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

Commit 9a5b183941b5 ("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: 9a5b183941b5 ("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>

0 files changed, 0 insertions, 0 deletions