From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 63B9D39061A for ; Tue, 12 May 2026 12:44:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778589840; cv=none; b=eyvSwZ5ogq6VDOPzxdEF95r/tJMqPB9QUyTpOvN60gWA4HUQrp5epJy84xaFzOHjbNERcAXvjN9lX69Lrl7QoUfgBPu12G8/lopselRcNJcfp8SSlO/V2OZ4gI6pasJnr2Gzz0p6ajmPeW9pyiDX2vpRcMFc0GSvZWG/n3n2vGc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778589840; c=relaxed/simple; bh=I5pqYbtb8YWMtcrOo1h5bGHH+X+Vvx6Hiv3RX7OtTcE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MzuiNBlYqVkFW/sb1nuNv1W/ah0S/Nl7xAVAt6iKvBtaj8jQFf47N0uPzjg50ZLNPHb8wSQBQIr5LhAPey7kGtYvlQSFFK2K2ClYPwi5ehHziXJ+MTwmo0CmY2iDQSTnT1qRd/enBKIxuhshGnvWFDyi1U8B+jwZndJc3hxpTPw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=OLtwiSVC; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="OLtwiSVC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 78994C2BCF6; Tue, 12 May 2026 12:43:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778589839; bh=I5pqYbtb8YWMtcrOo1h5bGHH+X+Vvx6Hiv3RX7OtTcE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=OLtwiSVCq7nWWP1cp2w5j+JfWqtuubgy8aNlU+xPV68CcKqmNxgfqCC2uUTWNZe/u 0bpQzQ6ITLuqlnUDD8vP0Y+ZsTfYyIMXteAWMPIL+S22EzLuajOnDkgrQngFGigcqT 1Xrty+V2AJCWjwIKk6rCU78k2azusczc0xTbUgYKtaUS5XBf8Uw+Mq7j96HoODA4nO IaE+u7lUO13Z78XIBRHdx8ynzTamZSKP/bXUxLDVtRhjh7R2yOAsRlDEFYovZuS6R0 sCB8174KOKqPSycPqcgBLG43aBdwxVmZJHKkeodDiESfTg1HcRmO9FxVFuOGafQNf8 tDt8JQ5TE44Ng== Message-ID: <0aab59b8-71c5-4059-8281-5dd876946528@kernel.org> Date: Tue, 12 May 2026 14:43:54 +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] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd To: Balbir Singh , Wei Yang , akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, sj@kernel.org, ziy@nvidia.com Cc: linux-mm@kvack.org, Lorenzo Stoakes , stable@vger.kernel.org References: <20260508013728.21285-1-richard.weiyang@gmail.com> <5e9ee072-b927-41e0-ba98-c9fdf11eccbc@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 5/9/26 00:48, Balbir Singh wrote: > On 5/8/26 11:37, Wei Yang wrote: >> For pmd_trans_huge() and pmd_is_migration_entry(), we does following >> before return the pmd entry: >> >> * re-validate pmd entry >> * check PVMW_MIGRATION >> * check_pmd() >> * handle on pte level if split under us >> >> But for device-private pmd, we just return after pmd_lock(). This may >> lead to inproper situation. >> > > Could you elaborate a more on the improper situation? > >> This patch fixes commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration >> support device-private entries") by following the same pattern as >> pmd_trans_huge() and pmd_is_migration_entry(). >> >> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") >> Signed-off-by: Wei Yang >> Cc: David Hildenbrand >> Cc: Balbir Singh >> Cc: SeongJae Park >> Cc: Zi Yan >> Cc: Lorenzo Stoakes >> Cc: >> --- >> mm/page_vma_mapped.c | 34 +++++++++++++++++++++++----------- >> 1 file changed, 23 insertions(+), 11 deletions(-) >> >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >> index a4d52fdb3056..5d337ea43019 100644 >> --- a/mm/page_vma_mapped.c >> +++ b/mm/page_vma_mapped.c >> @@ -269,21 +269,33 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >> spin_unlock(pvmw->ptl); >> pvmw->ptl = NULL; >> } else if (!pmd_present(pmde)) { >> - const softleaf_t entry = softleaf_from_pmd(pmde); >> + softleaf_t entry = softleaf_from_pmd(pmde); >> >> if (softleaf_is_device_private(entry)) { >> pvmw->ptl = pmd_lock(mm, pvmw->pmd); >> - return true; >> - } >> - >> - if ((pvmw->flags & PVMW_SYNC) && >> - thp_vma_suitable_order(vma, pvmw->address, >> - PMD_ORDER) && >> - (pvmw->nr_pages >= HPAGE_PMD_NR)) >> - sync_with_folio_pmd_zap(mm, pvmw->pmd); >> + entry = softleaf_from_pmd(*pvmw->pmd); >> + >> + if (softleaf_is_device_private(entry)) { > > Do we need to check softleaf_is_device_private() twice, can't we hold the pmd > lock and check once? I think what we try to do here is, is to only grab the lock if we verified that there is something of interest in there. I wonder if we should rewrite that whole thing to just do a pmd_same() check after grabbing the lock. Something a lot cleaner like: diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c index a4d52fdb3056..de6a255cc847 100644 --- a/mm/page_vma_mapped.c +++ b/mm/page_vma_mapped.c @@ -242,40 +242,28 @@ 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); + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) + return not_found(pvmw); + } else if (pmd_is_device_private_entry(pmde)) { + softleaf_t entry = softleaf_from_pmd(pmde); + if (pvmw->flags & PVMW_MIGRATION) + return not_found(pvmw); + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) + return not_found(pvmw); + } else { if ((pvmw->flags & PVMW_SYNC) && thp_vma_suitable_order(vma, pvmw->address, PMD_ORDER) && @@ -285,6 +273,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) step_forward(pvmw, PMD_SIZE); continue; } + + /* Double-check under PTL that the PMD didn't change. */ + pvmw->ptl = pmd_lock(mm, pvmw->pmd); + if (pmd_same(pmde, pmdp_get(pvmw->pmd))) + return true; + spin_unlock(pvmw->ptl); + pvmw->ptl = NULL; + goto restart; +pte_table: if (!map_pte(pvmw, &pmde, &ptl)) { if (!pvmw->pte) There is likely room to clean this up / compress it further. I'll note that this now also adds proper check_pmd() checks to pmd_is_device_private_entry(). The not_found(pvmw) if check_pmd() fails is rather weird ... but likely this works because THPs can really only be mapped through one PMD, and we always will look at the right spot ... -- Cheers, David