[Stable 4.4 - NEEDS REVIEW - 0/3] mm: working set fixes

[Stable 4.4 - NEEDS REVIEW - 0/3] mm: working set fixes

[Michal Hocko]

Hi Johannes,
this is my attempt to backport your 22f2ac51b6d6 ("mm: workingset: fix
crash in shadow node shrinker caused by replace_page_cache_page()")
which has been marked for 3.15+ stable trees. There are 2 follow up
fixes for this patch d3798ae8c6f3 ("mm: filemap: don't plant shadow
entries without radix tree node") and 3ddf40e8c319 ("mm: filemap: fix
mapping->nrpages double accounting in fuse") which are backported here
as well.

This is not an area I would feel really strongly so I would highly
appreciate if you could review these backports. The first two needed
quite some tweaking.

Thanks!

[Stable 4.4 - NEEDS REVIEW - 1/3] mm: workingset: fix crash in shadow node shrinker caused by replace_page_cache_page()

[Michal Hocko]

From: Johannes Weiner <hannes@cmpxchg.org>

Commit 22f2ac51b6d643666f4db093f13144f773ff3f3a upstream.

Antonio reports the following crash when using fuse under memory pressure:

  kernel BUG at /build/linux-a2WvEb/linux-4.4.0/mm/workingset.c:346!
  invalid opcode: 0000 [#1] SMP
  Modules linked in: all of them
  CPU: 2 PID: 63 Comm: kswapd0 Not tainted 4.4.0-36-generic #55-Ubuntu
  Hardware name: System manufacturer System Product Name/P8H67-M PRO, BIOS 3904 04/27/2013
  task: ffff88040cae6040 ti: ffff880407488000 task.ti: ffff880407488000
  RIP: shadow_lru_isolate+0x181/0x190
  Call Trace:
    __list_lru_walk_one.isra.3+0x8f/0x130
    list_lru_walk_one+0x23/0x30
    scan_shadow_nodes+0x34/0x50
    shrink_slab.part.40+0x1ed/0x3d0
    shrink_zone+0x2ca/0x2e0
    kswapd+0x51e/0x990
    kthread+0xd8/0xf0
    ret_from_fork+0x3f/0x70

which corresponds to the following sanity check in the shadow node
tracking:

  BUG_ON(node->count & RADIX_TREE_COUNT_MASK);

The workingset code tracks radix tree nodes that exclusively contain
shadow entries of evicted pages in them, and this (somewhat obscure)
line checks whether there are real pages left that would interfere with
reclaim of the radix tree node under memory pressure.

While discussing ways how fuse might sneak pages into the radix tree
past the workingset code, Miklos pointed to replace_page_cache_page(),
and indeed there is a problem there: it properly accounts for the old
page being removed - __delete_from_page_cache() does that - but then
does a raw raw radix_tree_insert(), not accounting for the replacement
page.  Eventually the page count bits in node->count underflow while
leaving the node incorrectly linked to the shadow node LRU.

To address this, make sure replace_page_cache_page() uses the tracked
page insertion code, page_cache_tree_insert().  This fixes the page
accounting and makes sure page-containing nodes are properly unlinked
from the shadow node LRU again.

Also, make the sanity checks a bit less obscure by using the helpers for
checking the number of pages and shadows in a radix tree node.

Fixes: 449dd6984d0e ("mm: keep page cache radix tree nodes in check")
Link: http://lkml.kernel.org/r/20160919155822.29498-1-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Antonio SJ Musumeci <trapexit@spawn.link>
Debugged-by: Miklos Szeredi <miklos@szeredi.hu>
Cc: <stable@vger.kernel.org>	[3.15+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 include/linux/swap.h |  2 ++
 mm/filemap.c         | 86 ++++++++++++++++++++++++++--------------------------
 mm/workingset.c      | 10 +++---
 3 files changed, 49 insertions(+), 49 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 7ba7dccaf0e7..b28de19aadbf 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -266,6 +266,7 @@ static inline void workingset_node_pages_inc(struct radix_tree_node *node)
 
 static inline void workingset_node_pages_dec(struct radix_tree_node *node)
 {
+	VM_BUG_ON(!workingset_node_pages(node));
 	node->count--;
 }
 
