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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 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 X-Rspam-User: X-Stat-Signature: 3rktm48fyubxqw53onbu7x6g7aq98r5h X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 77FD940004 X-HE-Tag: 1786468233-46778 X-HE-Meta: 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 y6PDV3HA uQrpQeBc9jXSHm7egDUUXj25n9wsBOAl/2+Way0p2B/zyBVLvYAo/+OI/ZIGhP9xWQ1VcFqrpXqDFq17+2Px5tBxYlvZGwT4sRWe/3bcy+O5txaz2v52ybePtFi7kzme5jrxJJBABFP/Ywicv9fLBj5ih+6Wby9soHb6JuJNAYBA3C5jM4aZlyVKrNL5Xyy+g1flY3Hy2nXEfbC/vjb+J8Zsqw1Jpz713iAPYBePFkIrngGIcvQn2XLXc1hEdRv34mH6n0918QZVxxirV54qKVEHfIC8SN59oWi/CZsd43tozMnmiL/gMdJaHvAM9Kx59oGoB4Ue8GPW8I6gTRhTj6Sys9Cimfn+4MzzY Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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