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 98EB93E1D05; Fri, 26 Jun 2026 13:51:42 +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=1782481903; cv=none; b=kDWfYMp/bkJpdAhAdMBGgLIYRZkHoYVgIJjsSOVPpU2wOBW/JjS/+Vf5Uab/eAD6Hxgvfd6KI1IDrK5PSLeOBZX2bE9/HZwxFCMy0XgQEDg42AAwS5+jVjTTsvyV/lYG78t6S7raGdRRqSV783wa6DLm4yR0RwYysTsZwj3gWVI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782481903; c=relaxed/simple; bh=0+hdvQLnAun10bg+4oAwwrNyo9ZCNRCyl1/B6sdkJeg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kmLIYlNpYF3PGJtA2Y9KpuYhoW6ksKpNpexxDBZPCMsWdNRocayAvnUchMnMdiBh5sVo35FrsuA2lvN93oQvVbtjQ1Fhu9l2MKl1GMiWdEMm7JX0D5RRHd6r/jsvKHKfA04/xbuR6aSGFT/9K4HTB1QTDzmjl8QDw/s7IBhsI0M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HK/Zrlu0; 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="HK/Zrlu0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 597461F00A3A; Fri, 26 Jun 2026 13:51:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782481902; bh=A4JY5U7hgmMwmoYLuJcVV6eGF+fJGev5ufEFJmKZ3vg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=HK/Zrlu0y/MbzFC1mzYqzA8l0ppdQsn5y3xuhlmR48qIl47dGPgiXM21LroMDBlXU EeoHu3qy7U2KKHnCmAvF0Cvg225munY7Z3aBSweQZCsu1q7cQfofJa2udVveiA4YRi uDAMX6CBowecIbvyBe58c4uVh7HINM4IdhRZvt/tws5BnVHLIYdwryO71J2TYD08hQ f5V+srAVFQI2gIclGKGritudWDZbcSbAuNXBDJANT+yur8OWEiUUzgaq9sUAm2vVEt zY9na4k8Pt1xx/Yvw+QBWzG+8wXh4hcg+CxLu58N2Q02wRH3P2e+h+4u8pyHFrT1sm 6/C10xF457MUw== Message-ID: <6feb9042-f9c0-4762-bd71-068d6ef5307c@kernel.org> Date: Fri, 26 Jun 2026 15:51:37 +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-hotfixes v4] mm/page_vma_mapped: fix device-private PMD handling To: Lance Yang Cc: richard.weiyang@gmail.com, akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, ziy@nvidia.com, sj@kernel.org, balbirs@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260626132728.77436-1-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 6/26/26 15:27, Lance Yang wrote: > > On Fri, Jun 26, 2026 at 12:07:56PM +0200, David Hildenbrand (Arm) wrote: >> On 6/24/26 08:53, Wei Yang wrote: >>> Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support >>> device-private entries") introduced the concept of device-private >>> PMD entries, but did not correctly update the rmap walk code to >>> account for them. >>> >>> As a result, when page_vma_mapped_walk() encounters device-private >>> PMD entries, it takes no action other than to acquire the PMD lock >>> and exit. >>> >>> However this is highly problematic for two reasons - firstly, >>> device private entries possess a PFN so check_pmd() needs to be >>> called to ensure an overlapping PFN range. >>> >>> Secondly, and more importantly, if PVMW_MIGRATION is set the >>> caller assumes the returned entry is a migration entry, resulting >>> in memory corruption when the caller tries to interpret the device >>> private entry as such. >>> >>> In addition, commit 146287290023 ("mm/huge_memory: implement >>> device-private THP splitting") allowed device private PMDs to be >>> split like THP mappings, but again did not update this code path. >>> >>> As a result, we might race a PMD split prior to acquiring the PMD >>> lock. >>> >>> This patch addresses all of these issues by invoking check_pmd(), >>> ensuring PMVW_MIGRATION is not set and checks whether a split raced >>> us we do for PMD THP and migration entries. >>> >>> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") >>> Cc: >>> Signed-off-by: Wei Yang >>> Suggested-by: David Hildenbrand >>> Cc: David Hildenbrand >>> Cc: Balbir Singh >>> Cc: SeongJae Park >>> Cc: Zi Yan >>> Cc: Lorenzo Stoakes >>> Cc: Lance Yang >>> >>> --- >>> v4: >>> * refine subject and commit log based on Lorenzo's suggestion >>> * put pmd device-private entry handling in its own if branch, >>> suggested by Lorenzo >>> >>> v3: >>> * remove cleanup part, only fix the issue for device-private entry >>> * refine user effect description based on Lorenzo's suggestion >>> >>> v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@gmail.com/T/#u >>> * specify the possible error case of current code and user visible effect >>> * besides fix, cleanup the pmd entry handling based on David's suggestion >>> >>> v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@gmail.com/ >>> --- >>> mm/page_vma_mapped.c | 20 +++++++++++++++----- >>> 1 