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 90E0E36165F for ; Wed, 1 Jul 2026 16:49:45 +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=1782924586; cv=none; b=gKpt/PHwmphLHs+x3Gf05LA4/jOOPrKdHrvt0MJi485uqwruXTurQ5WjSZ6QTvSbq4SVrvrh/HBwarl8R63ASsLO+jk6l8r768WJUEdAjaswEYKZGRgBJdukjYv40m8QRyhsFcW3LxFdaPUCLY7hH5JHL3mTYXt76+ZKLZQQvsI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782924586; c=relaxed/simple; bh=nrEMhNS6vNsbZ3fzSroV04Y39+XtItORXFn25u4ED/g=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=a6JemyIG0n4OfXOhJ9zZS2JrgGAiC8hPQIbxSh036rQue5eoajQwKt2peTpxLitycKwQZlv51sLYDgusJcmQGChxJxFE/QpugKOObaON2F0qJftkkgYh6gWoHoGGqVqqQ7SV3U3TXSNGL91918XoJuBiwf2kuN2aqlb3lWfWb/I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UZkTNtmp; 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="UZkTNtmp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DB2F81F00A3A; Wed, 1 Jul 2026 16:49:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782924585; bh=ewTcjC2YNzXZmdVVFzcmi3ViWYP+V41foEbY5CbQbA4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=UZkTNtmpKRaME4zjJl/r6A13mMXd1FfSdDxLmZ1795hN67MKNO4BVBvojv24PZjzM 1P6qSXhQZh3H/kawABE1PLrZDTKIO7roA/+MS9ddZz0k/DRiI+DWtjXk2dlvetM2Im hgV24I3m34Wg9VDKK65SlSJyXN9tM31ZRQf6mSjd8i0cZ85ihdamfUBW3ozh6mfBNY cy3nU46+8JQ/NWZCaSFohbKXtR4DEtlGxgH2NrctfLiWyK/kgAot0nwJEuUYlnQGVK eKIYogZ4WXRmBTvGZQ+4EZf9WJF8EsGX6/BCvMW+AqWPZyCCInwQ1H9ePeAzHdojKi 5riMsS/WKqOzw== Message-ID: <65671a65-a2de-4f12-994a-a96d2acbc92e@kernel.org> Date: Wed, 1 Jul 2026 18:49:38 +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_device: pin large folios before splitting To: Usama Arif , Andrew Morton , apopple@nvidia.com, byungchul@sk.com, gourry@gourry.net, joshua.hahnjy@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, matthew.brost@intel.com, rakie.kim@sk.com, ying.huang@linux.alibaba.com, ziy@nvidia.com Cc: shakeel.butt@linux.dev, hannes@cmpxchg.org, kernel-team@meta.com, sashiko-bot References: <20260701140638.840773-1-usama.arif@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 7/1/26 16:06, Usama Arif wrote: > migrate_vma_collect_pmd() can detect a large folio while holding the PTE > lock, then drop the PTE lock before calling migrate_vma_split_folio(). The > split helper took its own reference, but only after the lock had already > been dropped. > > One way to hit this is device migration over a range that contains a large > folio. The walker reads the PTE while holding the PTE lock and derives the > folio either from a present PTE via vm_normal_page(), or from a non-present > PTE that encodes a device-private softleaf entry. It then has to drop the > PTE lock because split_folio() can block. Before migrate_vma_split_folio() > gets a folio reference, concurrent reclaim, migration, or truncation can > replace or clear the entry and drop the last reference to the folio. The > split helper would then take a reference and lock on a stale folio pointer. > > Take a temporary reference before dropping the PTE lock and pass that > reference into migrate_vma_split_folio(). The helper consumes the > reference, so split_folio() still sees only the expected caller pin instead > of an extra pin that could make the split fail. > > Reported-by: sashiko-bot > Link: https://sashiko.dev/#/patchset/20260630164143.1595669-1-usama.arif%40linux.dev > Fixes: 022a12deda53 ("mm/migrate_device: handle partially mapped folios during collection") > Signed-off-by: Usama Arif > --- > mm/migrate_device.c | 21 ++++++++++++++++++--- > 1 file changed, 18 insertions(+), 3 deletions(-) > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > index 2f8b646302c2..f5a5f699e98e 100644 > --- a/mm/migrate_device.c > +++ b/mm/migrate_device.c > @@ -77,6 +77,9 @@ static int migrate_vma_collect_hole(unsigned long start, > * @folio: the folio to split > * @fault_page: struct page associated with the fault if any > * > + * If @folio is not the folio containing @fault_page, the caller must hold a > + * reference on @folio. The helper consumes that reference. > + * > * Returns 0 on success > */ > static int migrate_vma_split_folio(struct folio *folio, > @@ -86,10 +89,8 @@ static int migrate_vma_split_folio(struct folio *folio, > struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL; > struct folio *new_fault_folio = NULL; > > - if (folio != fault_folio) { > - folio_get(folio); > + if (folio != fault_folio) > folio_lock(folio); > - } > > ret = split_folio(folio); > if (ret) { > @@ -310,6 +311,13 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, > if (folio_test_large(folio)) { > int ret; > > + /* > + * Keep the folio stable after dropping the PTE > + * lock. migrate_vma_split_folio() consumes this > + * reference. > + */ Do we really need that comment (same below?). It's a common mechanism when keeping to refer to folios after dropping the PTL. > + if (folio != fault_folio) > + folio_get(folio); > lazy_mmu_mode_disable(); > pte_unmap_unlock(ptep, ptl); > ret = migrate_vma_split_folio(folio, > @@ -353,6 +361,13 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, > if (folio && folio_test_large(folio)) { > int ret; > > + /* > + * Keep the folio stable after dropping the > + * PTE lock. migrate_vma_split_folio() consumes > + * this reference. > + */ > + if (folio != fault_folio) > + folio_get(folio); > lazy_mmu_mode_disable(); > pte_unmap_unlock(ptep, ptl); > ret = migrate_vma_split_folio(folio, LGTM. The code duplication in this function is concerning. Acked-by: David Hildenbrand (Arm) -- Cheers, David