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mm/percpu: avoid IO/FS reclaim in backing allocations

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Commit 9a5b183941 ("mm, percpu: do not consider sleepable allocations
atomic") allows sleepable GFP_NOIO and GFP_NOFS percpu allocations to take
pcpu_alloc_mutex.  This avoids premature allocation failures, but it also
makes the mutex visible to callers from constrained IO/FS contexts.

Thread A calls pcpu_alloc_noprof() with GFP_KERNEL and takes
pcpu_alloc_mutex.  Since the internal allocation is not constrained by
NOFS, it may enter FS reclaim while still holding pcpu_alloc_mutex,
creating a dependency like: pcpu_alloc_mutex -> fs_reclaim -> FS lock

At the same time, Thread B may already hold an FS lock and then call
pcpu_alloc_noprof() with GFP_NOFS.  It will try to acquire
pcpu_alloc_mutex and block, creating the reverse dependency: FS lock ->
pcpu_alloc_mutex

This can still form a potential deadlock cycle.

Avoid the dependency by restricting percpu backing allocations to
GFP_NOIO.  The public allocation still uses the caller's GFP context to
decide whether it may block, but the internal memory allocations performed
while pcpu_alloc_mutex is held cannot recurse into IO or FS reclaim.

Link: https://lore.kernel.org/20260618130414.96383-5-kaitao.cheng@linux.dev
Fixes: 9a5b183941 ("mm, percpu: do not consider sleepable allocations atomic")
Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Shivam Kalra <shivamkalra98@zohomail.in>
Cc: Tejun Heo <tj@kernel.org>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Kaitao Cheng 2026-06-18 21:04:14 +08:00 committed by Andrew Morton
parent 17fcdd8699
commit 8725ae13f0
1 changed files with 13 additions and 5 deletions
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18
mm/percpu.c
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@ -1726,9 +1726,8 @@ static void pcpu_alloc_tag_free_hook(struct pcpu_chunk *chunk, int off, size_t s
* @gfp: allocation flags
*
* Allocate percpu area of @size bytes aligned at @align. If @gfp doesn't
* contain %GFP_KERNEL, the allocation is atomic. If @gfp has __GFP_NOWARN
* then no warning will be triggered on invalid or failed allocation
* requests.
* allow blocking, the allocation is atomic. If @gfp has __GFP_NOWARN then no
* warning will be triggered on invalid or failed allocation requests.
*
* RETURNS:
* Percpu pointer to the allocated area on success, NULL on failure.
@ -1749,8 +1748,17 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
size_t bits, bit_align;
gfp = current_gfp_context(gfp);
/* whitelisted flags that can be passed to the backing allocators */
pcpu_gfp = gfp & (GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN);
/*
* Allowlisted flags that can be passed to the backing allocators.
* Backing allocations under pcpu_alloc_mutex must not recurse into
* IO/FS reclaim. Otherwise a GFP_KERNEL caller holding the mutex can
* block on reclaim while a GFP_NOIO/NOFS caller holding an IO/FS lock
* waits for the same mutex.
*
* Do not pass __GFP_NOFAIL. A small percpu allocation may need many
* backing pages, making nofail reclaim too costly under NOIO/NOFS.
*/
pcpu_gfp = gfp & (GFP_NOIO | __GFP_NORETRY | __GFP_NOWARN);
is_atomic = !gfpflags_allow_blocking(gfp);
do_warn = !(gfp & __GFP_NOWARN);