file changed, 15 insertions(+), 5 deletions(-) >>> >>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >>> index 2ccbabfb2cc1..17dff8aab9f9 100644 >>> --- a/mm/page_vma_mapped.c >>> +++ b/mm/page_vma_mapped.c >>> @@ -269,14 +269,24 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >>> /* 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); >>> + } else if (pmd_is_device_private_entry(pmde)) { >>> + softleaf_t entry; >>> + >>> + pvmw->ptl = pmd_lock(mm, pvmw->pmd); >>> + pmde = *pvmw->pmd; >>> + entry = softleaf_from_pmd(pmde); >>> >>> - if (softleaf_is_device_private(entry)) { >>> - pvmw->ptl = pmd_lock(mm, pvmw->pmd); >>> + if (likely(softleaf_is_device_private(entry))) { >>> + if (pvmw->flags & PVMW_MIGRATION) >>> + return not_found(pvmw); >>> + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >>> + return not_found(pvmw); >>> return true; >>> } >>> - >>> + /* device-private pmd was split under us: handle on pte level */ >>> + spin_unlock(pvmw->ptl); >>> + pvmw->ptl = NULL; >>> + } else if (!pmd_present(pmde)) { >>> if ((pvmw->flags & PVMW_SYNC) && >>> thp_vma_suitable_order(vma, pvmw->address, >>> PMD_ORDER) && >> >> This is extremely hard to review given the existing crap handling here. I'm >> really sorry, but it makes my head hurt (I'm not kidding :) ). >> >> It's completely unclear why we only have to check for a subset of the cases >> after taking the lock. >> >> Could we simply extend the existing migration pmd handling and leave the >> !pmd_present() case for pmd_none()? >> >> That leaves no question to "which transitions are actually allowed", including >> "could we accidentally assume something is a page table when really it isn't". >> >> >> So what about something like the following? >> >> The "thp_migration_supported()" is not required when checking for >> pmd_is_migration_entry(), as that defaults to "false" when not compiled in. >> >> Untested: >> >> >> >From 048ecd33673ec649e168fbbb97749a7c0e344fcd Mon Sep 17 00:00:00 2001 >> From: "David Hildenbrand (Arm)" >> Date: Fri, 26 Jun 2026 12:03:40 +0200 >> Subject: [PATCH] tmp >> >> Signed-off-by: David Hildenbrand (Arm) >> --- >> mm/page_vma_mapped.c | 29 +++++++++++++++++------------ >> 1 file changed, 17 insertions(+), 12 deletions(-) >> >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >> index 2ccbabfb2cc17..ed2a23a90e8dd 100644 >> --- a/mm/page_vma_mapped.c >> +++ b/mm/page_vma_mapped.c >> @@ -243,21 +243,31 @@ 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)) { >> + if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) || >> + pmd_is_device_private_entry(pmde)) { >> pvmw->ptl = pmd_lock(mm, pvmw->pmd); >> pmde = *pvmw->pmd; >> - if (!pmd_present(pmde)) { >> + if (pmd_is_migration_entry(pmde)) { >> softleaf_t entry; >> >> - if (!thp_migration_supported() || >> - !(pvmw->flags & PVMW_MIGRATION)) >> + if (!(pvmw->flags & PVMW_MIGRATION)) >> return not_found(pvmw); >> entry = softleaf_from_pmd(pmde); >> + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >> + return not_found(pvmw); >> + return true; >> + } else if (pmd_is_device_private_entry(pmde)) { >> + softleaf_t entry; >> >> - if (!softleaf_is_migration(entry) || >> - !check_pmd(softleaf_to_pfn(entry), pvmw)) >> + if (pvmw->flags & PVMW_MIGRATION) >> + return not_found(pvmw); >> + entry = softleaf_from_pmd(pmde); >> + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >> return not_found(pvmw); >> return true; >> + } else if (!pmd_present(pmde) ){ >> + return not_found(pvmw); >> } >> if (likely(pmd_trans_huge(pmde))) { >> if (pvmw->flags & PVMW_MIGRATION) >> @@ -270,12 +280,7 @@ 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); >> - >> - 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, >> -- > > Might be good with this on top: > > ---8<--- > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index cfa1230c87bb..8b7c062bd81d 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -281,7 +281,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > return not_found(pvmw); > return true; > } > - /* THP pmd was split under us: handle on pte level */ > + /* THP/device-private pmd was split under us: handle on pte level */ > spin_unlock(pvmw->ptl); > pvmw->ptl = NULL; > } else if (!pmd_present(pmde)) { > -- > > Looks good to me as well, thanks! Makes sense, thanks! -- Cheers, David