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>

1 files changed, 13 insertions, 5 deletions

diff --git a/mm/percpu.c b/mm/percpu.c
index 4d89965cba16..a802d72c116f 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -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);