@@ -281,6 +282,7 @@ static inline void workingset_node_shadows_inc(struct radix_tree_node *node)
 
 static inline void workingset_node_shadows_dec(struct radix_tree_node *node)
 {
+	VM_BUG_ON(!workingset_node_shadows(node));
 	node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
 }
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 1bb007624b53..4cfe423d3e8a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -109,6 +109,48 @@
  *   ->tasklist_lock            (memory_failure, collect_procs_ao)
  */
 
+static int page_cache_tree_insert(struct address_space *mapping,
+				  struct page *page, void **shadowp)
+{
+	struct radix_tree_node *node;
+	void **slot;
+	int error;
+
+	error = __radix_tree_create(&mapping->page_tree, page->index,
+				    &node, &slot);
+	if (error)
+		return error;
+	if (*slot) {
+		void *p;
+
+		p = radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
+		if (!radix_tree_exceptional_entry(p))
+			return -EEXIST;
+		if (shadowp)
+			*shadowp = p;
+		mapping->nrshadows--;
+		if (node)
+			workingset_node_shadows_dec(node);
+	}
+	radix_tree_replace_slot(slot, page);
+	mapping->nrpages++;
+	if (node) {
+		workingset_node_pages_inc(node);
+		/*
+		 * Don't track node that contains actual pages.
+		 *
+		 * Avoid acquiring the list_lru lock if already
+		 * untracked.  The list_empty() test is safe as
+		 * node->private_list is protected by
+		 * mapping->tree_lock.
+		 */
+		if (!list_empty(&node->private_list))
+			list_lru_del(&workingset_shadow_nodes,
+				     &node->private_list);
+	}
+	return 0;
+}
+
 static void page_cache_tree_delete(struct address_space *mapping,
 				   struct page *page, void *shadow)
 {
@@ -538,7 +580,7 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
 		memcg = mem_cgroup_begin_page_stat(old);
 		spin_lock_irqsave(&mapping->tree_lock, flags);
 		__delete_from_page_cache(old, NULL, memcg);
-		error = radix_tree_insert(&mapping->page_tree, offset, new);
+		error = page_cache_tree_insert(mapping, new, NULL);
 		BUG_ON(error);
 		mapping->nrpages++;
 
@@ -562,48 +604,6 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
 }
 EXPORT_SYMBOL_GPL(replace_page_cache_page);
 
-static int page_cache_tree_insert(struct address_space *mapping,
-				  struct page *page, void **shadowp)
-{
-	struct radix_tree_node *node;
-	void **slot;
-	int error;
-
-	error = __radix_tree_create(&mapping->page_tree, page->index,
-				    &node, &slot);
-	if (error)
-		return error;
-	if (*slot) {
-		void *p;
-
-		p = radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
-		if (!radix_tree_exceptional_entry(p))
-			return -EEXIST;
-		if (shadowp)
-			*shadowp = p;
-		mapping->nrshadows--;
-		if (node)
-			workingset_node_shadows_dec(node);
-	}
-	radix_tree_replace_slot(slot, page);
-	mapping->nrpages++;
-	if (node) {
-		workingset_node_pages_inc(node);
-		/*
-		 * Don't track node that contains actual pages.
-		 *
-		 * Avoid acquiring the list_lru lock if already
-		 * untracked.  The list_empty() test is safe as
-		 * node->private_list is protected by
-		 * mapping->tree_lock.
-		 */
-		if (!list_empty(&node->private_list))
-			list_lru_del(&workingset_shadow_nodes,
-				     &node->private_list);
-	}
-	return 0;
-}
-
 static int __add_to_page_cache_locked(struct page *page,
 				      struct address_space *mapping,
 				      pgoff_t offset, gfp_t gfp_mask,
diff --git a/mm/workingset.c b/mm/workingset.c
index aa017133744b..df66f426fdcf 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -341,21 +341,19 @@ static enum lru_status shadow_lru_isolate(struct list_head *item,
 	 * no pages, so we expect to be able to remove them all and
 	 * delete and free the empty node afterwards.
 	 */
-
-	BUG_ON(!node->count);
-	BUG_ON(node->count & RADIX_TREE_COUNT_MASK);
+	BUG_ON(!workingset_node_shadows(node));
+	BUG_ON(workingset_node_pages(node));
 
