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Sun, 04 Oct 2026 08:19:22 +0000 X-Mizu-Trace-ID: e992c55305e9727b X-Migadu-Flow: FLOW_OUT From: Lance Yang To: usama.arif@linux.dev Cc: akpm@linux-foundation.org, david@kernel.org, chrisl@kernel.org, kasong@tencent.com, ljs@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, ying.huang@linux.alibaba.com, baoquan.he@linux.dev, willy@infradead.org, youngjun.park@lge.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, alex@ghiti.fr, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, nico.pache@linux.dev, liam@infradead.org, ryan.roberts@arm.com, vbabka@kernel.org, lance.yang@linux.dev, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, yosry@kernel.org, qi.zheng@linux.dev, luizcap@redhat.com, kernel-team@meta.com Subject: Re: [PATCH v8 26/30] mm: handle PMD swap entries in UFFDIO_MOVE Date: Sun, 4 Oct 2026 16:19:17 +0800 Message-ID: <20261004081917.20111-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261002095503.3585565-27-usama.arif@linux.dev> References: <20261002095503.3585565-27-usama.arif@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 6988A40007 X-Stat-Signature: 5ox9tab648t5hzqoa5bb9mpg9dp41dga X-Rspam-User: X-HE-Tag: 1791101964-74039 X-HE-Meta: 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 /9+riiMq 4Zk0eGq4nudR0xHHJTZbw4PUEXe0zBAC4GzytptaDt5aq53ak3DCIUKELc5+DfEpBu5wkBcrMvQfDDMqhyHO0QZ1OXQN1VGCvBo4YEm5JNxE7iO/dU1j3BwPD7Y1n6Dg5OGsaTDFOJ3CsPHzRxkAiTppkWB2KAexk/Qwe3Mf7kKRndUQa5RXLWHYfPKQoWAN+fSl18qJKwPI88P2eHiDHjKoP9vjW0lYdYn8eMaDy89zVW1f9oPcedV0ss4V3uEOTUminUItnDbeTiML0kFfq0qgt1Zq8ETy6Wp9N Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hey Usama, On Fri, Oct 02, 2026 at 02:52:40AM -0700, Usama Arif wrote: [...] >+static int move_swap_pmd(struct mm_struct *mm, struct vm_area_struct *dst_vma, >+ unsigned long dst_addr, unsigned long src_addr, >+ pmd_t *dst_pmd, pmd_t *src_pmd, >+ pmd_t orig_dst_pmd, pmd_t orig_src_pmd, >+ spinlock_t *dst_ptl, spinlock_t *src_ptl, >+ struct folio *src_folio, swp_entry_t entry) >+{ [...] >+ >+ moved_pmd = pmdp_huge_get_and_clear(mm, src_addr, src_pmd); >+ if (pgtable_supports_soft_dirty()) >+ moved_pmd = pmd_swp_mksoft_dirty(moved_pmd); Shouldn't we clear the source UFFD marker here? moved_pmd = pmd_swp_clear_uffd(moved_pmd); just like move_swap_pte(), no? >+ /* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */ >+ if (userfaultfd_rwp(dst_vma)) >+ moved_pmd = pmd_swp_mkuffd(moved_pmd); >+ set_pmd_at(mm, dst_addr, dst_pmd, moved_pmd); static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma, unsigned long dst_addr, unsigned long src_addr, pte_t *dst_pte, pte_t *src_pte, pte_t orig_dst_pte, pte_t orig_src_pte, pmd_t *dst_pmd, pmd_t dst_pmdval, spinlock_t *dst_ptl, spinlock_t *src_ptl, struct folio *src_folio, struct swap_info_struct *si, swp_entry_t entry) { ... orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte); ... orig_src_pte = pte_swp_clear_uffd(orig_src_pte); /* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */ if (userfaultfd_rwp(dst_vma)) orig_src_pte = pte_swp_mkuffd(orig_src_pte); set_pte_at(mm, dst_addr, dst_pte, orig_src_pte); ... } Suppose the source is an exclusive swap PMD carrying a UFFD-WP marker, and the destination is an empty hole registered for synchronous UFFD-WP without RWP. We haven't write-protected the destination, but a successful whole-PMD MOVE carries the source's UFFD-WP marker into it. On swap-in, that marker is restored in the present PMD, leaving it write-protected. The write fault then reaches handle_userfault(vmf, VM_UFFD_WP), even though we never write-protected the destination ... That would be quite a surprise for userspace, and could leave the writer stuck if the unexpected WP event goes unhandled :) vm_fault_t do_huge_pmd_swap_page(struct vm_fault *vmf) { struct vm_area_struct *vma = vmf->vma; ... bool exclusive, stable_writes, rwp_restore = false; bool write = vmf->flags & FAULT_FLAG_WRITE; ... pmd = folio_mk_pmd(folio, vma->vm_page_prot); ... if (pmd_swp_uffd(vmf->orig_pmd)) pmd = pmd_mkuffd(pmd); if (pmd_swp_uffd(vmf->orig_pmd) && userfaultfd_rwp(vma)) { pmd = pmd_modify(pmd, PAGE_NONE); rwp_restore = true; } ... if (exclusive) { if (!rwp_restore && (vma->vm_flags & VM_WRITE) && !userfaultfd_huge_pmd_wp(vma, pmd) && !pmd_needs_soft_dirty_wp(vma, pmd)) { pmd = pmd_mkwrite(pmd, vma); if (write) pmd = pmd_mkdirty(pmd); } rmap_flags |= RMAP_EXCLUSIVE; } ... vmf->orig_pmd = pmd; ... if (write && !pmd_write(pmd) && !rwp_restore) { vm_fault_t wp_ret = wp_huge_pmd(vmf); ... } ... } vm_fault_t wp_huge_pmd(struct vm_fault *vmf) { struct vm_area_struct *vma = vmf->vma; const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE; ... if (vma_is_anonymous(vma)) { if (likely(!unshare) && userfaultfd_huge_pmd_wp(vma, vmf->orig_pmd)) { if (userfaultfd_wp_async(vmf->vma)) goto split; return handle_userfault(vmf, VM_UFFD_WP); } return do_huge_pmd_wp_page(vmf); } ... } So, clearing the source UFFD-WP marker before the destination RWP check would match the PTE helper and keep the RWP re-arm :) Am I missing something? >+ src_pgtable = pgtable_trans_huge_withdraw(mm, src_pmd); >+ pgtable_trans_huge_deposit(mm, dst_pmd, src_pgtable); >+ >+ double_pt_unlock(dst_ptl, src_ptl); >+ return 0; >+} >+#endif /* CONFIG_THP_SWAP */ >+ [...] Cheers, Lance