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Thu, 05 Nov 2026 04:54:11 +0000 X-Mizu-Trace-ID: f63fd72cc0f28980 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: baolin.wang@linux.alibaba.com Cc: akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, lance.yang@linux.dev, qi.zheng@linux.dev, pfalcato@suse.de, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes Date: Thu, 5 Nov 2026 12:54:06 +0800 Message-ID: <20261105045406.39871-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: References: X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam09 X-Stat-Signature: bunqainxb4kftis8xcnwk6y9dy8cpwtk X-Rspamd-Queue-Id: 59C00A0005 X-Rspam-User: X-HE-Tag: 1793854453-180815 X-HE-Meta: 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 kL5aDEmO icOf4yLujzTL0OJ8ZC72Q6Ny49jHqXpCFjAiDM2xANK3OhrP2n8G0SGbOKZtQps2DrP7Sjl17Vxg9o5TbnT7UAljt49bKDs2wWGnsnUDzAAXh8nuQpzP54RznsDJx0ioMA4eyiyg4cdvuAPOYRCF9lEBB8lBMz/TSwuzvUGNh8VYqGhJ2sOGvV0vDDI71sOQ4VxWfZq12PDlqBUk9FbL3HvXh6RwERrvuSlcceQAR3xzfW60wbrps6+tDjgyxhW4IaERpTIvA90mG5XwptquzYllHcLWsMMb+3QRU Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Thu, Nov 05, 2026 at 10:56:28AM +0800, Baolin Wang wrote: >Similar to commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped >folios"), the '!pmd_present()' check in page_vma_mapped_walk() can also race >with concurrent page table reclamation via zap_empty_pte_table(). Good catch! >page_vma_mapped_walk() is used by many functions including try_to_unmap(), >try_to_migrate(), folio_mkclean(), pfn_mkclean_range(), remove_migration_ptes(), >page_mapped_in_vma(), and mapping_wrprotect_range(). Except for try_to_unmap() >and try_to_migrate(), the other callers do not perform post-walk accounting >checks, so the race has no observable impact on them. > >For migration, the race can cause unnecessary retries. For reclaim via >try_to_unmap(), the race can cause failure to reclaim a folio that is in >fact already unmapped (as shown below), which may lead to unexpected OOM >or increased refaults. > >CPU 0 (reclaim) CPU 1 (MADV_DONTNEED) >-------------------------------- -------------------------------- >folio_lock(F) >try_to_unmap(F, TTU_SYNC) > zap_pte_range(full PMD range) > spin_lock(ptl) > clear PTEs > zap_empty_pte_table() > spin_trylock(pml) > pmd_clear() > spin_unlock(pml) > > page_vma_mapped_walk() > observe !pmd_present(pmde) > nr_pages < HPAGE_PMD_NR > skip synchronization > return false > >try_to_unmap() returns >folio_mapped(F) == true >nr_unmap_fail += folio_nr_pages(F) > tlb_flush_mmu_tlbonly() > tlb_flush_rmaps() > remove F's rmaps > spin_unlock(ptl) > >Commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped folios") >has already moved the PMD clear operation (via zap_empty_pte_table()) to >after tlb_flush_rmaps(). Similarly, page_vma_mapped_walk() also needs to >use sync_with_folio_pmd_zap() to synchronize and avoid memory ordering >issues on weakly ordered CPUs. > >Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)") >Cc: stable@vger.kernel.org >Suggested-by: David Hildenbrand (Arm) >Signed-off-by: Baolin Wang >--- > mm/page_vma_mapped.c | 5 +---- > 1 file changed, 1 insertion(+), 4 deletions(-) > >diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >index e6b6170c2f4f..8797467b6f0c 100644 >--- a/mm/page_vma_mapped.c >+++ b/mm/page_vma_mapped.c >@@ -293,10 +293,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > 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) && >- (pvmw->nr_pages >= HPAGE_PMD_NR)) >+ if (pvmw->flags & PVMW_SYNC) Right. Whether the folio can be PMD-mapped doesn't tell us whether its PTE table can be reclaimed, so those extra checks can skip synchronization that the caller explicitly asked for ;) LGTM, thanks! Reviewed-by: Lance Yang > sync_with_folio_pmd_zap(mm, pvmw->pmd); > > goto next_pmd; >-- >2.47.3 > >