Re: improve the kmem_cache_alloc_bulk API

[Vlastimil Babka (SUSE)]

On 5/27/26 09:02, Christoph Hellwig wrote:
> Hi all,
> 
> kmem_cache_alloc_bulk has a very unintuitive and undocumented return
> value convention.  Fix that and add documentation.
> 
> Note that the few comments explaining it mention that the gfp flags
> must allow "spinning".  That's not really a term used in the memory
> allocator, is this supposed to mean "block" or "sleep"?

Page allocator now has alloc_pages_nolock() for when no spinning is
possible, and it uses ALLOC_TRYLOCK internally.

Slab has kmalloc_nolock() relying on that when it needs new pages.

In terms of gfp flags, such context is currently indicated by lack of
__GFP_KSWAPD_RECLAIM, where lack of __GFP_DIRECT_RECLAIM only means "no
sleeping" - see gfpflags_allow_spinning(). Slab uses it internally as
there's no ALLOC_TRYLOCK, but also there are callers from memcg and stackdepot.

Like the rest of gfp flags it's far from ideal, maybe we'll figure out a
better design eventually.

> Diffstat:
>  drivers/gpu/drm/msm/msm_iommu.c       |    6 +--
>  drivers/gpu/drm/panthor/panthor_mmu.c |   12 ++-----
>  include/linux/slab.h                  |    6 ++-
>  io_uring/io_uring.c                   |   23 +++++--------
>  lib/test_meminit.c                    |   19 +++++------
>  mm/kasan/kasan_test_c.c               |    5 +-
>  mm/kfence/kfence_test.c               |    9 ++---
>  mm/slub.c                             |   58 ++++++++++++++++++----------------
>  net/bpf/test_run.c                    |    7 +---
>  net/core/skbuff.c                     |   23 +++++++------
>  tools/include/linux/slab.h            |    2 -
>  tools/testing/shared/linux.c          |   19 ++++-------
>  12 files changed, 92 insertions(+), 97 deletions(-)