From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta1.migadu.com (out-179.mta1.migadu.com [95.215.58.179]) (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 5BA454ADD81 for ; Fri, 5 Jun 2026 08:49:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780649379; cv=none; b=WzIhSWjJnVNMffhvdHe4feMJPfX2+aW2UAJyhjttpdWr34spqj5ahK7ZBSIaaLAzC4c2saQAOV0dQTaNXk5ROpcXfQ6r3dMhtlrFtmLI5rd5g3LKZO7eZNI6573w3anuICtaR68VIvZY3ny61yMBHruODyEVUCY2h2U08jtk2eg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780649379; c=relaxed/simple; bh=525DidrRUguisOGviV352RpUUgN/U0MluVOTN6XIta4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=f+TvbFSmarsV3o9mL1d5sZSYTY2/eivX3aCj5G2SzhSIxwxXCGnh7TYaxW9P9iKLyIn0w+01Ih3utRe8uTWSu6If2M3cdzs3wen3fAtHm/19r85lSyUvFitbgg+IWhL7fKQOLrkgcHaDL4jSR5f9lkhy56R781nV45HQRAM1Yjc= 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=qjZBmLwJ; arc=none smtp.client-ip=95.215.58.179 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="qjZBmLwJ" Message-ID: <3de3a89b-92f0-4cd2-9f41-8e853eae4e78@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780649372; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=YOToXdxhhhdNK71DLVY4SYkIbODfD9tHfsxl+vivDR8=; b=qjZBmLwJgvsbuvq1fHyyIIcdo6WxEInJqqF3BNOBkvY1ArbkqE+G54aTWf86cZwwFBdXdn tr5j+DmrGHYFquCnV9slP+JnPAuV+Jgj5EuAXnT7DAck61kRPkW+plclGgVpgCzomSHQFO lsPlG9KHy3eJMYoD4IWUcfjiM2/d8MA= Date: Fri, 5 Jun 2026 16:48:35 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2 3/3] mm/percpu: Avoid IO/FS reclaim in backing allocations To: Andrew Morton Cc: Dennis Zhou , Tejun Heo , Christoph Lameter , Uladzislau Rezki , Pedro Falcato , Vlastimil Babka , Michal Hocko , muchun.song@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kaitao Cheng References: <20260604113101.89510-1-kaitao.cheng@linux.dev> <20260604113101.89510-4-kaitao.cheng@linux.dev> <20260604120709.445c027637b3ad72ad13279a@linux-foundation.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Kaitao Cheng In-Reply-To: <20260604120709.445c027637b3ad72ad13279a@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2026/6/5 03:07, Andrew Morton 写道: > On Thu, 4 Jun 2026 19:31:01 +0800 Kaitao Cheng wrote: > >> 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. >> >> ... >> >> --- 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. > > We're supposed to say "allowlist". > >> + * 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); > > AI review > (https://sashiko.dev/#/patchset/20260604113101.89510-1-kaitao.cheng@linux.dev) > asked why we're currently removing __GFP_NOFAIL here. There are > probably good reasons for this, but it would be good to describe them > in that comment. > This behavior has been present since commit 554fef1c39ee ("percpu: allow select gfp to be passed to underlying allocators"), which introduced the whitelist for GFP flags passed down to the backing allocators. I did a quick AI-assisted scan of the current tree and did not find any in-tree caller passing __GFP_NOFAIL to pcpu_alloc_noprof() or its wrappers. So the issue Sashiko described does not appear to be reachable with current callers. That said, I agree the semantics are somewhat incomplete: __GFP_NOFAIL is handled when taking pcpu_alloc_mutex, but it is not propagated through pcpu_gfp to the backing allocations. If we want to address this defensively, I think it would be better as a separate patch. Even though it touches the same line, it fixes a different issue from this change. -- Thanks Kaitao Cheng