote:
> > This patch introduces a concept of indirectly reclaimable memory
> > and adds the corresponding memory counter and /proc/vmstat item.
> > 
> > Indirectly reclaimable memory is any sort of memory, used by
> > the kernel (except of reclaimable slabs), which is actually
> > reclaimable, i.e. will be released under memory pressure.
> > 
> > The counter is in bytes, as it's not always possible to
> > count such objects in pages. The name contains BYTES
> > by analogy to NR_KERNEL_STACK_KB.
> > 
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: linux-fsdevel@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: linux-mm@kvack.org
> > Cc: kernel-team@fb.com
> 
> Hmm, looks like I'm late and this user-visible API change was just
> merged. But it's for rc1, so we can still change it, hopefully?
> 
> One problem I see with the counter is that it's in bytes, but among
> counters that use pages, and the name doesn't indicate it.

Here I just followed "nr_kernel_stack" path, which is measured in kB,
but this is not mentioned in the field name.

> Then, I don't
> see why users should care about the "indirectly" part, as that's just an
> implementation detail. It is reclaimable and that's what matters, right?
> (I also wanted to complain about lack of Documentation/... update, but
> looks like there's no general file about vmstat, ugh)

I agree, that it's a bit weird, and it's probably better to not expose
it at all; but this is how all vm counters work. We do expose them all
in /proc/vmstat. A good number of them is useless until you are not a
mm developer, so it's arguable more "debug info" rather than "api".
It's definitely not a reason to make them messy.
Does "nr_indirectly_reclaimable_bytes" look better to you?

> 
> I also kind of liked the idea from v1 rfc posting that there would be a
> separate set of reclaimable kmalloc-X caches for these kind of
> allocations. Besides accounting, it should also help reduce memory
> fragmentation. The right variant of cache would be detected via
> __GFP_RECLAIMABLE.

Well, the downside is that we have to introduce X new caches
just for this particular problem. I'm not strictly against the idea,
but not convinced that it's much better.

> 
> With that in mind, can we at least for now put the (manually maintained)
> byte counter in a variable that's not directly exposed via /proc/vmstat,
> and then when printing nr_slab_reclaimable, simply add the value
> (divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
> subtract the same value. This way we would be simply making the existing
> counters more precise, in line with their semantics.

Idk, I don't like the idea of adding a counter outside of the vm counters
infrastructure, and I definitely wouldn't touch the exposed
nr_slab_reclaimable and nr_slab_unreclaimable fields.
We do have some stats in /proc/slabinfo, /proc/meminfo and /sys/kernel/slab
and I think that we should keep it consistent.

Thanks!

> 
> Thoughts?
> Vlastimil
> 
> > ---
> >  include/linux/mmzone.h | 1 +
> >  mm/vmstat.c            | 1 +
> >  2 files changed, 2 insertions(+)
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index e09fe563d5dc..15e783f29e21 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -180,6 +180,7 @@ enum node_stat_item {
> >  	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
> >  	NR_DIRTIED,		/* page dirtyings since bootup */
> >  	NR_WRITTEN,		/* page writings since bootup */
> > +	NR_INDIRECTLY_RECLAIMABLE_BYTES, /* measured in bytes */
> >  	NR_VM_NODE_STAT_ITEMS
> >  };
> >  
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index 40b2db6db6b1..b6b5684f31fe 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -1161,6 +1161,7 @@ const char * const vmstat_text[] = {
> >  	"nr_vmscan_immediate_reclaim",
> >  	"nr_dirtied",
> >  	"nr_written",
> > +	"nr_indirectly_reclaimable",
> >  
> >  	/* enum writeback_stat_item counters */
> >  	"nr_dirty_threshold",
> > 
> 

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Vlastimil Babka]

On 04/11/2018 03:56 PM, Roman Gushchin wrote:
> On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:
>> [+CC linux-api]
>>
>> On 03/05/2018 02:37 PM, Roman Gushchin wrote:
>>> This patch introduces a concept of indirectly reclaimable memory
>>> and adds the corresponding memory counter and /proc/vmstat item.
>>>
>>> Indirectly reclaimable memory is any sort of memory, used by
>>> the kernel (except of reclaimable slabs), which is actually
>>> reclaimable, i.e. will be released under memory pressure.
>>>
>>> The counter is in bytes, as it's not always possible to
>>> count such objects in pages. The name contains BYTES
>>> by analogy to NR_KERNEL_STACK_KB.
>>>
>>> Signed-off-by: Roman Gushchin <guro@fb.com>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>> Cc: linux-fsdevel@vger.kernel.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Cc: linux-mm@kvack.org
>>> Cc: kernel-team@fb.com
>>
>> Hmm, looks like I'm late and this user-visible API change was just
>> merged. But it's for rc1, so we can still change it, hopefully?
>>
>> One problem I see with the counter is that it's in bytes, but among
>> counters that use pages, and the name doesn't indicate it.
> 
> Here I just followed "nr_kernel_stack" path, which is measured in kB,
> but this is not mentioned in the field name.

Oh, didn't know. Bad example to follow :P

>> Then, I don't
>> see why users should care about the "indirectly" part, as that's just an
>> implementation detail. It is reclaimable and that's what matters, right?
>> (I also wanted to complain about lack of Documentation/... update, but
>> looks like there's no general file about vmstat, ugh)
> 
> I agree, that it's a bit weird, and it's probably better to not expose
> it at all; but this is how all vm counters work. We do expose them all
> in /proc/vmstat. A good number of them is useless until you are not a
> mm developer, so it's arguable more "debug info" rather than "api".

Yeah the problem is that once tools start rely on them, they fall under
the "do not break userspace" rule, however we call them. So being
cautious and conservative can't hurt.

> It's definitely not a reason to make them messy.
> Does "nr_indirectly_reclaimable_bytes" look better to you?

It still has has the "indirecly" part and feels arbitrary :/

>>
>> I also kind of liked the idea from v1 rfc posting that there would be a
>> separate set of reclaimable kmalloc-X caches for these kind of
>> allocations. Besides accounting, it should also help reduce memory
>> fragmentation. The right variant of cache would be detected via
>> __GFP_RECLAIMABLE.
> 
> Well, the downside is that we have to introduce X new caches
> just for this particular problem. I'm not strictly against the idea,
> but not convinced that it's much better.

Maybe we can find more cases that would benefit from it. Heck, even slab
itself allocates some management structures from the generic kmalloc
caches, and if they are used for reclaimable caches, they could be
tracked as reclaimable as well.

>>
>> With that in mind, can we at least for now put the (manually maintained)
>> byte counter in a variable that's not directly exposed via /proc/vmstat,
>> and then when printing nr_slab_reclaimable, simply add the value
>> (divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
>> subtract the same value. This way we would be simply making the existing
>> counters more precise, in line with their semantics.
> 
> Idk, I don't like the idea of adding a counter outside of the vm counters
> infrastructure, and I definitely wouldn't touch the exposed
> nr_slab_reclaimable and nr_slab_unreclaimable fields.

We would be just making the reported values more precise wrt reality.

> We do have some stats in /proc/slabinfo, /proc/meminfo and /sys/kernel/slab
> and I think that we should keep it consistent.

Right, meminfo would be adjusted the same. slabinfo doesn't indicate
which caches are reclaimable, so there will be no change.
/sys/kernel/slab/cache/reclaim_account does, but I doubt anything will
break.

