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 BF9E0345757 for ; Tue, 16 Jun 2026 13:08:00 +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=1781615281; cv=none; b=B5auj6gt7PLx2+fuJQ5JlC5CJnWyWn5JAzn2n42BsQfd8X8qy7qOkosdxQzXOinNrEjFbIUeEvNodPtxmTVozqZ85jhjFyC3GE11nCy3+DFpbVDQCwXY3nOdVdQJv+f9bjYgoVMkJpeXKiMd84V2WcGYU1kdfyZIlVCAgJ9SELQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781615281; c=relaxed/simple; bh=HyLcNQbLsvNaHo5acFTUnjHxOApVJMkSPcGZ+qdf5l4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=sJk71gY9tOuuftVY2p0q49W9P8/VXJc+0GxaHV8F+poE2U3NLDPK9BS9eZxDlUpL+Y5HojPndGT1mB8uap3SB2/FChyL5AAzVFDG0qJyT7HdiMgvsgibwh+EY5GAUBDVRaZzKBZJ0lxhcq367tOhGmBgssRVLfi38RvCCYRVKD0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=U3LCMVeS; 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="U3LCMVeS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 760F01F000E9; Tue, 16 Jun 2026 13:07:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781615280; bh=44kGT5EsEATek0OwEmllUBkTdBMczZ5UTHP3Uzrhvfs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=U3LCMVeSvU194moAVojm2I397RFBkHK4PBp7fG54xBjccsk8nseP+p4yl5UDHqI/t qPjC/aNYF4BWcGm0/CNok0D9CbkBxX7LOE6iwe31Yl2WvaMGTOdFRlNR84ZUSUDaAl xCGLxlZmqFDqs6AdLcCpsOWq/m96hzIx4JaigC1MTvODdI9kuY2uXYsiCIXGXDlMbi ciq4Mudskv5z2T1CJzENOZqSZ48oJyKAyMBv+7sGHXxCP/TQdHPhdBzJXAUiKfjhp6 6lKaZldqjoKZRFlpJ85tY76FDuCssJ/4otz5eHXh2zgeRb1l2gsDoCLPg2XC2DJGoj HBpQWs3HtuNMQ== Message-ID: <666dc40b-e37a-46eb-af55-7a81bc1643f1@kernel.org> Date: Tue, 16 Jun 2026 15:07:53 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [Patch v2] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd To: Lance Yang , richard.weiyang@gmail.com Cc: akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, balbirs@nvidia.com, ziy@nvidia.com, sj@kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, stable@vger.kernel.org References: <20260616063436.20455-1-richard.weiyang@gmail.com> <20260616123001.6501-1-lance.yang@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 6/16/26 14:30, Lance Yang wrote: > > On Tue, Jun 16, 2026 at 06:34:36AM +0000, Wei Yang wrote: > [...] >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >> index 2ccbabfb2cc1..21635fab209c 100644 >> --- a/mm/page_vma_mapped.c >> +++ b/mm/page_vma_mapped.c >> @@ -243,40 +243,28 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >> */ >> pmde = pmdp_get_lockless(pvmw->pmd); >> >> - if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) { >> - pvmw->ptl = pmd_lock(mm, pvmw->pmd); >> - pmde = *pvmw->pmd; >> - if (!pmd_present(pmde)) { >> - softleaf_t entry; >> - >> - if (!thp_migration_supported() || >> - !(pvmw->flags & PVMW_MIGRATION)) >> - return not_found(pvmw); >> - entry = softleaf_from_pmd(pmde); >> - >> - if (!softleaf_is_migration(entry) || >> - !check_pmd(softleaf_to_pfn(entry), pvmw)) >> - return not_found(pvmw); >> - return true; >> - } >> - if (likely(pmd_trans_huge(pmde))) { >> - if (pvmw->flags & PVMW_MIGRATION) >> - return not_found(pvmw); >> - if (!check_pmd(pmd_pfn(pmde), pvmw)) >> - return not_found(pvmw); >> - return true; >> - } >> - /* THP pmd was split under us: handle on pte level */ >> - spin_unlock(pvmw->ptl); >> - pvmw->ptl = NULL; >> - } else if (!pmd_present(pmde)) { >> - const softleaf_t entry = softleaf_from_pmd(pmde); >> - >> - if (softleaf_is_device_private(entry)) { >> - pvmw->ptl = pmd_lock(mm, pvmw->pmd); >> - return true; >> - } >> + if (pmd_present(pmde)) { >> + if (!pmd_leaf(pmde)) >> + goto pte_table; >> + if (pvmw->flags & PVMW_MIGRATION) >> + return not_found(pvmw); >> + if (!check_pmd(pmd_pfn(pmde), pvmw)) >> + return not_found(pvmw); >> + } else if (pmd_is_migration_entry(pmde)) { >> + softleaf_t entry = softleaf_from_pmd(pmde); >> + >> + if (!(pvmw->flags & PVMW_MIGRATION)) >> + return not_found(pvmw); > > Looked at history a bit, and I wonder if this changed something old > here ... > > Since 616b8371539a ("mm: thp: enable thp migration in generic path"), PMD > migration handling took PTL before doing PVMW_MIGRATION/PFN checks, > including not_found() cases. So lockless PMD read was just a filter ... > > With this fix, true case gets final pmd_same() check, but this > not_found() case happens before taking PTL. > > So a !PVMW_MIGRATION walker could race with someone, e.g. > remove_migration_pmd(): we make the not_found() decision from old PMD > value that still says "migration", while real *pvmw->pmd may already be > present again. We return without ever taking PTL :) > > Not sure about practical fallout, but should these PMD-level not_found() > cases also take PTL and restart if PMD changed? I was hoping that we could so something similar to the PTE case. In map_pte(), we check whether the PMD changed, which is slightly different. The actual check happens in check_pte() after grabbing the PTL. For the case you describe, map_pte() would find !pte_none(ptent) ... !pte_present(ptent) and !is_migration, and effectively grab the lock and proceed to check_pte(). In check_pte() we re-check under lock indeed. -- Cheers, David