On Thu, Dec 6, 2018 at 11:32 AM Wei Yang [off-list ref] wrote:

On Thu, Dec 06, 2018 at 08:41:36AM +0800, Nicolas Boichat wrote:

quoted

On Wed, Dec 5, 2018 at 8:18 PM Wei Yang [off-list ref] wrote:

quoted

On Wed, Dec 05, 2018 at 03:39:51PM +0800, Nicolas Boichat wrote:

quoted

On Wed, Dec 5, 2018 at 3:25 PM Wei Yang [off-list ref] wrote:

quoted

On Wed, Dec 05, 2018 at 01:48:27PM +0800, Nicolas Boichat wrote:

quoted

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 <redacted>
---

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.

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?

I don't think CONFIG_ZONE_DMA32 can be enabled/disabled by
distro/user? IIUC this is a property of the architecture, some have it
enabled, some don't.

This behavior is what we expect?

quoted

Does that clarify?

quoted

--
Wei Yang
Help you, Help me

--
Wei Yang
Help you, Help me

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