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

[Wei Yang]

On Thu, Dec 06, 2018 at 08:41:36AM +0800, Nicolas Boichat wrote:
>On Wed, Dec 5, 2018 at 8:18 PM Wei Yang <richard.weiyang@xxxxxxxxx> wrote:
>>
>> On Wed, Dec 05, 2018 at 03:39:51PM +0800, Nicolas Boichat wrote:
>> >On Wed, Dec 5, 2018 at 3:25 PM Wei Yang <richard.weiyang@xxxxxxxxx> 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@xxxxxxxxxxxx>
>> >> >---
>> >> >
>> >> >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@xxxxxxxxxxxx>
>> >> >+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.
>> >
>>
>> Hmm, I get one confusion.
>>
>> This means if CONFIG_ZONE_DMA32 is not enabled, bug_mask will always
>> contains __GFP_DMA32. This will check with cachep->flags.
>>
>> If cachep->flags has GFP_DMA32, this always fail?
>>
>> Is this possible?
>
>Not fully sure to understand the question, but the code is:
>if (!IS_ENABLED(CONFIG_ZONE_DMA32) || !(slab_flags & SLAB_CACHE_DMA32))
>       bug_mask |= __GFP_DMA32;
>
>IS_ENABLED(CONFIG_ZONE_DMA32) == true:
> - (slab_flags & SLAB_CACHE_DMA32) => bug_mask untouched, __GFP_DMA32
>is allowed.
> - !(slab_flags & SLAB_CACHE_DMA32) => bug_mask |= __GFP_DMA32;,
>__GFP_DMA32 triggers warning
>IS_ENABLED(CONFIG_ZONE_DMA32) == false:
>  => bug_mask |= __GFP_DMA32;, __GFP_DMA32 triggers warning (as
>expected, GFP_DMA32 does not make sense if there is no DMA32 zone).

This is the case I am thinking.

The warning is reasonable since there is no DMA32. While the
kmem_cache_create() user is not easy to change their code.

For example, one writes code and wants to have a kmem_cache with DMA32
capability, so he writes kmem_cache_create(__GFP_DMA32). The code is
there and not easy to change. But one distro builder decides to disable
DMA32. This will leads to all the kmem_cache_create() through warning?

This behavior is what we expect?

>
>Does that clarify?
>
>>
>> --
>> Wei Yang
>> Help you, Help me

-- 
Wei Yang
Help you, Help me