On Wed, Dec 5, 2018 at 10:02 PM Vlastimil Babka [off-list ref] wrote:

On 12/5/18 6:48 AM, 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")

Same as my comment for 1/3.

I'll drop.

quoted

Signed-off-by: Nicolas Boichat <redacted>

In general,
Acked-by: Vlastimil Babka <redacted>

Some comments below:

quoted

---

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.

I don't have a strong opinion. It's a new file, yeah, but consistent
with already existing ones. I'd leave the decision with SL*B maintainers.

quoted

 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;

I'll point out that this is not even strictly needed AFAICS, as only
flags passed to kmem_cache_alloc() are checked - the cache->allocflags
derived from SLAB_CACHE_DMA32 are appended only after check_slab_flags()
(in both SLAB and SLUB AFAICS). And for a cache created with
SLAB_CACHE_DMA32, the caller of kmem_cache_alloc() doesn't need to also
include __GFP_DMA32, the allocation will be from ZONE_DMA32 regardless.

Yes, you're right. I also looked at existing users of SLAB_CACHE_DMA,
and there is one case in drivers/scsi/scsi_lib.c where GFP_DMA is not
be passed (all the other users pass it).

I can drop GFP_DMA32 from my call in io-pgtable-arm-v7s.c.

So it would be fine even unchanged. The check would anyway need some
more love to catch the same with __GFP_DMA to be consistent and cover
all corner cases.

Yes, the test is not complete. If we really wanted this to be
accurate, we'd need to check that GFP_* exactly matches SLAB_CACHE_*.

The only problem with dropping this is test that we should restore
GFP_DMA32 warning/errors somewhere else (as Christopher pointed out
here: https://lkml.org/lkml/2018/11/22/430), especially for kmalloc
case.

Maybe this can be done in kmalloc_slab.

quoted

      if (unlikely(flags & bug_mask)) {
              gfp_t invalid_mask = flags & bug_mask;

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