> Thanks!
> 
>>
>> Thoughts?
>> Vlastimil
>>
>>> ---
>>>  include/linux/mmzone.h | 1 +
>>>  mm/vmstat.c            | 1 +
>>>  2 files changed, 2 insertions(+)
>>>
>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>> index e09fe563d5dc..15e783f29e21 100644
>>> --- a/include/linux/mmzone.h
>>> +++ b/include/linux/mmzone.h
>>> @@ -180,6 +180,7 @@ enum node_stat_item {
>>>  	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
>>>  	NR_DIRTIED,		/* page dirtyings since bootup */
>>>  	NR_WRITTEN,		/* page writings since bootup */
>>> +	NR_INDIRECTLY_RECLAIMABLE_BYTES, /* measured in bytes */
>>>  	NR_VM_NODE_STAT_ITEMS
>>>  };
>>>  
>>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>>> index 40b2db6db6b1..b6b5684f31fe 100644
>>> --- a/mm/vmstat.c
>>> +++ b/mm/vmstat.c
>>> @@ -1161,6 +1161,7 @@ const char * const vmstat_text[] = {
>>>  	"nr_vmscan_immediate_reclaim",
>>>  	"nr_dirtied",
>>>  	"nr_written",
>>> +	"nr_indirectly_reclaimable",
>>>  
>>>  	/* enum writeback_stat_item counters */
>>>  	"nr_dirty_threshold",
>>>
>>
> 

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Michal Hocko]

On Thu 12-04-18 08:52:52, Vlastimil Babka wrote:
> On 04/11/2018 03:56 PM, Roman Gushchin wrote:
> > On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:
[...]
> >> With that in mind, can we at least for now put the (manually maintained)
> >> byte counter in a variable that's not directly exposed via /proc/vmstat,
> >> and then when printing nr_slab_reclaimable, simply add the value
> >> (divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
> >> subtract the same value. This way we would be simply making the existing
> >> counters more precise, in line with their semantics.
> > 
> > Idk, I don't like the idea of adding a counter outside of the vm counters
> > infrastructure, and I definitely wouldn't touch the exposed
> > nr_slab_reclaimable and nr_slab_unreclaimable fields.

Why?

> We would be just making the reported values more precise wrt reality.

I was suggesting something similar in an earlier discussion. I am not
really happy about the new exposed counter either. It is just arbitrary
by name yet very specific for this particular usecase.

What is a poor user supposed to do with the new counter? Can this be
used for any calculations?
-- 
Michal Hocko
SUSE Lab

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Roman Gushchin]

On Thu, Apr 12, 2018 at 01:52:17PM +0200, Michal Hocko wrote:
> On Thu 12-04-18 08:52:52, Vlastimil Babka wrote:
> > On 04/11/2018 03:56 PM, Roman Gushchin wrote:
> > > On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:
> [...]
> > >> With that in mind, can we at least for now put the (manually maintained)
> > >> byte counter in a variable that's not directly exposed via /proc/vmstat,
> > >> and then when printing nr_slab_reclaimable, simply add the value
> > >> (divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
> > >> subtract the same value. This way we would be simply making the existing
> > >> counters more precise, in line with their semantics.
> > > 
> > > Idk, I don't like the idea of adding a counter outside of the vm counters
> > > infrastructure, and I definitely wouldn't touch the exposed
> > > nr_slab_reclaimable and nr_slab_unreclaimable fields.
> 
> Why?

Both nr_slab_reclaimable and nr_slab_unreclaimable have a very simple
meaning: they are numbers of pages used by corresponding slab caches.

In the answer to the very first version of this patchset
Andrew suggested to generalize the idea to allow further
accounting of non-kmalloc() allocations.
I like the idea, even if don't have a good example right now.

The problem with external names existed for many years before
we've accidentally hit it, so if we don't have other examples
right now, it doesn't mean that we wouldn't have them in the future.

> 
> > We would be just making the reported values more precise wrt reality.
> 
> I was suggesting something similar in an earlier discussion. I am not
> really happy about the new exposed counter either. It is just arbitrary
> by name yet very specific for this particular usecase.
> 
> What is a poor user supposed to do with the new counter? Can this be
> used for any calculations?

For me the most important part is to fix the overcommit logic, because it's
a real security and production issue. Adjusting MemAvailable is important too.

I really open here for any concrete suggestions on how to do it without exporting
of a new value, and without adding too much complexity to the code
(e.g. skipping this particular mm counter on printing will be quite messy).

Thanks!

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Michal Hocko]

On Thu 12-04-18 15:38:33, Roman Gushchin wrote:
> On Thu, Apr 12, 2018 at 01:52:17PM +0200, Michal Hocko wrote:
> > On Thu 12-04-18 08:52:52, Vlastimil Babka wrote:
> > > On 04/11/2018 03:56 PM, Roman Gushchin wrote:
> > > > On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:
> > [...]
> > > >> With that in mind, can we at least for now put the (manually maintained)
> > > >> byte counter in a variable that's not directly exposed via /proc/vmstat,
> > > >> and then when printing nr_slab_reclaimable, simply add the value
> > > >> (divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
> > > >> subtract the same value. This way we would be simply making the existing
> > > >> counters more precise, in line with their semantics.
> > > > 
> > > > Idk, I don't like the idea of adding a counter outside of the vm counters
> > > > infrastructure, and I definitely wouldn't touch the exposed
> > > > nr_slab_reclaimable and nr_slab_unreclaimable fields.
> > 
> > Why?
> 
> Both nr_slab_reclaimable and nr_slab_unreclaimable have a very simple
> meaning: they are numbers of pages used by corresponding slab caches.

Right, but if names are reclaimable then they should end up in the
reclaimable slabs and to be accounted as such. Objects themselves are
not sufficient to reclaim the accounted memory.

> In the answer to the very first version of this patchset
> Andrew suggested to generalize the idea to allow further
> accounting of non-kmalloc() allocations.
> I like the idea, even if don't have a good example right now.

Well, I have to disagree here. It sounds completely ad-hoc without
a reasoable semantic. Or how does it help users when they do not know
what is the indirect dependency and how to trigger it.

> The problem with external names existed for many years before
> we've accidentally hit it, so if we don't have other examples
> right now, it doesn't mean that we wouldn't have them in the future.
> 
> > 
> > > We would be just making the reported values more precise wrt reality.
> > 
> > I was suggesting something similar in an earlier discussion. I am not
> > really happy about the new exposed counter either. It is just arbitrary
> > by name yet very specific for this particular usecase.
> > 
> > What is a poor user supposed to do with the new counter? Can this be
> > used for any calculations?
> 
> For me the most important part is to fix the overcommit logic, because it's
> a real security and production issue.

Sure, the problem is ugly. Not the first one when the unaccounted kernel
allocation can eat a lot of memory. We have many other such. The usual
answer was to use kmemcg accounting.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Roman Gushchin]

