From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B8713413D96 for ; Thu, 4 Jun 2026 11:32:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780572737; cv=none; b=dloeybzhUEeF92xAseXC1giU1GZ1OpThag/doUsSyAhPsX+aDGEB9xk6/N/zZpReHGKSw6gI2U8pzYUttw1u5rO4dSCzY2QFhmdju/vIPIijQpnPpEDodi2ZdxB4fFMJ3ZZ5YI0zaZg+MwkE+aaC4VU0w4RvCm7yHjFkZuANsRQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780572737; c=relaxed/simple; bh=CsFiYWxHXTn8/MSfUvt2g3xd5p/PPI6E7cyCeNILHKE=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=SbA/CopdyhIptfvXcVq1YZ9aT/phZuS0deDXTWzPmU7Pry3EZUnJ+57ppXVSLV6Bnaf7T/jKi0RuGSBgZhFk4m6FORPkwT9DgZnPwigVxGNmtDq5uwONt7u8XYxDFv2ca5rtNBajoehSobmAOpfqPqvtlh4vyGnWx+9MQW5VsdQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=Aq7frTa3; arc=none smtp.client-ip=91.218.175.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="Aq7frTa3" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. 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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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