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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780572733; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=a1GuPMUk5HjK1efq543yYrmfWbZP5fHMEwE2PeqlWRE=; b=Aq7frTa3xKlY0G3u8Xf7kVfMUeSdceuNDU0VEW/fswn5QlJmNaSTLJBnkwTnHpmxArAYI3 SA4/98OFIeqzbOAy06Y9+hdk74dYQEaaSFPzpB8z+rhC9aAw/W8Sii2vTLSUPJ/1IoI49j LqHYoi0w42eJu1uDpEbDe/X0bT1QumA= From: Kaitao Cheng To: Andrew Morton , Dennis Zhou , Tejun Heo , Christoph Lameter , Uladzislau Rezki , Pedro Falcato , Vlastimil Babka , Michal Hocko Cc: muchun.song@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kaitao Cheng Subject: [PATCH v2 3/3] mm/percpu: Avoid IO/FS reclaim in backing allocations Date: Thu, 4 Jun 2026 19:31:01 +0800 Message-ID: <20260604113101.89510-4-kaitao.cheng@linux.dev> In-Reply-To: <20260604113101.89510-1-kaitao.cheng@linux.dev> References: <20260604113101.89510-1-kaitao.cheng@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: C8E40180005 X-Stat-Signature: p4f83dos3e8xsss74qcxn94w8zo7wfe5 X-HE-Tag: 1780572737-411823 X-HE-Meta: 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 9XGF/tUg hQFMDfw+DKTcb7/JvD3uY1N3Esh45F/rhFyoJMeCX/6SAd1yIY9/JIGMyihlybsydh64KFRij3A8uQBT7n7glCMJbtPGR6E+Zmpf7Srgmza6b/VVvwi+G38xJAYsxJu7RY5pe6pokvX+3Wi5zD13xLJMUpBCJd8e5IIJM+84MwCYSrbySCWe5VsiJwBSOkW3GyLxQKQue1KgTxHY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Kaitao Cheng 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 --- 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