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 F39ED37DAD5; Sun, 13 Sep 2026 18:48:51 +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=1789325333; cv=none; b=WA5+pwjRIX2B+BhPs63of++TeOv7yMaSLPuB+TZf+DU87HdixaVdca4fD0eU9Uc+EYfnv1ohMdHLM3oRunKLxYPyoPKNNBSWhA3rlnrACQw6JwAKNx8I7PArsc28AhjkqR723Cf2hFst6HBj6EIqbBXVulqVAW4maD9hwyi1bz8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789325333; c=relaxed/simple; bh=YpHQsc2Iy2Bk0FTSZfSYiNgSZKWCUQ3aj1Ad5T/x6Os=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=CGJR7a2FT23XtmmL/idjCtMvVcpOLVQXODkfpoIFNCrtc4hbwwGqzIN5Z0ZVC77QQM671Df+pcMFccwEzcx5mnmlOQw3OvPt8rMsL3+74+sn4ISrbCnSoGpeFAziNMQmgtXKuNC+99Dn24Qol9uz385HfCYzzQbBt04KEVnyphw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ItY4COLM; 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="ItY4COLM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E79701F000FF; Sun, 13 Sep 2026 18:48:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789325331; bh=jb/hhwiK/UcdPAYGlSzJUb1U4vkmWKdi+D5nmTCyiwc=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=ItY4COLMkAOPfq5upFo8+NRTEgTDJTWe71lMpMoYvx0ForD2G2ynjMLUv2Mnqq/Jo C6CeqeBAIXxivARXiF0/4f1ecs//FHJoa5wN0EJkQq+rGGS9/FtWk79Jh/I5fzCZqr hwZMMnyXadBtXtpmEgWarpdPi/AIA3J8c2CWRh5gPKngo4zKqcGydp/CKNne6/zAAA xhozv6PlZTBw0YRIbYlFzREHen/VOfCFQhbe5rxQ9EHEZqVlV/OGyByRAIrjc6ugl4 2+ME+jKjjsts8211xwWo0ZHPTCfQl7vVJN1Ht8x9Tq0kbu4rKkT8V0GtXuT6+HfY08 SX9dpAjXFMf0g== Date: Sun, 13 Sep 2026 19:48:44 +0100 From: "Lorenzo Stoakes (ARM)" To: Andrew Morton Cc: Nguyen Ngoc Thang , David Hildenbrand , Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Matthew Wilcox , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Jan Kara , Hugh Dickins Subject: Re: [PATCH v2] khugepaged: hold invalidate_lock across collapse_file() readahead Message-ID: References: <20260913163644.122133-1-ngocthang2710.1999@gmail.com> <20260913111700.6de78b5834842f7dd3f12d07@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260913111700.6de78b5834842f7dd3f12d07@linux-foundation.org> (Nguyen - do not send v2 in reply to v1, look across mm and see how things are done here). somebody who has a On Sun, Sep 13, 2026 at 11:17:00AM -0700, Andrew Morton wrote: > On Sun, 13 Sep 2026 23:36:44 +0700 Nguyen Ngoc Thang wrote: > > > collapse_file() calls page_cache_sync_readahead() to fault in missing > > pages before collapsing them into a THP. That helper takes > > mapping->invalidate_lock itself for the duration of the call, then > > drops it -- but truncate (e.g. ext4_setattr() -> truncate_pagecache()) > > takes invalidate_lock and then waits on each page's folio lock while > > holding it. If collapse_file() has already locked one of those folios > > by the time truncate reaches it, and then tries to acquire > > invalidate_lock again (e.g. on the first readahead call, since > > invalidate_lock is not yet held at that point), the two paths can > > deadlock/hang on each other's lock: truncate blocked on the folio lock > > collapse holds, and collapse blocked waiting for invalidate_lock that > > truncate holds. > > > > Reproducing this over ~150,000 collapse iterations with truncate > > racing concurrently reliably hits hung_task: blocked tasks within > > about 20 seconds on an unpatched kernel. > > > > Fix it by taking invalidate_lock_shared once for the whole scan, after > > alloc_charge_folio() succeeds and before locking any folio, and using > > page_cache_ra_unbounded() directly in the readahead call site instead > > of page_cache_sync_readahead(), since the latter would try to retake > > the lock we already hold. page_cache_ra_unbounded() does not clamp to > > EOF like the helper it replaces, so clamp the requested range > > explicitly. > > > > 730633f0b7f9 added invalidate_lock acquisition around readahead but > > missed collapse_file(), which already locks pages while calling > > readahead; later filesystem conversions made the deadlock reachable by > > taking invalidate_lock before waiting on page locks during truncate. > > > > Reported-by: syzbot+16bf7cd0ebeb1de93aa5@syzkaller.appspotmail.com > > Closes: https://syzkaller.appspot.com/bug?extid=16bf7cd0ebeb1de93aa5 > > Fixes: 730633f0b7f9 ("mm: Protect operations adding pages to page cache with invalidate_lock") > > (You forgot to cc the original author) Also a change log, and that mm doesn't like sending a respin in-reply-to a previous version. Which is all consistent with somebody using e.g. openclaw to pepper generated patches across the kernel... > > Five years. > > I wonder why this hasn't been discovered by lockdep, AI, syzbot or any > other of the tools we've been using for so long. > > Thanks for doing all this. See https://lore.kernel.org/linux-mm/aqbtfms0_2ULBIT7@gremlin/ I am not really entirely happy with somebody who has sent a flurry of patches across disparate subsystems with no previous track record being in charge of a potentially backported fix. It's David's decision but I think this fix should be taken over by somebody else. See https://lore.kernel.org/all/?q=f%3ANguyen+Ngoc+Thang > > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -2257,6 +2257,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, > > enum scan_result result = SCAN_SUCCEED; > > int nr_none = 0; > > bool is_shmem = shmem_file(file); > > + bool need_unlock = false; > > > > /* > > * MADV_COLLAPSE ignores shmem huge config, so do not check shmem > > @@ -2271,6 +2272,15 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, > > if (result != SCAN_SUCCEED) > > goto out; > > > > + /* > > + * Take invalidate_lock before any folio lock: the readahead below > > + * needs it, and truncate holds it while waiting on folio locks. > > + */ > > + if (!is_shmem) { > > Is the shmem special-case a red flag? > > Probably this fix an acceptable minimal-thing-for-backporting. > > Question for maintainers as well as for yourself: but does this > indicate a need for a more architected redo? > -- Cheers, Lorenzo