 	for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) {
 		if (node->slots[i]) {
 			BUG_ON(!radix_tree_exceptional_entry(node->slots[i]));
 			node->slots[i] = NULL;
-			BUG_ON(node->count < (1U << RADIX_TREE_COUNT_SHIFT));
-			node->count -= 1U << RADIX_TREE_COUNT_SHIFT;
+			workingset_node_shadows_dec(node);
 			BUG_ON(!mapping->nrshadows);
 			mapping->nrshadows--;
 		}
 	}
-	BUG_ON(node->count);
+	BUG_ON(workingset_node_shadows(node));
 	inc_zone_state(page_zone(virt_to_page(node)), WORKINGSET_NODERECLAIM);
 	if (!__radix_tree_delete_node(&mapping->page_tree, node))
 		BUG();
-- 
2.9.3

Re: [Stable 4.4 - NEEDS REVIEW - 1/3] mm: workingset: fix crash in shadow node shrinker caused by replace_page_cache_page()

[Johannes Weiner]

On Mon, Oct 24, 2016 at 05:26:03PM +0200, Michal Hocko wrote:
> From: Johannes Weiner <hannes@cmpxchg.org>
> 
> Commit 22f2ac51b6d643666f4db093f13144f773ff3f3a upstream.
> 
> Antonio reports the following crash when using fuse under memory pressure:
> 
>   kernel BUG at /build/linux-a2WvEb/linux-4.4.0/mm/workingset.c:346!
>   invalid opcode: 0000 [#1] SMP
>   Modules linked in: all of them
>   CPU: 2 PID: 63 Comm: kswapd0 Not tainted 4.4.0-36-generic #55-Ubuntu
>   Hardware name: System manufacturer System Product Name/P8H67-M PRO, BIOS 3904 04/27/2013
>   task: ffff88040cae6040 ti: ffff880407488000 task.ti: ffff880407488000
>   RIP: shadow_lru_isolate+0x181/0x190
>   Call Trace:
>     __list_lru_walk_one.isra.3+0x8f/0x130
>     list_lru_walk_one+0x23/0x30
>     scan_shadow_nodes+0x34/0x50
>     shrink_slab.part.40+0x1ed/0x3d0
>     shrink_zone+0x2ca/0x2e0
>     kswapd+0x51e/0x990
>     kthread+0xd8/0xf0
>     ret_from_fork+0x3f/0x70
> 
> which corresponds to the following sanity check in the shadow node
> tracking:
> 
>   BUG_ON(node->count & RADIX_TREE_COUNT_MASK);
> 
> The workingset code tracks radix tree nodes that exclusively contain
> shadow entries of evicted pages in them, and this (somewhat obscure)
> line checks whether there are real pages left that would interfere with
> reclaim of the radix tree node under memory pressure.
> 
> While discussing ways how fuse might sneak pages into the radix tree
> past the workingset code, Miklos pointed to replace_page_cache_page(),
> and indeed there is a problem there: it properly accounts for the old
> page being removed - __delete_from_page_cache() does that - but then
> does a raw raw radix_tree_insert(), not accounting for the replacement
> page.  Eventually the page count bits in node->count underflow while
> leaving the node incorrectly linked to the shadow node LRU.
> 
> To address this, make sure replace_page_cache_page() uses the tracked
> page insertion code, page_cache_tree_insert().  This fixes the page
> accounting and makes sure page-containing nodes are properly unlinked
> from the shadow node LRU again.
> 
> Also, make the sanity checks a bit less obscure by using the helpers for
> checking the number of pages and shadows in a radix tree node.
> 
> Fixes: 449dd6984d0e ("mm: keep page cache radix tree nodes in check")
> Link: http://lkml.kernel.org/r/20160919155822.29498-1-hannes@cmpxchg.org
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Reported-by: Antonio SJ Musumeci <trapexit@spawn.link>
> Debugged-by: Miklos Szeredi <miklos@szeredi.hu>
> Cc: <stable@vger.kernel.org>	[3.15+]
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  include/linux/swap.h |  2 ++
>  mm/filemap.c         | 86 ++++++++++++++++++++++++++--------------------------
>  mm/workingset.c      | 10 +++---
>  3 files changed, 49 insertions(+), 49 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 7ba7dccaf0e7..b28de19aadbf 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -266,6 +266,7 @@ static inline void workingset_node_pages_inc(struct radix_tree_node *node)
>  
>  static inline void workingset_node_pages_dec(struct radix_tree_node *node)
>  {
> +	VM_BUG_ON(!workingset_node_pages(node));
>  	node->count--;
>  }

