Re: [PATCH v4 2/3] mm: Add support for kmem caches in DMA32 zone

[Nicolas Boichat <drinkcat@chromium.org>]

On Wed, Dec 5, 2018 at 3:25 PM Wei Yang <richard.weiyang@gmail.com> wrote:
>
> On Wed, Dec 05, 2018 at 01:48:27PM +0800, Nicolas Boichat wrote:
> >In some cases (e.g. IOMMU ARMv7s page allocator), we need to allocate
> >data structures smaller than a page with GFP_DMA32 flag.
> >
> >This change makes it possible to create a custom cache in DMA32 zone
> >using kmem_cache_create, then allocate memory using kmem_cache_alloc.
> >
> >We do not create a DMA32 kmalloc cache array, as there are currently
> >no users of kmalloc(..., GFP_DMA32). The new test in check_slab_flags
> >ensures that such calls still fail (as they do before this change).
> >
> >Fixes: ad67f5a6545f ("arm64: replace ZONE_DMA with ZONE_DMA32")
> >Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
> >---
> >
> >Changes since v2:
> > - Clarified commit message
> > - Add entry in sysfs-kernel-slab to document the new sysfs file
> >
> >(v3 used the page_frag approach)
> >
> >Documentation/ABI/testing/sysfs-kernel-slab |  9 +++++++++
> > include/linux/slab.h                        |  2 ++
> > mm/internal.h                               |  8 ++++++--
> > mm/slab.c                                   |  4 +++-
> > mm/slab.h                                   |  3 ++-
> > mm/slab_common.c                            |  2 +-
> > mm/slub.c                                   | 18 +++++++++++++++++-
> > 7 files changed, 40 insertions(+), 6 deletions(-)
> >
> >diff --git a/Documentation/ABI/testing/sysfs-kernel-slab b/Documentation/ABI/testing/sysfs-kernel-slab
> >index 29601d93a1c2ea..d742c6cfdffbe9 100644
> >--- a/Documentation/ABI/testing/sysfs-kernel-slab
> >+++ b/Documentation/ABI/testing/sysfs-kernel-slab
> >@@ -106,6 +106,15 @@ Description:
> >               are from ZONE_DMA.
> >               Available when CONFIG_ZONE_DMA is enabled.
> >
> >+What:         /sys/kernel/slab/cache/cache_dma32
> >+Date:         December 2018
> >+KernelVersion:        4.21
> >+Contact:      Nicolas Boichat <drinkcat@chromium.org>
> >+Description:
> >+              The cache_dma32 file is read-only and specifies whether objects
> >+              are from ZONE_DMA32.
> >+              Available when CONFIG_ZONE_DMA32 is enabled.
> >+
> > What:         /sys/kernel/slab/cache/cpu_slabs
> > Date:         May 2007
> > KernelVersion:        2.6.22
> >diff --git a/include/linux/slab.h b/include/linux/slab.h
> >index 11b45f7ae4057c..9449b19c5f107a 100644
> >--- a/include/linux/slab.h
> >+++ b/include/linux/slab.h
> >@@ -32,6 +32,8 @@
> > #define SLAB_HWCACHE_ALIGN    ((slab_flags_t __force)0x00002000U)
> > /* Use GFP_DMA memory */
> > #define SLAB_CACHE_DMA                ((slab_flags_t __force)0x00004000U)
> >+/* Use GFP_DMA32 memory */
> >+#define SLAB_CACHE_DMA32      ((slab_flags_t __force)0x00008000U)
> > /* DEBUG: Store the last owner for bug hunting */
> > #define SLAB_STORE_USER               ((slab_flags_t __force)0x00010000U)
> > /* Panic if kmem_cache_create() fails */
> >diff --git a/mm/internal.h b/mm/internal.h
> >index a2ee82a0cd44ae..fd244ad716eaf8 100644
> >--- a/mm/internal.h
> >+++ b/mm/internal.h
> >@@ -14,6 +14,7 @@
> > #include <linux/fs.h>
> > #include <linux/mm.h>
> > #include <linux/pagemap.h>
> >+#include <linux/slab.h>
> > #include <linux/tracepoint-defs.h>
> >
> > /*
> >@@ -34,9 +35,12 @@
> > #define GFP_CONSTRAINT_MASK (__GFP_HARDWALL|__GFP_THISNODE)
> >
> > /* Check for flags that must not be used with a slab allocator */
> >-static inline gfp_t check_slab_flags(gfp_t flags)
> >+static inline gfp_t check_slab_flags(gfp_t flags, slab_flags_t slab_flags)
> > {
> >-      gfp_t bug_mask = __GFP_DMA32 | __GFP_HIGHMEM | ~__GFP_BITS_MASK;
> >+      gfp_t bug_mask = __GFP_HIGHMEM | ~__GFP_BITS_MASK;
> >+
> >+      if (!IS_ENABLED(CONFIG_ZONE_DMA32) || !(slab_flags & SLAB_CACHE_DMA32))
> >+              bug_mask |= __GFP_DMA32;
>
> The original version doesn't check CONFIG_ZONE_DMA32.
>
> Do we need to add this condition here?
> Could we just decide the bug_mask based on slab_flags?

We can. The reason I did it this way is that when we don't have
CONFIG_ZONE_DMA32, the compiler should be able to simplify to:

bug_mask = __GFP_HIGHMEM | ~__GFP_BITS_MASK;
if (true || ..) => if (true)
   bug_mask |= __GFP_DMA32;

Then just
bug_mask = __GFP_HIGHMEM | ~__GFP_BITS_MASK | __GFP_DMA32;

And since the function is inline, slab_flags would not even need to be
accessed at all.

