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 EDEE03382E5 for ; Wed, 1 Apr 2026 10:15:39 +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=1775038540; cv=none; b=kl7Je86b2BgwCAqSRcfX/N4VDgKQ0C/A0bJSe6RwpLHsp1sW41l5OL9xQBuLLzIpLXtIlEY5KezplWG7FSkiYmHFoMphkeVCHUcbCVC9/vzK1NnJjMaqsRmB/WdXPhDUwoNqRLmX0+i3jZytqlQIxCHLta31AcnYRmuyrVanTnE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775038540; c=relaxed/simple; bh=gPfGpsPWtFNdEOnKKfHw5AfsvOOAcxMLCXyvPk3Q6Cc=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=ULXfXKYom3fZAZz+VRxI96v5I0W3McWuivd2dc6GZ5OdnlXX72BywS/a5ZJVfhi5O8bsM+ugZizyndgvgPeT10t/tQgBwO7QYr1wU8mjFH+YireoQj+8jnSA8qFaCqW6qdi/sO3ge6IdDeqMDilbJY9CdOvESXsFM0/1DdOkEkg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=C87ht2XU; 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="C87ht2XU" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 67CACC4CEF7; Wed, 1 Apr 2026 10:15:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775038539; bh=gPfGpsPWtFNdEOnKKfHw5AfsvOOAcxMLCXyvPk3Q6Cc=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=C87ht2XU1p1ey1swk04IfzogavurvJ5V8TQ8YgI1FffaGA64aro2WYLEj5yHRd36V h2CDWj3+M/DEChZcelqV6WMOYHlDDYvUx+BWgdMywl46ggRJSipBnCpybJVrELs2ZU 7Gz80GC/Q+PXSpWWRz7Gcz2HXbDMAzYMP7xVwaFFaV//fp2CFdOrl23NnKB2T4MUPG w0+CtylbAGSFGpiYQzmD739xdKyVIJrH7rMwCpzwBO7teXqRDdQ4gt7Sb7dobh7RP3 CYucX69y+K7/j64jx8hRw9l4SLKZnr6Av+XAqi4ZoulG4M7TAPqA+pBGbNDw/ex6XW ++rjEazkEKfdQ== Message-ID: Date: Wed, 1 Apr 2026 12:15:32 +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] mm/migrate: fix stale partially_mapped arg to deferred_split_folio() From: "David Hildenbrand (Arm)" To: Deepanshu Kartikey , akpm@linux-foundation.org, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com Cc: usama.arif@linux.dev, richard.weiyang@gmail.com, sj@kernel.org, hannes@cmpxchg.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com, Lance Yang References: <20260401084116.22219-1-kartikey406@gmail.com> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/1/26 12:10, David Hildenbrand (Arm) wrote: > On 4/1/26 10:41, Deepanshu Kartikey wrote: >> In migrate_folio_move(), src_partially_mapped is sampled from the source >> folio before move_to_new_folio() is called: >> >> if (folio_order(src) > 1 && >> !data_race(list_empty(&src->_deferred_list))) { >> src_deferred_split = true; >> src_partially_mapped = folio_test_partially_mapped(src); >> } >> >> A concurrent thread can unmap pages from the source folio between this >> read and the actual migration, making the sampled value stale. > > Trying to make sense of this. > > In migrate_folio_move() don't we have the folio completely unmapped > because there are only migration entries referencing the folio? > > See migrate_folio_unmap(), where we check !folio_mapped(). > > Why should we suddenly have mapped folio here? Something is off. > > Unmapping a migration entry will not involve rmap code and not mess with > the partially-mapped flag. > Okay, concluding that the above reasoning is all wrong. Let's discuss it with Lance's proposal, which makes more sense. https://lore.kernel.org/r/20260401085932.20945-1-lance.yang@linux.dev -- Cheers, David