We should also pull 21f54ddae449 ("Using BUG_ON() as an assert() is
_never_ acceptable") into stable on top of this patch to replace the
BUG_ONs with warnings.

Otherwise this looks good to me.

Re: [Stable 4.4 - NEEDS REVIEW - 1/3] mm: workingset: fix crash in shadow node shrinker caused by replace_page_cache_page()

[Michal Hocko]

On Mon 24-10-16 14:52:23, Johannes Weiner wrote:
> On Mon, Oct 24, 2016 at 05:26:03PM +0200, Michal Hocko wrote:
[...]
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 7ba7dccaf0e7..b28de19aadbf 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -266,6 +266,7 @@ static inline void workingset_node_pages_inc(struct radix_tree_node *node)
> >  
> >  static inline void workingset_node_pages_dec(struct radix_tree_node *node)
> >  {
> > +	VM_BUG_ON(!workingset_node_pages(node));
> >  	node->count--;
> >  }
> 
> We should also pull 21f54ddae449 ("Using BUG_ON() as an assert() is
> _never_ acceptable") into stable on top of this patch to replace the
> BUG_ONs with warnings.
> 
> Otherwise this looks good to me.

OK, I have put that one on top and after the review will post it in one
series. Thanks for the review Johannes!

-- 
Michal Hocko
SUSE Labs

[Stable 4.4 - NEEDS REVIEW - 2/3] mm: filemap: don't plant shadow entries without radix tree node

[Michal Hocko]

From: Johannes Weiner <hannes@cmpxchg.org>

Commit d3798ae8c6f3767c726403c2ca6ecc317752c9dd upstream.

When the underflow checks were added to workingset_node_shadow_dec(),
they triggered immediately:

  kernel BUG at ./include/linux/swap.h:276!
  invalid opcode: 0000 [#1] SMP
  Modules linked in: isofs usb_storage fuse xt_CHECKSUM ipt_MASQUERADE nf_nat_masquerade_ipv4 tun nf_conntrack_netbios_ns nf_conntrack_broadcast ip6t_REJECT nf_reject_ipv6
   soundcore wmi acpi_als pinctrl_sunrisepoint kfifo_buf tpm_tis industrialio acpi_pad pinctrl_intel tpm_tis_core tpm nfsd auth_rpcgss nfs_acl lockd grace sunrpc dm_crypt
  CPU: 0 PID: 20929 Comm: blkid Not tainted 4.8.0-rc8-00087-gbe67d60ba944 #1
  Hardware name: System manufacturer System Product Name/Z170-K, BIOS 1803 05/06/2016
  task: ffff8faa93ecd940 task.stack: ffff8faa7f478000
  RIP: page_cache_tree_insert+0xf1/0x100
  Call Trace:
    __add_to_page_cache_locked+0x12e/0x270
    add_to_page_cache_lru+0x4e/0xe0
    mpage_readpages+0x112/0x1d0
    blkdev_readpages+0x1d/0x20
    __do_page_cache_readahead+0x1ad/0x290
    force_page_cache_readahead+0xaa/0x100
    page_cache_sync_readahead+0x3f/0x50
    generic_file_read_iter+0x5af/0x740
    blkdev_read_iter+0x35/0x40
    __vfs_read+0xe1/0x130
    vfs_read+0x96/0x130
    SyS_read+0x55/0xc0
    entry_SYSCALL_64_fastpath+0x13/0x8f
  Code: 03 00 48 8b 5d d8 65 48 33 1c 25 28 00 00 00 44 89 e8 75 19 48 83 c4 18 5b 41 5c 41 5d 41 5e 5d c3 0f 0b 41 bd ef ff ff ff eb d7 <0f> 0b e8 88 68 ef ff 0f 1f 84 00
  RIP  page_cache_tree_insert+0xf1/0x100

This is a long-standing bug in the way shadow entries are accounted in
the radix tree nodes. The shrinker needs to know when radix tree nodes
contain only shadow entries, no pages, so node->count is split in half
to count shadows in the upper bits and pages in the lower bits.

Unfortunately, the radix tree implementation doesn't know of this and
assumes all entries are in node->count. When there is a shadow entry
directly in root->rnode and the tree is later extended, the radix tree
implementation will copy that entry into the new node and and bump its
node->count, i.e. increases the page count bits. Once the shadow gets
removed and we subtract from the upper counter, node->count underflows
and triggers the warning. Afterwards, without node->count reaching 0
again, the radix tree node is leaked.

Limit shadow entries to when we have actual radix tree nodes and can
count them properly. That means we lose the ability to detect refaults
from files that had only the first page faulted in at eviction time.

Fixes: 449dd6984d0e ("mm: keep page cache radix tree nodes in check")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-and-tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 include/linux/radix-tree.h |  3 +++
 lib/radix-tree.c           | 14 ++++++++++++++
 mm/filemap.c               | 48 +++++++++++++++++++++-------------------------
 3 files changed, 39 insertions(+), 26 deletions(-)

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 5d5174b59802..1fe61d2a3875 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -271,6 +271,9 @@ bool __radix_tree_delete_node(struct radix_tree_root *root,
 			      struct radix_tree_node *node);
 void *radix_tree_delete_item(struct radix_tree_root *, unsigned long, void *);
 void *radix_tree_delete(struct radix_tree_root *, unsigned long);
+void radix_tree_clear_tags(struct radix_tree_root *root,
+			   struct radix_tree_node *node,
+			   void **slot);
 unsigned int
 radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
 			unsigned long first_index, unsigned int max_items);
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 6b79e9026e24..3ca8be1d1c70 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -1430,6 +1430,20 @@ void *radix_tree_delete(struct radix_tree_root *root, unsigned long index)
 }
 EXPORT_SYMBOL(radix_tree_delete);
 
+void radix_tree_clear_tags(struct radix_tree_root *root,
+			   struct radix_tree_node *node,
+			   void **slot)
+{
+	if (node) {
+		unsigned int tag, offset = get_slot_offset(node, slot);
+		for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++)
+			node_tag_clear(root, node, tag, offset);
+	} else {
+		/* Clear root node tags */
+		root->gfp_mask &= __GFP_BITS_MASK;
+	}
+}
+
 /**
  *	radix_tree_tagged - test whether any items in the tree are tagged
  *	@root:		radix tree root
diff --git a/mm/filemap.c b/mm/filemap.c
index 4cfe423d3e8a..c9c19c12f61d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -155,44 +155,27 @@ static void page_cache_tree_delete(struct address_space *mapping,
 				   struct page *page, void *shadow)
 {
 	struct radix_tree_node *node;
-	unsigned long index;
-	unsigned int offset;
-	unsigned int tag;
 	void **slot;
 