On Thu, Apr 12, 2018 at 08:52:52AM +0200, Vlastimil Babka wrote:
> On 04/11/2018 03:56 PM, Roman Gushchin wrote:
> > On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:
> >> [+CC linux-api]
> >>
> >> On 03/05/2018 02:37 PM, Roman Gushchin wrote:
> >>> This patch introduces a concept of indirectly reclaimable memory
> >>> and adds the corresponding memory counter and /proc/vmstat item.
> >>>
> >>> Indirectly reclaimable memory is any sort of memory, used by
> >>> the kernel (except of reclaimable slabs), which is actually
> >>> reclaimable, i.e. will be released under memory pressure.
> >>>
> >>> The counter is in bytes, as it's not always possible to
> >>> count such objects in pages. The name contains BYTES
> >>> by analogy to NR_KERNEL_STACK_KB.
> >>>
> >>> Signed-off-by: Roman Gushchin <guro@fb.com>
> >>> Cc: Andrew Morton <akpm@linux-foundation.org>
> >>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> >>> Cc: Michal Hocko <mhocko@suse.com>
> >>> Cc: Johannes Weiner <hannes@cmpxchg.org>
> >>> Cc: linux-fsdevel@vger.kernel.org
> >>> Cc: linux-kernel@vger.kernel.org
> >>> Cc: linux-mm@kvack.org
> >>> Cc: kernel-team@fb.com
> >>
> >> Hmm, looks like I'm late and this user-visible API change was just
> >> merged. But it's for rc1, so we can still change it, hopefully?
> >>
> >> One problem I see with the counter is that it's in bytes, but among
> >> counters that use pages, and the name doesn't indicate it.
> > 
> > Here I just followed "nr_kernel_stack" path, which is measured in kB,
> > but this is not mentioned in the field name.
> 
> Oh, didn't know. Bad example to follow :P
> 
> >> Then, I don't
> >> see why users should care about the "indirectly" part, as that's just an
> >> implementation detail. It is reclaimable and that's what matters, right?
> >> (I also wanted to complain about lack of Documentation/... update, but
> >> looks like there's no general file about vmstat, ugh)
> > 
> > I agree, that it's a bit weird, and it's probably better to not expose
> > it at all; but this is how all vm counters work. We do expose them all
> > in /proc/vmstat. A good number of them is useless until you are not a
> > mm developer, so it's arguable more "debug info" rather than "api".
> 
> Yeah the problem is that once tools start rely on them, they fall under
> the "do not break userspace" rule, however we call them. So being
> cautious and conservative can't hurt.
> 
> > It's definitely not a reason to make them messy.
> > Does "nr_indirectly_reclaimable_bytes" look better to you?
> 
> It still has has the "indirecly" part and feels arbitrary :/
> 
> >>
> >> I also kind of liked the idea from v1 rfc posting that there would be a
> >> separate set of reclaimable kmalloc-X caches for these kind of
> >> allocations. Besides accounting, it should also help reduce memory
> >> fragmentation. The right variant of cache would be detected via
> >> __GFP_RECLAIMABLE.
> > 
> > Well, the downside is that we have to introduce X new caches
> > just for this particular problem. I'm not strictly against the idea,
> > but not convinced that it's much better.
> 
> Maybe we can find more cases that would benefit from it. Heck, even slab
> itself allocates some management structures from the generic kmalloc
> caches, and if they are used for reclaimable caches, they could be
> tracked as reclaimable as well.

This is a good catch!

> 
> >>
> >> With that in mind, can we at least for now put the (manually maintained)
> >> byte counter in a variable that's not directly exposed via /proc/vmstat,
> >> and then when printing nr_slab_reclaimable, simply add the value
> >> (divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
> >> subtract the same value. This way we would be simply making the existing
> >> counters more precise, in line with their semantics.
> > 
> > Idk, I don't like the idea of adding a counter outside of the vm counters
> > infrastructure, and I definitely wouldn't touch the exposed
> > nr_slab_reclaimable and nr_slab_unreclaimable fields.
> 
> We would be just making the reported values more precise wrt reality.

It depends on if we believe that only slab memory can be reclaimable
or not. If yes, this is true, otherwise not.

My guess is that some drivers (e.g. networking) might have buffers,
which are reclaimable under mempressure, and are allocated using
the page allocator. But I have to look closer...

> > We do have some stats in /proc/slabinfo, /proc/meminfo and /sys/kernel/slab
> > and I think that we should keep it consistent.
> 
> Right, meminfo would be adjusted the same. slabinfo doesn't indicate
> which caches are reclaimable, so there will be no change.
> /sys/kernel/slab/cache/reclaim_account does, but I doubt anything will
> break.

It also can be found out of the corresponding directory name in sysfs:
$ ls -la /sys/kernel/slab/dentr*
lrwxrwxrwx. 1 root root 0 Apr 11 14:45 /sys/kernel/slab/dentry -> :aA-0000192
                                                                   ^
						this is the "reclaimable" flag
Not saying that something will break.

Thanks!

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Michal Hocko]

On Thu 12-04-18 15:57:03, Roman Gushchin wrote:
> On Thu, Apr 12, 2018 at 08:52:52AM +0200, Vlastimil Babka wrote:
[...]
> > We would be just making the reported values more precise wrt reality.
> 
> It depends on if we believe that only slab memory can be reclaimable
> or not. If yes, this is true, otherwise not.
> 
> My guess is that some drivers (e.g. networking) might have buffers,
> which are reclaimable under mempressure, and are allocated using
> the page allocator. But I have to look closer...

Well, we have many direct page allocator users which are not accounted
in vmstat. Some of those use their specific accounting (e.g. network
buffers, some fs metadata a many others). In the ideal world MM layer
would know about those but...

Anyway, this particular case is quite clear, no? We _use_ kmalloc so
this is slab allocator. We just misaccount it.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[vinayak menon]

On Thu, Apr 12, 2018 at 8:27 PM, Roman Gushchin <guro@fb.com> wrote:
> On Thu, Apr 12, 2018 at 08:52:52AM +0200, Vlastimil Babka wrote:
>> On 04/11/2018 03:56 PM, Roman Gushchin wrote:
>> > On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:
>> >> [+CC linux-api]
>> >>
>> >> On 03/05/2018 02:37 PM, Roman Gushchin wrote:
>> >>> This patch introduces a concept of indirectly reclaimable memory
>> >>> and adds the corresponding memory counter and /proc/vmstat item.
>> >>>
>> >>> Indirectly reclaimable memory is any sort of memory, used by
>> >>> the kernel (except of reclaimable slabs), which is actually
>> >>> reclaimable, i.e. will be released under memory pressure.
>> >>>
>> >>> The counter is in bytes, as it's not always possible to
>> >>> count such objects in pages. The name contains BYTES
>> >>> by analogy to NR_KERNEL_STACK_KB.
>> >>>
>> >>> Signed-off-by: Roman Gushchin <guro@fb.com>
>> >>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> >>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> >>> Cc: Michal Hocko <mhocko@suse.com>
>> >>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> >>> Cc: linux-fsdevel@vger.kernel.org
>> >>> Cc: linux-kernel@vger.kernel.org
>> >>> Cc: linux-mm@kvack.org
>> >>> Cc: kernel-team@fb.com
>> >>
>> >> Hmm, looks like I'm late and this user-visible API change was just
>> >> merged. But it's for rc1, so we can still change it, hopefully?
>> >>
>> >> One problem I see with the counter is that it's in bytes, but among
>> >> counters that use pages, and the name doesn't indicate it.
>> >
>> > Here I just followed "nr_kernel_stack" path, which is measured in kB,
>> > but this is not mentioned in the field name.
>>
>> Oh, didn't know. Bad example to follow :P
>>
>> >> Then, I don't
>> >> see why users should care about the "indirectly" part, as that's just an
>> >> implementation detail. It is reclaimable and that's what matters, right?
>> >> (I also wanted to complain about lack of Documentation/... update, but
>> >> looks like there's no general file about vmstat, ugh)
>> >
>> > I agree, that it's a bit weird, and it's probably better to not expose
>> > it at all; but this is how all vm counters work. We do expose them all
>> > in /proc/vmstat. A good number of them is useless until you are not a
>> > mm developer, so it's arguable more "debug info" rather than "api".
>>
>> Yeah the problem is that once tools start rely on them, they fall under
>> the "do not break userspace" rule, however we call them. So being
>> cautious and conservative can't hurt.
>>
>> > It's definitely not a reason to make them messy.
>> > Does "nr_indirectly_reclaimable_bytes" look better to you?
>>
>> It still has has the "indirecly" part and feels arbitrary :/
>>
>> >>
>> >> I also kind of liked the idea from v1 rfc posting that there would be a
>> >> separate set of reclaimable kmalloc-X caches for these kind of
>> >> allocations. Besides accounting, it should also help reduce memory
>> >> fragmentation. The right variant of cache would be detected via
>> >> __GFP_RECLAIMABLE.
>> >
>> > Well, the downside is that we have to introduce X new caches
>> > just for this particular problem. I'm not strictly against the idea,
>> > but not convinced that it's much better.
>>
>> Maybe we can find more cases that would benefit from it. Heck, even slab
>> itself allocates some management structures from the generic kmalloc
>> caches, and if they are used for reclaimable caches, they could be
>> tracked as reclaimable as well.
>
> This is a good catch!
>
>>
>> >>
>> >> With that in mind, can we at least for now put the (manually maintained)
>> >> byte counter in a variable that's not directly exposed via /proc/vmstat,
>> >> and then when printing nr_slab_reclaimable, simply add the value
>> >> (divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
>> >> subtract the same value. This way we would be simply making the existing
>> >> counters more precise, in line with their semantics.
>> >
>> > Idk, I don't like the idea of adding a counter outside of the vm counters
>> > infrastructure, and I definitely wouldn't touch the exposed
>> > nr_slab_reclaimable and nr_slab_unreclaimable fields.
>>
>> We would be just making the reported values more precise wrt reality.
>
> It depends on if we believe that only slab memory can be reclaimable
> or not. If yes, this is true, otherwise not.
>
> My guess is that some drivers (e.g. networking) might have buffers,
> which are reclaimable under mempressure, and are allocated using
> the page allocator. But I have to look closer...
>

