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Mon, 22 Jun 2026 07:21:02 -0700 (PDT) Date: Mon, 22 Jun 2026 14:21:02 +0000 From: Wei Yang To: Lorenzo Stoakes Cc: Wei Yang , akpm@linux-foundation.org, david@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, sj@kernel.org, ziy@nvidia.com, balbirs@nvidia.com, linux-mm@kvack.org, stable@vger.kernel.org, Lance Yang , linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd Message-ID: <20260622142102.pcmr5pftshj5lvju@master> Reply-To: Wei Yang References: <20260622130651.23359-1-richard.weiyang@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) On Mon, Jun 22, 2026 at 02:46:40PM +0100, Lorenzo Stoakes wrote: >+cc Lance, linux-kernel > >Your subject line is 83 characters long and is way too detailed how about 'fix >device-private PMD handling'? > Got it. >You forgot to include linux-kernel@vger.kernel.org on the mail, lore seems to be >a bit broken atm but in general it's helpful to include that. Got it. So usually we send a patch to both linux-mm and linux-kernel? If so, I remember is later actions. > >Also is useful to make this [PATCH mm-hotfixes] to make it really clear it's >intended as a hotfix. > Got it. >Some commit msg language nits: > >On Mon, Jun 22, 2026 at 01:06:51PM +0000, Wei Yang wrote: >> For pmd_trans_huge() and pmd_is_migration_entry(), we does following >> before return the pmd entry: > >Sounds better as: > > For PMD entries that satisfy pmd_trans_huge() or pmd_is_migration_entry(), we > perform the following actions: > Sure. >> >> * re-validate pmd entry after PTL >> * check PVMW_MIGRATION >> * check_pmd() >> * handle on pte level if split under us >> >> But for device-private pmd, we just return after pmd_lock(). > >-> > > However, for device-private PMD entries, we simply acquire the PMD lock > and return. > Sure. >Also can you please give some justification here as to why all this also applies >to device-private PMD? Right now it sounds hand wavey. > I thought below paragraph explain it. Not sure what justification is preferred. >> If a softleaf entry is present, e.g. device-private pmd, the existing >> code simply acquires the PMD lock and returns success even if >> PVMW_MIGRATION is set (indicating a migration entry is sought), meaning >> that the caller can incorrectly interpret the entry as something it is >> not, causing data corruption. > >This is repetitive, you already mentioned device-private PMD, you already >mentioned that it simply acquires the PMD lock. > Ah, I copied your suggestion from [1]. Hope I don't misunderstand it. [1]: https://lore.kernel.org/linux-mm/ajUXNjRMraKb6k2n@lucifer/ >You should talk about what issue it caused and why: > > This is particularly problematic when PVMW_MIGRATION is set (meaning a > migration entry is sought), as it causes a device-private PMD entry to > be returned with a different data layout, causing memory corruption. > This looks good. I would take this one, if you prefer. >> >> 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() for device private entry. > >This is pretty useless. We see what patch it fixes in the Fixes tag, and you're >just repeating things you said above, I'd drop it. > Got it. >> 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 >> >> --- >> 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 | 32 ++++++++++++++++++++++---------- >> 1 file changed, 22 insertions(+), 10 deletions(-) >> >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >> index 2ccbabfb2cc1..8de3c6b82df6 100644 >> --- a/mm/page_vma_mapped.c >> +++ b/mm/page_vma_mapped.c >> @@ -270,21 +270,33 @@ 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); >> >> - step_forward(pvmw, PMD_SIZE); >> - continue; >> + if (softleaf_is_device_private(entry)) { > >This is all very horrible. You have an example of how pmde is re-got in the >pmd_trans_huge() branch and pmd_is_device_private_entry() exists... > >We can just make this another branch and do the re-check more neatly. > I plan to keep the change small, but yeah it is ugly. >I enclose a patch that does that (untested, please check). > > >> + 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 ((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); >> + >> + step_forward(pvmw, PMD_SIZE); >> + continue; >> + } >> } >> if (!map_pte(pvmw, &pmde, &ptl)) { >> if (!pvmw->pte) >> -- >> 2.34.1 >> > >Thanks, Lorenzo > >----8<---- >>>From e6a3c1c782714ed831c4d46a14bb99226423bf59 Mon Sep 17 00:00:00 2001 >From: Wei Yang >Date: Mon, 22 Jun 2026 13:06:51 +0000 >Subject: [PATCH] refactored > >Signed-off-by: Lorenzo Stoakes >--- > 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) && >-- >2.54.0 If we prefer this way, I will check and take it. -- Wei Yang Help you, Help me