 	VM_BUG_ON(!PageLocked(page));
 
 	__radix_tree_lookup(&mapping->page_tree, page->index, &node, &slot);
 
-	if (shadow) {
-		mapping->nrshadows++;
-		/*
-		 * Make sure the nrshadows update is committed before
-		 * the nrpages update so that final truncate racing
-		 * with reclaim does not see both counters 0 at the
-		 * same time and miss a shadow entry.
-		 */
-		smp_wmb();
-	}
-	mapping->nrpages--;
+	radix_tree_clear_tags(&mapping->page_tree, node, slot);
 
 	if (!node) {
-		/* Clear direct pointer tags in root node */
-		mapping->page_tree.gfp_mask &= __GFP_BITS_MASK;
-		radix_tree_replace_slot(slot, shadow);
-		return;
-	}
-
-	/* Clear tree tags for the removed page */
-	index = page->index;
-	offset = index & RADIX_TREE_MAP_MASK;
-	for (tag = 0; tag < RADIX_TREE_MAX_TAGS; tag++) {
-		if (test_bit(offset, node->tags[tag]))
-			radix_tree_tag_clear(&mapping->page_tree, index, tag);
+		/*
+		 * We need a node to properly account shadow
+		 * entries. Don't plant any without. XXX
+		 */
+		shadow = NULL;
 	}
 
 	/* Delete page, swap shadow entry */
 	radix_tree_replace_slot(slot, shadow);
+	if (!node)
+		goto out;
+
 	workingset_node_pages_dec(node);
 	if (shadow)
 		workingset_node_shadows_inc(node);
@@ -212,6 +195,19 @@ static void page_cache_tree_delete(struct address_space *mapping,
 		node->private_data = mapping;
 		list_lru_add(&workingset_shadow_nodes, &node->private_list);
 	}
+
+out:
+	if (shadow) {
+		mapping->nrshadows++;
+		/*
+		 * Make sure the nrshadows update is committed before
+		 * the nrpages update so that final truncate racing
+		 * with reclaim does not see both counters 0 at the
+		 * same time and miss a shadow entry.
+		 */
+		smp_wmb();
+	}
+	mapping->nrpages--;
 }
 
 /*
-- 
2.9.3

Re: [Stable 4.4 - NEEDS REVIEW - 2/3] mm: filemap: don't plant shadow entries without radix tree node

[Johannes Weiner]

Hi Michal,

On Mon, Oct 24, 2016 at 05:26:04PM +0200, Michal Hocko wrote:
> @@ -155,44 +155,27 @@ static void page_cache_tree_delete(struct address_space *mapping,
>  				   struct page *page, void *shadow)
>  {
>  	struct radix_tree_node *node;
> -	unsigned long index;
> -	unsigned int offset;
> -	unsigned int tag;
>  	void **slot;
>  
>  	VM_BUG_ON(!PageLocked(page));
>  
>  	__radix_tree_lookup(&mapping->page_tree, page->index, &node, &slot);
>  
> -	if (shadow) {
> -		mapping->nrshadows++;
> -		/*
> -		 * Make sure the nrshadows update is committed before
> -		 * the nrpages update so that final truncate racing
> -		 * with reclaim does not see both counters 0 at the
> -		 * same time and miss a shadow entry.
> -		 */
> -		smp_wmb();
> -	}
> -	mapping->nrpages--;
> +	radix_tree_clear_tags(&mapping->page_tree, node, slot);
>  
>  	if (!node) {
> -		/* Clear direct pointer tags in root node */
> -		mapping->page_tree.gfp_mask &= __GFP_BITS_MASK;
> -		radix_tree_replace_slot(slot, shadow);
> -		return;
> -	}

There is no need to include the refactoring of the tag clearing in the
stable backport. I already sent a simpler backport of this patch for
4.4 to Greg, attached here for reference:

>From ba2dae8c4ddcc70a1d7bb1478f4c7c6ffe130b6f Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Fri, 21 Oct 2016 13:40:41 -0400
Subject: [PATCH] mm: filemap: don't plant shadow entries without radix tree
 node

commit d3798ae8c6f3767c726403c2ca6ecc317752c9dd upstream.

