[PATCH v2 3/3] mm/percpu: Avoid IO/FS reclaim in backing allocations

[Kaitao Cheng]

From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>

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.

Fixes: 9a5b183941b5 ("mm, percpu: do not consider sleepable allocations atomic")
Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
---
 mm/percpu.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 4d89965cba16..e6f449323064 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 +1748,14 @@ 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);
+	/*
+	 * Whitelisted 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.
+	 */
+	pcpu_gfp = gfp & (GFP_NOIO | __GFP_NORETRY | __GFP_NOWARN);
 	is_atomic = !gfpflags_allow_blocking(gfp);
 	do_warn = !(gfp & __GFP_NOWARN);
 
-- 
2.43.0