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 73CF2402BA1 for ; Wed, 8 Jul 2026 08:43:37 +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=1783500219; cv=none; b=EyGw6553sbIy3y4MQqo4r7un+WA7kwcz6Gg5QXFDG3GQQwZyupmS/3OEFuvs328Cmx8HVf+6U3V3veKRRgfTMCyK79DLZKqb23+GV8HzUAsdkg07wVDzf2vCb23Sucq2V9BE4RMagaGVMa/LriCXcUX5xYt48oIpCTNM3GZ2xYU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783500219; c=relaxed/simple; bh=24cBfy51LgvYNru2BAnQwztCecGa3pX9gP8iZDrutdQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tdZpOx+QpEAIOmnQzLK+P3MGCS53YrhS45J4p9dP/n/aVMTuT/KUWukMCrORMqhVRPw+jVKbxpf60d6b1gnN4xwc+KhGE2Da4FkHUgX4N+XGCesAYG0tT/nTKY1sJhbkT2GuQ74226ZgRaAm95r9ngqeGlRzPNM1Wfh+Azn8LK4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JooeV2Ah; 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="JooeV2Ah" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2A4B31F000E9; Wed, 8 Jul 2026 08:43:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783500215; bh=iylKqZxKYvA+a+i91mDQinLsIvH7MQriBJaD7bqvwKk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=JooeV2AhyukQcKllXv3qlcIRIL/p8bVgh/uB0cuwFguDijyU3xwg6iKIygYOTss3a XjQbTLK5F3zbOFSiUEZ4L/9KpJWI380qKf5HL3Le4VEwxBsvNDyk3UQzx043e8JnLW WRrNmP6Y5sRm0d13vAt60HR3WHZODKU4pBzGmxrk9wvAzpOLmhA9hnwXUExJ38eIjK vCjk0qnuH0xJXnQqsTcF5+uxWOaX2bRO63+GkV7xTE5DC67XOgr42T67fg8cEsQ8yw PsATXEjzNxR51yYUsGzaK6nl8C4ntRTKH5NBFCjUbSU7EzGkU7gKDkdoBy9vEEenos R4kaN5fSKKRrQ== Message-ID: Date: Wed, 8 Jul 2026 10:43:30 +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: avoid overflowing migrate_vma collection arrays To: Zi Yan , Andrew Morton , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Balbir Singh , Kefeng Wang Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sashiko References: <20260707-fix-array-overflow-in-migrate_vma_collect_pmd-v1-1-ce3ff4627653@nvidia.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 7/8/26 03:50, Zi Yan wrote: > migrate_vma_collect_pmd() can drop pte lock to split a large folio and > restart. But the code does not handle restart properly when pmd becomes > huge or cleared. It can overflow migrate->dst and migrate->src arrays > during the hole or skip collection. Fix it by: > 1. avoiding migrate_vma_collect_huge_pmd() if some collection is done, > 2. skipping the rest of the range if pmd no longer points to a pte page > table. > > Fixes: a30b48bf1b244 ("mm/migrate_device: implement THP migration of zone device pages") > Reported-by: Sashiko > Closes: https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefeng.wang@huawei.com > Assisted-by: Claude:claude-opus-4-8 > Signed-off-by: Zi Yan > --- > The issue is spot by Sashiko during a patch[1] review as a pre-existing one. > This patch has the minimal change. An alternative is to reset > migrate->cpages and migrate->npages and restart the whole range from the > beginning, but that also requires a restoration of no-longer-present PTEs. > > Link: https://lore.kernel.org/all/20260706111958.3649651-1-wangkefeng.wang@huawei.com/ [1] > --- > mm/migrate_device.c | 19 +++++++++++++++++-- > 1 file changed, 17 insertions(+), 2 deletions(-) > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > index 2fffeb1f99694..6ceb47ec1da24 100644 > --- a/mm/migrate_device.c > +++ b/mm/migrate_device.c > @@ -257,7 +257,12 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, > pte_t *ptep; > > again: > - if (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)) { > + /* > + * Only check pmd when addr is at start, namely no pte is collected. > + * It avoids collecting the same address range [start, addr) twice > + * and overflowing the collection arrays. > + */ > + if (addr == start && (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp))) { > int ret = migrate_vma_collect_huge_pmd(pmdp, start, end, walk, fault_folio); > > if (ret == -EAGAIN) > @@ -267,8 +272,18 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, > } > > ptep = pte_offset_map_lock(mm, pmdp, start, &ptl); > - if (!ptep) > + if (!ptep) { > + /* > + * Skip the rest if pmd becomes huge or cleared. Flush if any > + * pte is modified > + */ > + if (addr != start) { > + if (unmapped) > + flush_tlb_range(walk->vma, start, end); > + return migrate_vma_collect_skip(addr, end, walk); > + } > goto again; > + } > lazy_mmu_mode_enable(); > ptep += (addr - start) / PAGE_SIZE; It's hard to express which feelings reading migrate_vma_collect_pmd() gives me, haha :) In case we split ... couldn't we just undo what we already did, before doing the "goto again" ? -- Cheers, David