When the underflow checks were added to workingset_node_shadow_dec(),
they triggered immediately:

  kernel BUG at ./include/linux/swap.h:276!
  invalid opcode: 0000 [#1] SMP
  Modules linked in: isofs usb_storage fuse xt_CHECKSUM ipt_MASQUERADE nf_nat_masquerade_ipv4 tun nf_conntrack_netbios_ns nf_conntrack_broadcast ip6t_REJECT nf_reject_ipv6
   soundcore wmi acpi_als pinctrl_sunrisepoint kfifo_buf tpm_tis industrialio acpi_pad pinctrl_intel tpm_tis_core tpm nfsd auth_rpcgss nfs_acl lockd grace sunrpc dm_crypt
  CPU: 0 PID: 20929 Comm: blkid Not tainted 4.8.0-rc8-00087-gbe67d60ba944 #1
  Hardware name: System manufacturer System Product Name/Z170-K, BIOS 1803 05/06/2016
  task: ffff8faa93ecd940 task.stack: ffff8faa7f478000
  RIP: page_cache_tree_insert+0xf1/0x100
  Call Trace:
    __add_to_page_cache_locked+0x12e/0x270
    add_to_page_cache_lru+0x4e/0xe0
    mpage_readpages+0x112/0x1d0
    blkdev_readpages+0x1d/0x20
    __do_page_cache_readahead+0x1ad/0x290
    force_page_cache_readahead+0xaa/0x100
    page_cache_sync_readahead+0x3f/0x50
    generic_file_read_iter+0x5af/0x740
    blkdev_read_iter+0x35/0x40
    __vfs_read+0xe1/0x130
    vfs_read+0x96/0x130
    SyS_read+0x55/0xc0
    entry_SYSCALL_64_fastpath+0x13/0x8f
  Code: 03 00 48 8b 5d d8 65 48 33 1c 25 28 00 00 00 44 89 e8 75 19 48 83 c4 18 5b 41 5c 41 5d 41 5e 5d c3 0f 0b 41 bd ef ff ff ff eb d7 <0f> 0b e8 88 68 ef ff 0f 1f 84 00
  RIP  page_cache_tree_insert+0xf1/0x100

This is a long-standing bug in the way shadow entries are accounted in
the radix tree nodes. The shrinker needs to know when radix tree nodes
contain only shadow entries, no pages, so node->count is split in half
to count shadows in the upper bits and pages in the lower bits.

Unfortunately, the radix tree implementation doesn't know of this and
assumes all entries are in node->count. When there is a shadow entry
directly in root->rnode and the tree is later extended, the radix tree
implementation will copy that entry into the new node and and bump its
node->count, i.e. increases the page count bits. Once the shadow gets
removed and we subtract from the upper counter, node->count underflows
and triggers the warning. Afterwards, without node->count reaching 0
again, the radix tree node is leaked.

Limit shadow entries to when we have actual radix tree nodes and can
count them properly. That means we lose the ability to detect refaults
from files that had only the first page faulted in at eviction time.

