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

Nicolas Boichat
2018-12-06T03:55:34Z

On Thu, Dec 6, 2018 at 11:32 AM Wei Yang <richard.weiyang@gmail.com> wrote:
>
> 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@gmail.com> 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@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.
> >> >
> >>
> >> 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?
>
> >
> >Does that clarify?
> >
> >>
> >> --
> >> Wei Yang
> >> Help you, Help me
>
> --
> Wei Yang
> Help you, Help me