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 E59D94562B6; Tue, 11 Aug 2026 17:10:32 +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=1786468234; cv=none; b=tPsaTe/H1mzb5DS/5ONbAujevb3NjEzSdIq+Rld7OvCux32rZvV7RjgfSK7QMj9v4kdIC8SXi4gdlYAYvJ2YnSw+iVcf1703S0lQyH08lrPCQGhhWbsC5uklodn7+jMiKXW550HMAJTCT2AXTc7fBOftWnA7kuwxO84FDcU7E04= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786468234; c=relaxed/simple; bh=jSslxTyMsgju1erDFakraY4oEDJMxL4FlbfA9WOoMgk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=sARhZLPcTZIt55PymM6Ug9ws/fO3XyieiFwt83GIXMbDucInB6nQX4Edq1xOWoIKbfsRxqrl6+PR4RG/LG4n0ltA6nUf998+ivMfOh313t760uvcB5xUElP8yoOaWlcXWsEEMZ3hcwxnlObkEaJeJwczCju0Nl4guepvIlK0Smc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=I7X2++rf; 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="I7X2++rf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2F401F000E9; Tue, 11 Aug 2026 17:10:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786468232; bh=MCBNxDcN3O2GpPTZ53sd40013NC+rNNPo2MxMRjb1tM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=I7X2++rfnldfSGFK+xk3u9oWU2zI6ibiB9i1kKaeSqwd52oaecMd1kwAbXY5ClATB zsgKLAKZbdRi2FdBGdkUkT5Q7bPpdvLJGBI6m0v60qeglppT8ylvq34hxD4Kh+QC8p 87X6jBXozMN+0rikYQHRm2O69QcbDA5hZmSP/8ZW2yxo136a6K0GjihIcqoWTGjm/n ndEZ2Ak2332uGblLtgv4TQjTDlMntknShroH+5t5JD2Jj29EWHH1vKOsTzO7qJwKMr pFsX3EOuy6r3UEjYexlgJPClsObzPAx7lZxY4Kr6UArsDn5s3yySzIp+VrDda2RW+r ouUysHo7dNA2w== Message-ID: <26ca1c70-8526-42e4-82f3-0de90ba773ce@kernel.org> Date: Tue, 11 Aug 2026 19:10:27 +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 v2 1/2] mm/pagewalk: fix stale walk->action escaping walk_pmd_range() To: Hyunwoo Kim , akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260811161949.3879321-1-imv4bel@gmail.com> <20260811161949.3879321-2-imv4bel@gmail.com> 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 8/11/26 18:18, Hyunwoo Kim wrote: > If ->pmd_entry() sets walk->action = ACTION_AGAIN, the pmd_none() > check is retried. The PMD entry may be cleared at the point of retry. > > In this case, if walk->ops->install_pte is not specified, the code > continues to the next PMD entry in the range without resetting > walk->action to ACTION_SUBTREE. > > This leaves walk->action erroneously set to ACTION_AGAIN, which is > incorrect. > > This was incorrect but not problematic up until commit 3b89863c3fa4 > ("mm/pagewalk: fix race between concurrent split and refault") > which updated walk_pud_range() to check for walk->action == > ACTION_AGAIN upon walk_pmd_range()'s return, causing the PUD walk > to be retried. > > In this case this results in duplicate walk callbacks being > invoked, which is erroneous and will break any caller that is not > idempotent with respect to this (and waste time for those which > are). > > A specific example of this breaking things is mincore which walks > an internal cursor data structure a byte at a time on assumption > that page table entry callbacks are called only once for each > entry. > > Fix the problem by resetting walk->action to ACTION_SUBTREE prior > to the none check. > > The pattern also exists in walk_pud_range() so fix it there too. > > This issue was found through AI-based fuzzing. > > Fixes: 3b89863c3fa4 ("mm/pagewalk: fix race between concurrent split and refault") > Cc: stable@vger.kernel.org > Assisted-by: Claude:claude-opus-5 > Signed-off-by: Hyunwoo Kim > --- Acked-by: David Hildenbrand (Arm) -- Cheers, David