Fixes: 449dd6984d0e ("mm: keep page cache radix tree nodes in check")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-and-tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 mm/filemap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 1bb007624b53..fb52502d12f4 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -122,6 +122,14 @@ static void page_cache_tree_delete(struct address_space *mapping,
 
 	__radix_tree_lookup(&mapping->page_tree, page->index, &node, &slot);
 
+	if (!node) {
+		/*
+		 * We need a node to properly account shadow
+		 * entries. Don't plant any without. XXX
+		 */
+		shadow = NULL;
+	}
+
 	if (shadow) {
 		mapping->nrshadows++;
 		/*
-- 
2.10.0

Re: [Stable 4.4 - NEEDS REVIEW - 2/3] mm: filemap: don't plant shadow entries without radix tree node

[Michal Hocko]

On Mon 24-10-16 14:56:00, Johannes Weiner wrote:
> Hi Michal,
> 
> On Mon, Oct 24, 2016 at 05:26:04PM +0200, Michal Hocko wrote:
> > @@ -155,44 +155,27 @@ static void page_cache_tree_delete(struct address_space *mapping,
> >  				   struct page *page, void *shadow)
> >  {
> >  	struct radix_tree_node *node;
> > -	unsigned long index;
> > -	unsigned int offset;
> > -	unsigned int tag;
> >  	void **slot;
> >  
> >  	VM_BUG_ON(!PageLocked(page));
> >  
> >  	__radix_tree_lookup(&mapping->page_tree, page->index, &node, &slot);
> >  
> > -	if (shadow) {
> > -		mapping->nrshadows++;
> > -		/*
> > -		 * Make sure the nrshadows update is committed before
> > -		 * the nrpages update so that final truncate racing
> > -		 * with reclaim does not see both counters 0 at the
> > -		 * same time and miss a shadow entry.
> > -		 */
> > -		smp_wmb();
> > -	}
> > -	mapping->nrpages--;
> > +	radix_tree_clear_tags(&mapping->page_tree, node, slot);
> >  
> >  	if (!node) {
> > -		/* Clear direct pointer tags in root node */
> > -		mapping->page_tree.gfp_mask &= __GFP_BITS_MASK;
> > -		radix_tree_replace_slot(slot, shadow);
> > -		return;
> > -	}
> 
> There is no need to include the refactoring of the tag clearing in the
> stable backport. I already sent a simpler backport of this patch for
> 4.4 to Greg, attached here for reference:

I do not see this in 4.4 so maybe it's fallen through cracks. Yours
definitely looks easier and I will use it. I will post all 4 patches for
inclusion for stable tomorrow unless something else pops out.

Thanks for the review again!
-- 
Michal Hocko
SUSE Labs

[Stable 4.4 - NEEDS REVIEW - 3/3] mm: filemap: fix mapping->nrpages double accounting in fuse

[Michal Hocko]

From: Johannes Weiner <hannes@cmpxchg.org>

Commit 3ddf40e8c31964b744ff10abb48c8e36a83ec6e7 upstream.

Commit 22f2ac51b6d6 ("mm: workingset: fix crash in shadow node shrinker
caused by replace_page_cache_page()") switched replace_page_cache() from
raw radix tree operations to page_cache_tree_insert() but didn't take
into account that the latter function, unlike the raw radix tree op,
handles mapping->nrpages.  As a result, that counter is bumped for each
page replacement rather than balanced out even.

The mapping->nrpages counter is used to skip needless radix tree walks
when invalidating, truncating, syncing inodes without pages, as well as
statistics for userspace.  Since the error is positive, we'll do more
page cache tree walks than necessary; we won't miss a necessary one.
And we'll report more buffer pages to userspace than there are.  The
error is limited to fuse inodes.

Fixes: 22f2ac51b6d6 ("mm: workingset: fix crash in shadow node shrinker caused by replace_page_cache_page()")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/filemap.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index c9c19c12f61d..8675ce8a8f87 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -578,7 +578,6 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
 		__delete_from_page_cache(old, NULL, memcg);
 		error = page_cache_tree_insert(mapping, new, NULL);
 		BUG_ON(error);
-		mapping->nrpages++;
 
 		/*
 		 * hugetlb pages do not participate in page cache accounting.
-- 
2.9.3

Re: [Stable 4.4 - NEEDS REVIEW - 3/3] mm: filemap: fix mapping->nrpages double accounting in fuse

[Johannes Weiner]

This one looks good to me.

Thanks