One such case I have encountered is that of the ION page pool. The page pool
registers a shrinker. When not in any memory pressure page pool can go high
and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.

Thanks,
Vinayak

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Minchan Kim]

On Mon, Mar 05, 2018 at 01:37:43PM +0000, Roman Gushchin wrote:
> I was reported about suspicious growth of unreclaimable slabs
> on some machines. I've found that it happens on machines
> with low memory pressure, and these unreclaimable slabs
> are external names attached to dentries.
> 
> External names are allocated using generic kmalloc() function,
> so they are accounted as unreclaimable. But they are held
> by dentries, which are reclaimable, and they will be reclaimed
> under the memory pressure.
> 
> In particular, this breaks MemAvailable calculation, as it
> doesn't take unreclaimable slabs into account.
> This leads to a silly situation, when a machine is almost idle,
> has no memory pressure and therefore has a big dentry cache.
> And the resulting MemAvailable is too low to start a new workload.
> 
> To address the issue, the NR_INDIRECTLY_RECLAIMABLE_BYTES counter
> is used to track the amount of memory, consumed by external names.
> The counter is increased in the dentry allocation path, if an external
> name structure is allocated; and it's decreased in the dentry freeing
> path.
> 
> To reproduce the problem I've used the following Python script:
>   import os
> 
>   for iter in range (0, 10000000):
>       try:
>           name = ("/some_long_name_%d" % iter) + "_" * 220
>           os.stat(name)
>       except Exception:
>           pass
> 
> Without this patch:
>   $ cat /proc/meminfo | grep MemAvailable
>   MemAvailable:    7811688 kB
>   $ python indirect.py
>   $ cat /proc/meminfo | grep MemAvailable
>   MemAvailable:    2753052 kB
> 
> With the patch:
>   $ cat /proc/meminfo | grep MemAvailable
>   MemAvailable:    7809516 kB
>   $ python indirect.py
>   $ cat /proc/meminfo | grep MemAvailable
>   MemAvailable:    7749144 kB
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: kernel-team@fb.com
> ---
>  fs/dcache.c | 29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 5c7df1df81ff..a0312d73f575 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -273,8 +273,16 @@ static void __d_free(struct rcu_head *head)
>  static void __d_free_external(struct rcu_head *head)
>  {
>  	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
> -	kfree(external_name(dentry));
> -	kmem_cache_free(dentry_cache, dentry); 
> +	struct external_name *name = external_name(dentry);
> +	unsigned long bytes;
> +
> +	bytes = dentry->d_name.len + offsetof(struct external_name, name[1]);
> +	mod_node_page_state(page_pgdat(virt_to_page(name)),
> +			    NR_INDIRECTLY_RECLAIMABLE_BYTES,
> +			    -kmalloc_size(kmalloc_index(bytes)));
> +
> +	kfree(name);
> +	kmem_cache_free(dentry_cache, dentry);
>  }
>  
>  static inline int dname_external(const struct dentry *dentry)
> @@ -1598,6 +1606,7 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
>  	struct dentry *dentry;
>  	char *dname;
>  	int err;
> +	size_t reclaimable = 0;
>  
>  	dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);
>  	if (!dentry)
> @@ -1614,9 +1623,11 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
>  		name = &slash_name;
>  		dname = dentry->d_iname;
>  	} else if (name->len > DNAME_INLINE_LEN-1) {
> -		size_t size = offsetof(struct external_name, name[1]);
> -		struct external_name *p = kmalloc(size + name->len,
> -						  GFP_KERNEL_ACCOUNT);
> +		struct external_name *p;
> +
> +		reclaimable = offsetof(struct external_name, name[1]) +
> +			name->len;
> +		p = kmalloc(reclaimable, GFP_KERNEL_ACCOUNT);

Can't we use kmem_cache_alloc with own cache created with SLAB_RECLAIM_ACCOUNT
if they are reclaimable? 
With that, it would help fragmentation problem with __GFP_RECLAIMABLE for
page allocation as well as counting problem, IMHO.


>  		if (!p) {
>  			kmem_cache_free(dentry_cache, dentry); 
>  			return NULL;
> @@ -1665,6 +1676,14 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
>  		}
>  	}
>  
> +	if (unlikely(reclaimable)) {
> +		pg_data_t *pgdat;
> +
> +		pgdat = page_pgdat(virt_to_page(external_name(dentry)));
> +		mod_node_page_state(pgdat, NR_INDIRECTLY_RECLAIMABLE_BYTES,
> +				    kmalloc_size(kmalloc_index(reclaimable)));
> +	}
> +
>  	this_cpu_inc(nr_dentry);
>  
>  	return dentry;
> -- 
> 2.14.3
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Michal Hocko]

On Fri 13-04-18 22:35:19, Minchan Kim wrote:
> On Mon, Mar 05, 2018 at 01:37:43PM +0000, Roman Gushchin wrote:
[...]
> > @@ -1614,9 +1623,11 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
> >  		name = &slash_name;
> >  		dname = dentry->d_iname;
> >  	} else if (name->len > DNAME_INLINE_LEN-1) {
> > -		size_t size = offsetof(struct external_name, name[1]);
> > -		struct external_name *p = kmalloc(size + name->len,
> > -						  GFP_KERNEL_ACCOUNT);
> > +		struct external_name *p;
> > +
> > +		reclaimable = offsetof(struct external_name, name[1]) +
> > +			name->len;
> > +		p = kmalloc(reclaimable, GFP_KERNEL_ACCOUNT);
> 
> Can't we use kmem_cache_alloc with own cache created with SLAB_RECLAIM_ACCOUNT
> if they are reclaimable? 

No, because names have different sizes and so we would basically have to
duplicate many caches.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Vlastimil Babka]

On 04/13/2018 03:59 PM, Michal Hocko wrote:
> On Fri 13-04-18 22:35:19, Minchan Kim wrote:
>> On Mon, Mar 05, 2018 at 01:37:43PM +0000, Roman Gushchin wrote:
> [...]
>>> @@ -1614,9 +1623,11 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
>>>  		name = &slash_name;
>>>  		dname = dentry->d_iname;
>>>  	} else if (name->len > DNAME_INLINE_LEN-1) {
>>> -		size_t size = offsetof(struct external_name, name[1]);
>>> -		struct external_name *p = kmalloc(size + name->len,
>>> -						  GFP_KERNEL_ACCOUNT);
>>> +		struct external_name *p;
>>> +
>>> +		reclaimable = offsetof(struct external_name, name[1]) +
>>> +			name->len;
>>> +		p = kmalloc(reclaimable, GFP_KERNEL_ACCOUNT);
>>
>> Can't we use kmem_cache_alloc with own cache created with SLAB_RECLAIM_ACCOUNT
>> if they are reclaimable? 
> 
> No, because names have different sizes and so we would basically have to
> duplicate many caches.

