From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 EC1663F7E8D for ; Wed, 1 Apr 2026 11:23:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775042585; cv=none; b=VISimkbQGpiMXzUNUQ2wDwuk1pWBcTMYwNPN3RqLH3SycTZShMIJveF5nC+qrJGEL+cR6vzZzC6jKmEG8foL82IdNlHluRJaBLGV/zXEIUhGxjkSt9LdZw4CDJLM+E3dZFRQNNCAPPhGEQZ09A0nrBdbqyv8fxOYz/C/MDQhMag= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775042585; c=relaxed/simple; bh=J0EmuxBv/Kz9NWrvoGinSM8UV0p+xOj5OhofK5if/u8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UBgEvNFfBeEueIdKwIUqNJ7DGncxqJoZ0ORn2R0hC0NFYLTa1IlGiADFV1JnmGKHbzCFAbVjQYEuUcfsNhdMmEho4dhFJH0x6lioxOqk3XsPp0/jjG64K0HqbeooKSF/CNIfLNV1+xUyo3nHkG8CbIzg6L7R/38nMEKIHxwIru0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=OAYiie9x; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="OAYiie9x" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DBA60C4CEF7; Wed, 1 Apr 2026 11:23:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775042584; bh=J0EmuxBv/Kz9NWrvoGinSM8UV0p+xOj5OhofK5if/u8=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=OAYiie9xMgfT6uDxgfp0lBvoMUkXBKZEv9Ad+PONHhE3osiVap6EDg7S83N6dOxG4 BOhBKX9EW+wWtspp3GHKQuJPhXcHMrHwSiOen7j+dXdYfQoLg+HvR+Rlut+b4Vs8Y9 hiHL0/gZ1FGKpgzoDI84Zt103PIGfijevKrRKZSnaoN/8zLX978yPK3Zt6nJ6mIjFH SitOr1lq0lBz/bhpbeYlZJ6XyfWqbAlu4zC2eWxfL5grIzkHHS9Y0GWFOl3ZXCacq0 yHrBkUWwSILHjsbB0dxQqzMEYIc6D57gX7vk1/uEhLHkYkPJEHILGqeeA8RyWUX6cl gjPALjSfzUSzg== Message-ID: <38a347dc-8a1c-4d04-926d-7a6318e93ca2@kernel.org> Date: Wed, 1 Apr 2026 13:22:58 +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: [syzbot] [mm?] WARNING in deferred_split_folio To: Lance Yang Cc: kartikey406@gmail.com, usama.arif@linux.dev, Liam.Howlett@oracle.com, ziy@nvidia.com, syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com, akpm@linux-foundation.org, baohua@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, npache@redhat.com, ryan.roberts@arm.com, syzkaller-bugs@googlegroups.com References: <20260401112049.18634-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/1/26 13:20, Lance Yang wrote: > > On Wed, Apr 01, 2026 at 01:00:13PM +0200, David Hildenbrand (Arm) wrote: >> On 4/1/26 12:53, Lance Yang wrote: >>> >>> +Cc Deepanshu >>> >>> >>> Afraid not, it closes the remove_migration_ptes() -> >>> deferred_split_folio() race, but opens a new one with the shrinker, IIUC >>> >>> Once dst is on the deferred split queue, deferred_split_scan() can >>> pick it up immediately. The shrinker unconditionally dequeues every >>> folio it visits: >>> >>> list_del_init(&folio->_deferred_list); /* always */ >>> >>> Then for a non-partially-mapped folio, if folio_trylock() fails >>> (dst is still locked by migration), it falls through to: >>> >>> next: >>> if (did_split || !folio_test_partially_mapped(folio)) >>> continue; /* not requeued, dst silently lost */ >>> >>> so it is *not* requeued. >> >> How is that different to the shrinker just trying to lock the folio before we >> unlock it and failing? The race already exists? > > Ouch, you're right, I was wrong - the trylock drop is a pre-existing > issue, not caused by the reorder ;) > >> >> To sort out that race a trylock must not result in the folio getting >> discarded. > > Nice, LGTM! > > Given that the "trylock -> drop" behavior seems to exist already today, > do you think it's worth fixing that together with the reorder? I'd do it in a single shot if possible. Can you craft something? (cc stable etc) -- Cheers, David