Re: [PATCH] mm/slab: improve kmem_cache_alloc_bulk

[Vlastimil Babka (SUSE)]

On 5/28/26 11:34, Christoph Hellwig wrote:
> The kmem_cache_alloc_bulk return value is weird.  It returns the number
> of allocated objects, but that must always be 0 or the requested number
> based on the implementations and the handling in the callers, but that
> assumption is not actually documented anywhere, which confuses automated
> review tools.
> 
> Fix this by returning a bool if the allocation succeeded and adding a
> kerneldoc comment explaining the API.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> Reviewed-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx> # skbuff
> ---
>  drivers/gpu/drm/msm/msm_iommu.c       |  6 +--
>  drivers/gpu/drm/panthor/panthor_mmu.c | 13 +++---
>  include/linux/slab.h                  |  6 ++-
>  io_uring/io_uring.c                   | 23 ++++-------
>  lib/test_meminit.c                    | 23 +++++------
>  mm/kasan/kasan_test_c.c               |  5 +--
>  mm/kfence/kfence_test.c               |  9 ++--
>  mm/slub.c                             | 59 +++++++++++++++------------
>  net/bpf/test_run.c                    |  7 ++--
>  net/core/skbuff.c                     | 24 ++++++-----
>  tools/include/linux/slab.h            |  2 +-
>  tools/testing/shared/linux.c          | 19 ++++-----
>  12 files changed, 97 insertions(+), 99 deletions(-)

Thanks, I applied it to slab/for-7.2/alloc_bulk and merged to slab/for-next
(it's still yankable in case of issues)

Did some fixups below (the comment was stale prior to the patch; restored
unlikely(), simplified one line).

A test merge into yesterday's -next found a conflict in drivers/gpu/drm/
panthor/panthor_mmu.c. Commit 1013bf53650e ("drm/panthor: Split
panthor_vm_prepare_map_op_ctx() to prepare for reclaim") moved the changed
codeto a new function panthor_vm_op_ctx_prealloc_pts().
But it's solvable so no need for a complicated coordination I think.

diff --git a/mm/slub.c b/mm/slub.c
index 6caf6f3ceeed..711df528c9a6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -7372,10 +7372,7 @@ bool kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags,
        }
 
 out:
-       /*
-        * memcg and kmem_cache debug support and memory initialization.
-        * Done outside of the IRQ disabled fastpath loop.
-        */
+       /* memcg and kmem_cache debug support and memory initialization */
        return likely(slab_post_alloc_hook(s, NULL, flags, size, p,
                        slab_want_init_on_alloc(flags, s), s->object_size));
 }
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 99ab9ddb05e3..dbf0d8eae8d8 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -246,8 +246,8 @@ static int xdp_recv_frames(struct xdp_frame **frames, int nframes,
        int i;
        LIST_HEAD(list);
 
-       if (!kmem_cache_alloc_bulk(net_hotdata.skbuff_cache, gfp, nframes,
-                                  (void **)skbs)) {
+       if (unlikely(!kmem_cache_alloc_bulk(net_hotdata.skbuff_cache, gfp,
+                                           nframes, (void **)skbs))) {
                for (i = 0; i < nframes; i++)
                        xdp_return_frame(frames[i]);
                return -ENOMEM;
diff --git a/tools/testing/shared/linux.c b/tools/testing/shared/linux.c
index e9c3bc9b3272..e0a0693df08f 100644
--- a/tools/testing/shared/linux.c
+++ b/tools/testing/shared/linux.c
@@ -301,7 +301,7 @@ int kmem_cache_refill_sheaf(struct kmem_cache *s, gfp_t gfp,
        if (!kmem_cache_alloc_bulk(s, gfp, size - sheaf->size,
                        &sheaf->objects[sheaf->size]))
                return -ENOMEM;
-       sheaf->size += (size - sheaf->size);
+       sheaf->size = size;
        return 0;
 }