We would need kmalloc-reclaimable-X variants. It could be worth it,
especially if we find more similar usages. I suspect they would be more
useful than the existing dma-kmalloc-X :)

Maybe create both (dma and reclaimable) on demand?

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Michal Hocko]

On Fri 13-04-18 16:20:00, Vlastimil Babka wrote:
> On 04/13/2018 03:59 PM, Michal Hocko wrote:
> > On Fri 13-04-18 22:35:19, Minchan Kim wrote:
> >> On Mon, Mar 05, 2018 at 01:37:43PM +0000, Roman Gushchin wrote:
> > [...]
> >>> @@ -1614,9 +1623,11 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
> >>>  		name = &slash_name;
> >>>  		dname = dentry->d_iname;
> >>>  	} else if (name->len > DNAME_INLINE_LEN-1) {
> >>> -		size_t size = offsetof(struct external_name, name[1]);
> >>> -		struct external_name *p = kmalloc(size + name->len,
> >>> -						  GFP_KERNEL_ACCOUNT);
> >>> +		struct external_name *p;
> >>> +
> >>> +		reclaimable = offsetof(struct external_name, name[1]) +
> >>> +			name->len;
> >>> +		p = kmalloc(reclaimable, GFP_KERNEL_ACCOUNT);
> >>
> >> Can't we use kmem_cache_alloc with own cache created with SLAB_RECLAIM_ACCOUNT
> >> if they are reclaimable? 
> > 
> > No, because names have different sizes and so we would basically have to
> > duplicate many caches.
> 
> We would need kmalloc-reclaimable-X variants. It could be worth it,
> especially if we find more similar usages. I suspect they would be more
> useful than the existing dma-kmalloc-X :)

I am still not sure why __GFP_RECLAIMABLE cannot be made work as
expected and account slab pages as SLAB_RECLAIMABLE
-- 
Michal Hocko
SUSE Labs

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Johannes Weiner]

On Fri, Apr 13, 2018 at 04:28:21PM +0200, Michal Hocko wrote:
> On Fri 13-04-18 16:20:00, Vlastimil Babka wrote:
> > We would need kmalloc-reclaimable-X variants. It could be worth it,
> > especially if we find more similar usages. I suspect they would be more
> > useful than the existing dma-kmalloc-X :)
> 
> I am still not sure why __GFP_RECLAIMABLE cannot be made work as
> expected and account slab pages as SLAB_RECLAIMABLE

Can you outline how this would work without separate caches?

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Michal Hocko]

On Fri 13-04-18 10:37:16, Johannes Weiner wrote:
> On Fri, Apr 13, 2018 at 04:28:21PM +0200, Michal Hocko wrote:
> > On Fri 13-04-18 16:20:00, Vlastimil Babka wrote:
> > > We would need kmalloc-reclaimable-X variants. It could be worth it,
> > > especially if we find more similar usages. I suspect they would be more
> > > useful than the existing dma-kmalloc-X :)
> > 
> > I am still not sure why __GFP_RECLAIMABLE cannot be made work as
> > expected and account slab pages as SLAB_RECLAIMABLE
> 
> Can you outline how this would work without separate caches?

I thought that the cache would only maintain two sets of slab pages
depending on the allocation reuquests. I am pretty sure there will be
other details to iron out and maybe it will turn out that such a large
portion of the chache would need to duplicate the state that a
completely new cache would be more reasonable. Is this worth exploring
at least? I mean something like this should help with the fragmentation
already AFAIU. Accounting would be just free on top.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Vlastimil Babka]

On 04/16/2018 01:41 PM, Michal Hocko wrote:
> On Fri 13-04-18 10:37:16, Johannes Weiner wrote:
>> On Fri, Apr 13, 2018 at 04:28:21PM +0200, Michal Hocko wrote:
>>> On Fri 13-04-18 16:20:00, Vlastimil Babka wrote:
>>>> We would need kmalloc-reclaimable-X variants. It could be worth it,
>>>> especially if we find more similar usages. I suspect they would be more
>>>> useful than the existing dma-kmalloc-X :)
>>>
>>> I am still not sure why __GFP_RECLAIMABLE cannot be made work as
>>> expected and account slab pages as SLAB_RECLAIMABLE
>>
>> Can you outline how this would work without separate caches?
> 
> I thought that the cache would only maintain two sets of slab pages
> depending on the allocation reuquests. I am pretty sure there will be
> other details to iron out and

For example the percpu (and other) array caches...

> maybe it will turn out that such a large
> portion of the chache would need to duplicate the state that a
> completely new cache would be more reasonable.

I'm afraid that's the case, yes.

> Is this worth exploring
> at least? I mean something like this should help with the fragmentation
> already AFAIU. Accounting would be just free on top.

Yep. It could be also CONFIG_urable so smaller systems don't need to
deal with the memory overhead of this.

So do we put it on LSF/MM agenda?

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Michal Hocko]

On Mon 16-04-18 14:06:21, Vlastimil Babka wrote:
> On 04/16/2018 01:41 PM, Michal Hocko wrote:
> > On Fri 13-04-18 10:37:16, Johannes Weiner wrote:
> >> On Fri, Apr 13, 2018 at 04:28:21PM +0200, Michal Hocko wrote:
> >>> On Fri 13-04-18 16:20:00, Vlastimil Babka wrote:
> >>>> We would need kmalloc-reclaimable-X variants. It could be worth it,
> >>>> especially if we find more similar usages. I suspect they would be more
> >>>> useful than the existing dma-kmalloc-X :)
> >>>
> >>> I am still not sure why __GFP_RECLAIMABLE cannot be made work as
> >>> expected and account slab pages as SLAB_RECLAIMABLE
> >>
> >> Can you outline how this would work without separate caches?
> > 
> > I thought that the cache would only maintain two sets of slab pages
> > depending on the allocation reuquests. I am pretty sure there will be
> > other details to iron out and
> 
> For example the percpu (and other) array caches...
> 
> > maybe it will turn out that such a large
> > portion of the chache would need to duplicate the state that a
> > completely new cache would be more reasonable.
> 
> I'm afraid that's the case, yes.
> 
> > Is this worth exploring
> > at least? I mean something like this should help with the fragmentation
> > already AFAIU. Accounting would be just free on top.
> 
> Yep. It could be also CONFIG_urable so smaller systems don't need to
> deal with the memory overhead of this.
> 
> So do we put it on LSF/MM agenda?

If you volunteer to lead the discussion, then I do not have any
objections.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Matthew Wilcox]

On Mon, Apr 16, 2018 at 02:06:21PM +0200, Vlastimil Babka wrote:
> On 04/16/2018 01:41 PM, Michal Hocko wrote:
> > On Fri 13-04-18 10:37:16, Johannes Weiner wrote:
> >> On Fri, Apr 13, 2018 at 04:28:21PM +0200, Michal Hocko wrote:
> >>> On Fri 13-04-18 16:20:00, Vlastimil Babka wrote:
> >>>> We would need kmalloc-reclaimable-X variants. It could be worth it,
> >>>> especially if we find more similar usages. I suspect they would be more
> >>>> useful than the existing dma-kmalloc-X :)
> >>>
> >>> I am still not sure why __GFP_RECLAIMABLE cannot be made work as
> >>> expected and account slab pages as SLAB_RECLAIMABLE
> >>
> >> Can you outline how this would work without separate caches?
> > 
> > I thought that the cache would only maintain two sets of slab pages
> > depending on the allocation reuquests. I am pretty sure there will be
> > other details to iron out and
> 
> For example the percpu (and other) array caches...
> 
> > maybe it will turn out that such a large
> > portion of the chache would need to duplicate the state that a
> > completely new cache would be more reasonable.
> 
> I'm afraid that's the case, yes.

