From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 E9DF537883D for ; Tue, 2 Jun 2026 07:16:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780384590; cv=none; b=CFFR3Y0U1OaPD5u3VqetddnQP6hELADSqEkjmCPh72eRGjZaJdG9KvvvRatDgnF3pr7uj72RI9VWrP+z7aSZ/vteiJZ7WnHaESMCgDXJe2nPhv3Pw5zS8GKrkB/n7KM8RvoIWZcL+vW1VriCEV2Q1oUrxLwvsN0xwDPFKwZ9l7E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780384590; c=relaxed/simple; bh=DUNovQ5PvuBOWf+sJmmRt5hhQzubPZsHDzevLJe9ICI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=dK6Kr+NCmwfVcZs4pMAMttPku4959ENkLs38RltmaayXTD7BOmLhmtxqHx/l7b7obdvyqQHmqk6ogk881njkOKnN4bIj/NG31Jmiifo+64fEcRwOvgOYiH6Cs4FMjcaP0YYmRWuczU4VzhRdRgXPCDqq4Geb3RlRvKO0J7DIBM0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=N+W2wLPV; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="N+W2wLPV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2F011F00898; Tue, 2 Jun 2026 07:16:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780384588; bh=cUNbZYbt9aI1mcjTwuR9tZLk6dg4eovk1Fkh3D9nCH8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=N+W2wLPVXpHphVHfpDvXlLwOzwIHKEOpSQNJYGgyaMi3txVRj3uPMsrh8FZG4M5y3 gysbcy2bu/9T+X+KuzD6fbjLDOOqdJmXXbexEez/EOTH8aExgtDMt2Aenl3vAxPKV8 rjcV+uM+jBZ+hU5nd0Wa3Rn8625buOpguXzBbmhwqn2pP/4v9Lnix8GQIfg6gLIVCA HdxwaIPe1CTjPCjzjgOZ0KJMupg24vtMPk56M/bg5OqynOyZE2dqd45km+qUUvpTMu U++QLyWUQvDfdB6zJwiU4gk6qw9N0AcxAKsJCqt97DJIaeQwKc02jMa/+3XiLSXScG mHuats6hLJKhQ== Message-ID: <536ea40b-8501-4a81-84c7-de5f12f4eaf3@kernel.org> Date: Tue, 2 Jun 2026 09:16:24 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm/percpu: Preserve NOFS/NOIO scope during chunk create and populate Content-Language: en-US To: Kaitao Cheng , Michal Hocko , Dennis Zhou Cc: Pedro Falcato , akpm@linux-foundation.org, tj@kernel.org, cl@gentwo.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, muchun.song@linux.dev, Kaitao Cheng References: <20260528132917.81123-1-kaitao.cheng@linux.dev> <20260528132917.81123-2-kaitao.cheng@linux.dev> <5a4aa532-77a0-436a-8f5e-1bbcf2db6bbb@linux.dev> <7e913ba8-fa91-4916-a871-66de7c80cd29@linux.dev> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 6/2/26 05:03, Kaitao Cheng wrote: > > > 在 2026/6/1 23:45, Michal Hocko 写道: >> On Mon 01-06-26 10:27:53, Kaitao Cheng wrote: >>> However, if we revert 9a5b183941b, it seems that all of these issues would >>> be resolved. The only downside is that the failure rate of pcpu_alloc_noprof() >>> allocations may increase, which might be acceptable. >> >> That has practical impact on some versions of iscsid which do not have >> PR_SET_IO_FLUSHER. And maybe some more so I would rather not revert >> based on a theoretical concerns which I believe is the case here. >> > > Based on the previous discussion, I think we have a way to address most > of the concurrency issues around percpu allocation. > > However, there still seems to be one remaining case that I do not yet > have a good way to solve. For example: > > 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. > > Does anyone have a good suggestion for how to handle this remaining case? > Or should we simply treat all GFP_KERNEL/GFP_NOFS allocation behavior in > pcpu_alloc_noprof() as GFP_NOIO? > > If there is no clear solution for now, would it be acceptable to first > fix some of the issues introduced by commit 9a5b183941b, and leave this > remaining case as a pre-existing historical issue to be handled separately > later? We don't need to solve any issues that are only theoretical and based on scenarios that nobody sane should be doing, i.e. Pedro already pointed out "As in no reclaim path should be insane^W daring enough to do pcpu allocations?" If anyone would (start to) do that, we would likely have lockdep reports from the testing bots, which warn that the scenario can now exist, even before it results in an actual deadlock. Elsewhere Pedro said "The proper way of fixing this would probably be to release pcpu_alloc_mutex (or not have it in the first place!) while you're allocating memory." Such a refactoring might be worth it (if it's feasible to do cleanly and doesn't come with downsides) just to eliminate these lock dependencies properly for good. Patching over individual theoretical issues is IMHO not worth it.