> >
> >       if (unlikely(flags & bug_mask)) {
> >               gfp_t invalid_mask = flags & bug_mask;
> >diff --git a/mm/slab.c b/mm/slab.c
> >index 65a774f05e7836..2fd3b9a996cbe6 100644
> >--- a/mm/slab.c
> >+++ b/mm/slab.c
> >@@ -2109,6 +2109,8 @@ int __kmem_cache_create(struct kmem_cache *cachep, slab_flags_t flags)
> >       cachep->allocflags = __GFP_COMP;
> >       if (flags & SLAB_CACHE_DMA)
> >               cachep->allocflags |= GFP_DMA;
> >+      if (flags & SLAB_CACHE_DMA32)
> >+              cachep->allocflags |= GFP_DMA32;
> >       if (flags & SLAB_RECLAIM_ACCOUNT)
> >               cachep->allocflags |= __GFP_RECLAIMABLE;
> >       cachep->size = size;
> >@@ -2643,7 +2645,7 @@ static struct page *cache_grow_begin(struct kmem_cache *cachep,
> >        * Be lazy and only check for valid flags here,  keeping it out of the
> >        * critical path in kmem_cache_alloc().
> >        */
> >-      flags = check_slab_flags(flags);
> >+      flags = check_slab_flags(flags, cachep->flags);
> >       WARN_ON_ONCE(cachep->ctor && (flags & __GFP_ZERO));
> >       local_flags = flags & (GFP_CONSTRAINT_MASK|GFP_RECLAIM_MASK);
> >
> >diff --git a/mm/slab.h b/mm/slab.h
> >index 4190c24ef0e9df..fcf717e12f0a86 100644
> >--- a/mm/slab.h
> >+++ b/mm/slab.h
> >@@ -127,7 +127,8 @@ static inline slab_flags_t kmem_cache_flags(unsigned int object_size,
> >
> >
> > /* Legal flag mask for kmem_cache_create(), for various configurations */
> >-#define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | SLAB_PANIC | \
> >+#define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
> >+                       SLAB_CACHE_DMA32 | SLAB_PANIC | \
> >                        SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS )
> >
> > #if defined(CONFIG_DEBUG_SLAB)
> >diff --git a/mm/slab_common.c b/mm/slab_common.c
> >index 70b0cc85db67f8..18b7b809c8d064 100644
> >--- a/mm/slab_common.c
> >+++ b/mm/slab_common.c
> >@@ -53,7 +53,7 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
> >               SLAB_FAILSLAB | SLAB_KASAN)
> >
> > #define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \
> >-                       SLAB_ACCOUNT)
> >+                       SLAB_CACHE_DMA32 | SLAB_ACCOUNT)
> >
> > /*
> >  * Merge control. If this is set then no merging of slab caches will occur.
> >diff --git a/mm/slub.c b/mm/slub.c
> >index 21a3f6866da472..6d47765a82d150 100644
> >--- a/mm/slub.c
> >+++ b/mm/slub.c
> >@@ -1685,7 +1685,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> >
> > static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
> > {
> >-      flags = check_slab_flags(flags);
> >+      flags = check_slab_flags(flags, s->flags);
> >
> >       return allocate_slab(s,
> >               flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK), node);
> >@@ -3577,6 +3577,9 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> >       if (s->flags & SLAB_CACHE_DMA)
> >               s->allocflags |= GFP_DMA;
> >
> >+      if (s->flags & SLAB_CACHE_DMA32)
> >+              s->allocflags |= GFP_DMA32;
> >+
> >       if (s->flags & SLAB_RECLAIM_ACCOUNT)
> >               s->allocflags |= __GFP_RECLAIMABLE;
> >
> >@@ -5095,6 +5098,14 @@ static ssize_t cache_dma_show(struct kmem_cache *s, char *buf)
> > SLAB_ATTR_RO(cache_dma);
> > #endif
> >
> >+#ifdef CONFIG_ZONE_DMA32
> >+static ssize_t cache_dma32_show(struct kmem_cache *s, char *buf)
> >+{
> >+      return sprintf(buf, "%d\n", !!(s->flags & SLAB_CACHE_DMA32));
> >+}
> >+SLAB_ATTR_RO(cache_dma32);
> >+#endif
> >+
> > static ssize_t usersize_show(struct kmem_cache *s, char *buf)
> > {
> >       return sprintf(buf, "%u\n", s->usersize);
> >@@ -5435,6 +5446,9 @@ static struct attribute *slab_attrs[] = {
> > #ifdef CONFIG_ZONE_DMA
> >       &cache_dma_attr.attr,
> > #endif
> >+#ifdef CONFIG_ZONE_DMA32
> >+      &cache_dma32_attr.attr,
> >+#endif
> > #ifdef CONFIG_NUMA
> >       &remote_node_defrag_ratio_attr.attr,
> > #endif
> >@@ -5665,6 +5679,8 @@ static char *create_unique_id(struct kmem_cache *s)
> >        */
> >       if (s->flags & SLAB_CACHE_DMA)
> >               *p++ = 'd';
> >+      if (s->flags & SLAB_CACHE_DMA32)
> >+              *p++ = 'D';
> >       if (s->flags & SLAB_RECLAIM_ACCOUNT)
> >               *p++ = 'a';
> >       if (s->flags & SLAB_CONSISTENCY_CHECKS)
> >--
> >2.20.0.rc1.387.gf8505762e3-goog
> >
> >_______________________________________________
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> >iommu@lists.linux-foundation.org
> >https://lists.linuxfoundation.org/mailman/listinfo/iommu
>
> --
> Wei Yang
> Help you, Help me