I'm not sure it'll be so bad, at least for SLUB ... I think everything
we need to duplicate is already percpu, and if we combine GFP_DMA
and GFP_RECLAIMABLE into this, we might even get more savings.  Also,
we only need to do this for the kmalloc slabs; currently 13 of them.
So we eliminate 13 caches and in return allocate 13 * 2 * NR_CPU pointers.
That'll be a win on some machines and a loss on others, but the machines
where it's consuming more memory should have more memory to begin with,
so I'd count it as a win.

The node partial list probably wants to be trebled in size to have one
list per memory type.  But I think the allocation path only changes
like this:

@@ -2663,10 +2663,13 @@ static __always_inline void *slab_alloc_node(struct kmem
_cache *s,
        struct kmem_cache_cpu *c;
        struct page *page;
        unsigned long tid;
+       unsigned int offset = 0;
 
        s = slab_pre_alloc_hook(s, gfpflags);
        if (!s)
                return NULL;
        if (s->flags & SLAB_KMALLOC)
                offset = flags_to_slab_id(gfpflags);
 redo:
        /*
         * Must read kmem_cache cpu data via this cpu ptr. Preemption is
@@ -2679,8 +2682,8 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
         * to check if it is matched or not.
         */
        do {
-               tid = this_cpu_read(s->cpu_slab->tid);
-               c = raw_cpu_ptr(s->cpu_slab);
+               tid = this_cpu_read((&s->cpu_slab[offset])->tid);
+               c = raw_cpu_ptr(&s->cpu_slab[offset]);
        } while (IS_ENABLED(CONFIG_PREEMPT) &&
                 unlikely(tid != READ_ONCE(c->tid)));
 

> > Is this worth exploring
> > at least? I mean something like this should help with the fragmentation
> > already AFAIU. Accounting would be just free on top.
> 
> Yep. It could be also CONFIG_urable so smaller systems don't need to
> deal with the memory overhead of this.
> 
> So do we put it on LSF/MM agenda?

We have an agenda?  :-)

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Vlastimil Babka]

On 04/16/2018 02:27 PM, Michal Hocko wrote:
> On Mon 16-04-18 14:06:21, Vlastimil Babka wrote:
>>
>> For example the percpu (and other) array caches...
>>
>>> maybe it will turn out that such a large
>>> portion of the chache would need to duplicate the state that a
>>> completely new cache would be more reasonable.
>>
>> I'm afraid that's the case, yes.
>>
>>> Is this worth exploring
>>> at least? I mean something like this should help with the fragmentation
>>> already AFAIU. Accounting would be just free on top.
>>
>> Yep. It could be also CONFIG_urable so smaller systems don't need to
>> deal with the memory overhead of this.
>>
>> So do we put it on LSF/MM agenda?
> 
> If you volunteer to lead the discussion, then I do not have any
> objections.

Sure, let's add the topic of SLAB_MINIMIZE_WASTE [1] as well.

Something like "Supporting reclaimable kmalloc caches and large
non-buddy-sized objects in slab allocators" ?

[1] https://marc.info/?l=linux-mm&m=152156671614796&w=2

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Michal Hocko]

[the head of the thread is http://lkml.kernel.org/r/08524819-14ef-81d0-fa90-d7af13c6b9d5@suse.cz]

On Mon 16-04-18 21:57:50, Vlastimil Babka wrote:
> On 04/16/2018 02:27 PM, Michal Hocko wrote:
> > On Mon 16-04-18 14:06:21, Vlastimil Babka wrote:
> >>
> >> For example the percpu (and other) array caches...
> >>
> >>> maybe it will turn out that such a large
> >>> portion of the chache would need to duplicate the state that a
> >>> completely new cache would be more reasonable.
> >>
> >> I'm afraid that's the case, yes.
> >>
> >>> Is this worth exploring
> >>> at least? I mean something like this should help with the fragmentation
> >>> already AFAIU. Accounting would be just free on top.
> >>
> >> Yep. It could be also CONFIG_urable so smaller systems don't need to
> >> deal with the memory overhead of this.
> >>
> >> So do we put it on LSF/MM agenda?
> > 
> > If you volunteer to lead the discussion, then I do not have any
> > objections.
> 
> Sure, let's add the topic of SLAB_MINIMIZE_WASTE [1] as well.
> 
> Something like "Supporting reclaimable kmalloc caches and large
> non-buddy-sized objects in slab allocators" ?
> 
> [1] https://marc.info/?l=linux-mm&m=152156671614796&w=2

OK, noted.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory

[Roman Gushchin]

On Mon, Apr 16, 2018 at 01:41:44PM +0200, Michal Hocko wrote:
> On Fri 13-04-18 10:37:16, Johannes Weiner wrote:
> > On Fri, Apr 13, 2018 at 04:28:21PM +0200, Michal Hocko wrote:
> > > On Fri 13-04-18 16:20:00, Vlastimil Babka wrote:
> > > > We would need kmalloc-reclaimable-X variants. It could be worth it,
> > > > especially if we find more similar usages. I suspect they would be more
> > > > useful than the existing dma-kmalloc-X :)
> > > 
> > > I am still not sure why __GFP_RECLAIMABLE cannot be made work as
> > > expected and account slab pages as SLAB_RECLAIMABLE
> > 
> > Can you outline how this would work without separate caches?
> 
> I thought that the cache would only maintain two sets of slab pages
> depending on the allocation reuquests. I am pretty sure there will be
> other details to iron out and maybe it will turn out that such a large
> portion of the chache would need to duplicate the state that a
> completely new cache would be more reasonable. Is this worth exploring
> at least? I mean something like this should help with the fragmentation
> already AFAIU. Accounting would be just free on top.

IMO, this approach is much better than duplicating all kmalloc caches.
It's definitely has to be explored and discussed.

Thank you!

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Vijayanand Jitta]

On 4/13/2018 5:43 PM, vinayak menon wrote:
> On Thu, Apr 12, 2018 at 8:27 PM, Roman Gushchin <guro@fb.com> wrote:
>> On Thu, Apr 12, 2018 at 08:52:52AM +0200, Vlastimil Babka wrote:
>>> On 04/11/2018 03:56 PM, Roman Gushchin wrote:
>>>> On Wed, Apr 11, 2018 at 03:16:08PM +0200, Vlastimil Babka wrote:
>>>>> [+CC linux-api]
>>>>>
>>>>> On 03/05/2018 02:37 PM, Roman Gushchin wrote:
>>>>>> This patch introduces a concept of indirectly reclaimable memory
>>>>>> and adds the corresponding memory counter and /proc/vmstat item.
>>>>>>
>>>>>> Indirectly reclaimable memory is any sort of memory, used by
>>>>>> the kernel (except of reclaimable slabs), which is actually
>>>>>> reclaimable, i.e. will be released under memory pressure.
>>>>>>
>>>>>> The counter is in bytes, as it's not always possible to
>>>>>> count such objects in pages. The name contains BYTES
>>>>>> by analogy to NR_KERNEL_STACK_KB.
>>>>>>
>>>>>> Signed-off-by: Roman Gushchin <guro@fb.com>
>>>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>>>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>>>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>>>>> Cc: linux-fsdevel@vger.kernel.org
>>>>>> Cc: linux-kernel@vger.kernel.org
>>>>>> Cc: linux-mm@kvack.org
>>>>>> Cc: kernel-team@fb.com
>>>>>
>>>>> Hmm, looks like I'm late and this user-visible API change was just
>>>>> merged. But it's for rc1, so we can still change it, hopefully?
>>>>>
>>>>> One problem I see with the counter is that it's in bytes, but among
>>>>> counters that use pages, and the name doesn't indicate it.
>>>>
>>>> Here I just followed "nr_kernel_stack" path, which is measured in kB,
>>>> but this is not mentioned in the field name.
>>>
>>> Oh, didn't know. Bad example to follow :P
>>>
>>>>> Then, I don't
>>>>> see why users should care about the "indirectly" part, as that's just an
>>>>> implementation detail. It is reclaimable and that's what matters, right?
>>>>> (I also wanted to complain about lack of Documentation/... update, but
>>>>> looks like there's no general file about vmstat, ugh)
>>>>
>>>> I agree, that it's a bit weird, and it's probably better to not expose
>>>> it at all; but this is how all vm counters work. We do expose them all
>>>> in /proc/vmstat. A good number of them is useless until you are not a
>>>> mm developer, so it's arguable more "debug info" rather than "api".
>>>
>>> Yeah the problem is that once tools start rely on them, they fall under
>>> the "do not break userspace" rule, however we call them. So being
>>> cautious and conservative can't hurt.
>>>
>>>> It's definitely not a reason to make them messy.
>>>> Does "nr_indirectly_reclaimable_bytes" look better to you?
>>>
>>> It still has has the "indirecly" part and feels arbitrary :/
>>>
>>>>>
>>>>> I also kind of liked the idea from v1 rfc posting that there would be a
>>>>> separate set of reclaimable kmalloc-X caches for these kind of
>>>>> allocations. Besides accounting, it should also help reduce memory
>>>>> fragmentation. The right variant of cache would be detected via
>>>>> __GFP_RECLAIMABLE.
>>>>
>>>> Well, the downside is that we have to introduce X new caches
>>>> just for this particular problem. I'm not strictly against the idea,
>>>> but not convinced that it's much better.
>>>
>>> Maybe we can find more cases that would benefit from it. Heck, even slab
>>> itself allocates some management structures from the generic kmalloc
>>> caches, and if they are used for reclaimable caches, they could be
>>> tracked as reclaimable as well.
>>
>> This is a good catch!
>>
>>>
>>>>>
>>>>> With that in mind, can we at least for now put the (manually maintained)
>>>>> byte counter in a variable that's not directly exposed via /proc/vmstat,
>>>>> and then when printing nr_slab_reclaimable, simply add the value
>>>>> (divided by PAGE_SIZE), and when printing nr_slab_unreclaimable,
>>>>> subtract the same value. This way we would be simply making the existing
>>>>> counters more precise, in line with their semantics.
>>>>
>>>> Idk, I don't like the idea of adding a counter outside of the vm counters
>>>> infrastructure, and I definitely wouldn't touch the exposed
>>>> nr_slab_reclaimable and nr_slab_unreclaimable fields.
>>>
>>> We would be just making the reported values more precise wrt reality.
>>
>> It depends on if we believe that only slab memory can be reclaimable
>> or not. If yes, this is true, otherwise not.
>>
>> My guess is that some drivers (e.g. networking) might have buffers,
>> which are reclaimable under mempressure, and are allocated using
>> the page allocator. But I have to look closer...
>>
> 
> One such case I have encountered is that of the ION page pool. The page pool
> registers a shrinker. When not in any memory pressure page pool can go high
> and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
> a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.
> 
> Thanks,
> Vinayak
> 

As Vinayak mentioned NR_INDIRECTLY_RECLAIMABLE_BYTES can be used to solve the issue
with ION page pool when OVERCOMMIT_GUESS is set, the patch for the same can be 
found here https://lkml.org/lkml/2018/4/24/1288

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Roman Gushchin]

On Wed, Apr 25, 2018 at 09:19:29AM +0530, Vijayanand Jitta wrote:
> >>>> Idk, I don't like the idea of adding a counter outside of the vm counters
> >>>> infrastructure, and I definitely wouldn't touch the exposed
> >>>> nr_slab_reclaimable and nr_slab_unreclaimable fields.
> >>>
> >>> We would be just making the reported values more precise wrt reality.
> >>
> >> It depends on if we believe that only slab memory can be reclaimable
> >> or not. If yes, this is true, otherwise not.
> >>
> >> My guess is that some drivers (e.g. networking) might have buffers,
> >> which are reclaimable under mempressure, and are allocated using
> >> the page allocator. But I have to look closer...
> >>
> > 
> > One such case I have encountered is that of the ION page pool. The page pool
> > registers a shrinker. When not in any memory pressure page pool can go high
> > and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
> > a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.

Perfect!
This is exactly what I've expected.

> > 
> > Thanks,
> > Vinayak
> > 
> 
> As Vinayak mentioned NR_INDIRECTLY_RECLAIMABLE_BYTES can be used to solve the issue
> with ION page pool when OVERCOMMIT_GUESS is set, the patch for the same can be 
> found here https://lkml.org/lkml/2018/4/24/1288

This makes perfect sense to me.

Please, fell free to add:
Acked-by: Roman Gushchin <guro@fb.com>

Thank you!

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Vlastimil Babka]

On 04/25/2018 02:52 PM, Roman Gushchin wrote:
> On Wed, Apr 25, 2018 at 09:19:29AM +0530, Vijayanand Jitta wrote:
>>>>>> Idk, I don't like the idea of adding a counter outside of the vm counters
>>>>>> infrastructure, and I definitely wouldn't touch the exposed
>>>>>> nr_slab_reclaimable and nr_slab_unreclaimable fields.
>>>>>
>>>>> We would be just making the reported values more precise wrt reality.
>>>>
>>>> It depends on if we believe that only slab memory can be reclaimable
>>>> or not. If yes, this is true, otherwise not.
>>>>
>>>> My guess is that some drivers (e.g. networking) might have buffers,
>>>> which are reclaimable under mempressure, and are allocated using
>>>> the page allocator. But I have to look closer...
>>>>
>>>
>>> One such case I have encountered is that of the ION page pool. The page pool
>>> registers a shrinker. When not in any memory pressure page pool can go high
>>> and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
>>> a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.

FYI, we have discussed this at LSF/MM and agreed to try the kmalloc
reclaimable caches idea. The existing counter could then remain for page
allocator users such as ION. It's a bit weird to have it in bytes and
not pages then, IMHO. What if we hid it from /proc/vmstat now so it
doesn't become ABI, and later convert it to page granularity and expose
it under a name such as "nr_other_reclaimable" ?

Vlastimil

> Perfect!
> This is exactly what I've expected.
> 
>>>
>>> Thanks,
>>> Vinayak
>>>
>>
>> As Vinayak mentioned NR_INDIRECTLY_RECLAIMABLE_BYTES can be used to solve the issue
>> with ION page pool when OVERCOMMIT_GUESS is set, the patch for the same can be 
>> found here https://lkml.org/lkml/2018/4/24/1288
> 
> This makes perfect sense to me.
> 
> Please, fell free to add:
> Acked-by: Roman Gushchin <guro@fb.com>
> 
> Thank you!
> 

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Matthew Wilcox]

On Fri, Apr 13, 2018 at 05:43:39PM +0530, vinayak menon wrote:
> One such case I have encountered is that of the ION page pool. The page pool
> registers a shrinker. When not in any memory pressure page pool can go high
> and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
> a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.

Why not just account them as NR_SLAB_RECLAIMABLE?  I know it's not slab, but
other than that mis-naming, it seems like it'll do the right thing.

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Roman Gushchin]

On Wed, Apr 25, 2018 at 05:47:26PM +0200, Vlastimil Babka wrote:
> On 04/25/2018 02:52 PM, Roman Gushchin wrote:
> > On Wed, Apr 25, 2018 at 09:19:29AM +0530, Vijayanand Jitta wrote:
> >>>>>> Idk, I don't like the idea of adding a counter outside of the vm counters
> >>>>>> infrastructure, and I definitely wouldn't touch the exposed
> >>>>>> nr_slab_reclaimable and nr_slab_unreclaimable fields.
> >>>>>
> >>>>> We would be just making the reported values more precise wrt reality.
> >>>>
> >>>> It depends on if we believe that only slab memory can be reclaimable
> >>>> or not. If yes, this is true, otherwise not.
> >>>>
> >>>> My guess is that some drivers (e.g. networking) might have buffers,
> >>>> which are reclaimable under mempressure, and are allocated using
> >>>> the page allocator. But I have to look closer...
> >>>>
> >>>
> >>> One such case I have encountered is that of the ION page pool. The page pool
> >>> registers a shrinker. When not in any memory pressure page pool can go high
> >>> and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
> >>> a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.
> 
> FYI, we have discussed this at LSF/MM and agreed to try the kmalloc
> reclaimable caches idea. The existing counter could then remain for page
> allocator users such as ION. It's a bit weird to have it in bytes and
> not pages then, IMHO. What if we hid it from /proc/vmstat now so it
> doesn't become ABI, and later convert it to page granularity and expose
> it under a name such as "nr_other_reclaimable" ?

I've nothing against hiding it from /proc/vmstat, as long as we keep
the counter in place and the main issue resolved.

Maybe it's better to add nr_reclaimable = nr_slab_reclaimable + nr_other_reclaimable,
which will have a simpler meaning that nr_other_reclaimable (what is other?).

Thanks!

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Vlastimil Babka]

On 04/25/2018 05:55 PM, Matthew Wilcox wrote:
> On Fri, Apr 13, 2018 at 05:43:39PM +0530, vinayak menon wrote:
>> One such case I have encountered is that of the ION page pool. The page pool
>> registers a shrinker. When not in any memory pressure page pool can go high
>> and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
>> a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.
> 
> Why not just account them as NR_SLAB_RECLAIMABLE?  I know it's not slab, but
> other than that mis-naming, it seems like it'll do the right thing.

Hm I think it would be confusing for anyone trying to correlate the
number with /proc/slabinfo - the numbers there wouldn't add up.

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Vlastimil Babka]

On 04/25/2018 06:48 PM, Roman Gushchin wrote:
> On Wed, Apr 25, 2018 at 05:47:26PM +0200, Vlastimil Babka wrote:
>> On 04/25/2018 02:52 PM, Roman Gushchin wrote:
>>> On Wed, Apr 25, 2018 at 09:19:29AM +0530, Vijayanand Jitta wrote:
>>>>>>>> Idk, I don't like the idea of adding a counter outside of the vm counters
>>>>>>>> infrastructure, and I definitely wouldn't touch the exposed
>>>>>>>> nr_slab_reclaimable and nr_slab_unreclaimable fields.
>>>>>>>
>>>>>>> We would be just making the reported values more precise wrt reality.
>>>>>>
>>>>>> It depends on if we believe that only slab memory can be reclaimable
>>>>>> or not. If yes, this is true, otherwise not.
>>>>>>
>>>>>> My guess is that some drivers (e.g. networking) might have buffers,
>>>>>> which are reclaimable under mempressure, and are allocated using
>>>>>> the page allocator. But I have to look closer...
>>>>>>
>>>>>
>>>>> One such case I have encountered is that of the ION page pool. The page pool
>>>>> registers a shrinker. When not in any memory pressure page pool can go high
>>>>> and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
>>>>> a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.
>>
>> FYI, we have discussed this at LSF/MM and agreed to try the kmalloc
>> reclaimable caches idea. The existing counter could then remain for page
>> allocator users such as ION. It's a bit weird to have it in bytes and
>> not pages then, IMHO. What if we hid it from /proc/vmstat now so it
>> doesn't become ABI, and later convert it to page granularity and expose
>> it under a name such as "nr_other_reclaimable" ?
> 
> I've nothing against hiding it from /proc/vmstat, as long as we keep
> the counter in place and the main issue resolved.

Sure.

> Maybe it's better to add nr_reclaimable = nr_slab_reclaimable + nr_other_reclaimable,
> which will have a simpler meaning that nr_other_reclaimable (what is other?).

"other" can be changed, sure. nr_reclaimable is possible if we change
slab to adjust that counter as well - vmstat code doesn't support
arbitrary calculations when printing.

> Thanks!
> 

Re: [PATCH 1/3] mm: introduce NR_INDIRECTLY_RECLAIMABLE_BYTES

[Roman Gushchin]

On Wed, Apr 25, 2018 at 07:02:42PM +0200, Vlastimil Babka wrote:
> On 04/25/2018 06:48 PM, Roman Gushchin wrote:
> > On Wed, Apr 25, 2018 at 05:47:26PM +0200, Vlastimil Babka wrote:
> >> On 04/25/2018 02:52 PM, Roman Gushchin wrote:
> >>> On Wed, Apr 25, 2018 at 09:19:29AM +0530, Vijayanand Jitta wrote:
> >>>>>>>> Idk, I don't like the idea of adding a counter outside of the vm counters
> >>>>>>>> infrastructure, and I definitely wouldn't touch the exposed
> >>>>>>>> nr_slab_reclaimable and nr_slab_unreclaimable fields.
> >>>>>>>
> >>>>>>> We would be just making the reported values more precise wrt reality.
> >>>>>>
> >>>>>> It depends on if we believe that only slab memory can be reclaimable
> >>>>>> or not. If yes, this is true, otherwise not.
> >>>>>>
> >>>>>> My guess is that some drivers (e.g. networking) might have buffers,
> >>>>>> which are reclaimable under mempressure, and are allocated using
> >>>>>> the page allocator. But I have to look closer...
> >>>>>>
> >>>>>
> >>>>> One such case I have encountered is that of the ION page pool. The page pool
> >>>>> registers a shrinker. When not in any memory pressure page pool can go high
> >>>>> and thus cause an mmap to fail when OVERCOMMIT_GUESS is set. I can send
> >>>>> a patch to account ION page pool pages in NR_INDIRECTLY_RECLAIMABLE_BYTES.
> >>
> >> FYI, we have discussed this at LSF/MM and agreed to try the kmalloc
> >> reclaimable caches idea. The existing counter could then remain for page
> >> allocator users such as ION. It's a bit weird to have it in bytes and
> >> not pages then, IMHO. What if we hid it from /proc/vmstat now so it
> >> doesn't become ABI, and later convert it to page granularity and expose
> >> it under a name such as "nr_other_reclaimable" ?
> > 
> > I've nothing against hiding it from /proc/vmstat, as long as we keep
> > the counter in place and the main issue resolved.
> 
> Sure.
> 
> > Maybe it's better to add nr_reclaimable = nr_slab_reclaimable + nr_other_reclaimable,
> > which will have a simpler meaning that nr_other_reclaimable (what is other?).
> 
> "other" can be changed, sure. nr_reclaimable is possible if we change
> slab to adjust that counter as well - vmstat code doesn't support
> arbitrary calculations when printing.

Sure, but even just hiding a value isn't that easy now.
So we have to touch the vmstat printing code anyway.

Thanks!