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Sun, 08 Nov 2026 08:40:41 +0000 X-Mizu-Trace-ID: 8756b258539f3c21 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org Cc: akpm@linux-foundation.org, rppt@kernel.org, peterx@redhat.com, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, qi.zheng@linux.dev, tz2294@columbia.edu, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm/mprotect: don't skip uffd marker install when racing with PTE table reclaim Date: Sun, 8 Nov 2026 16:40:35 +0800 Message-ID: <20261108084035.85404-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261107-uffd_shmem_marker-v1-1-91fa2d653cfe@kernel.org> References: <20261107-uffd_shmem_marker-v1-1-91fa2d653cfe@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: imcq37auhs3use3bz3aqeaj91k9symdw X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: CC68E40002 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-HE-Tag: 1794127244-315177 X-HE-Meta: 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 H3KOy5J9 oW7uGuh2wxYsmHy9r3bq2E4veP23Jt1eJT/OW8jorRlMyb0b8VhjVyzl0jpNneao6DCiLAEBJoHRGH7enRlfblWvRP+Xshz4ruJ9V7FeQ6Vi74jaxY3sF6QmkKlGrwT8xsxlViQf/d+rahdCfF4F3vtBuuFC/HfbW3sYfMIIsVwvlAvfvWIj8wtbaCW0AXrnHLvNQyXdvcLMiVISgthASbGum5UBaqK2n9cC0sMzNbZK+gKJax89IONcGZgivIaiHZ1LW 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 Sat, Nov 07, 2026 at 12:28:55AM +0100, David Hildenbrand (Arm) wrote: >change_pmd_range() populates PTE tables before installing uffd-wp >markers. An empty PTE table can be removed after change_pmd_prepare() >returns but before the pmd_none() check. > >That check then skips the range, allowing UFFDIO_WRITEPROTECT to report >success without installing the required markers. Subsequent writes by >userspace to these areas will not trigger uffd events. > >PTE table reclaim during MADV_DONTNEED and page table retraction during >khugepaged can trigger this. > >Retry page-table allocation in that case. Simplify the code, getting >rid of change_pmd_prepare() by routing all code through the "again" label. Cool, nice work1 >It's a small race though, so likely this is not very urgent. > >Fixes: fe2567eb5521 ("mm/shmem: allow uffd wr-protect none pte for file-backed mem") >Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)") >Cc: stable@vger.kernel.org >Signed-off-by: David Hildenbrand (Arm) >--- >I expect Andrew to pick this up. >--- > mm/mprotect.c | 48 +++++++++++------------------------------------- > 1 file changed, 11 insertions(+), 37 deletions(-) > >diff --git a/mm/mprotect.c b/mm/mprotect.c >index e18d4e12fc341..ef4e7188e24da 100644 >--- a/mm/mprotect.c >+++ b/mm/mprotect.c >@@ -470,25 +470,8 @@ pgtable_populate_needed(struct vm_area_struct *vma, unsigned long cp_flags) > } > > /* >- * Populate the pgtable underneath for whatever reason if requested. >- * When {pte|pmd|...}_alloc() failed we treat it the same way as pgtable >- * allocation failures during page faults by kicking OOM and returning >- * error. >- */ >-#define change_pmd_prepare(vma, pmd, cp_flags) \ >- ({ \ >- long err = 0; \ >- if (unlikely(pgtable_populate_needed(vma, cp_flags))) { \ >- if (pte_alloc(vma->vm_mm, pmd)) \ >- err = -ENOMEM; \ >- } \ >- err; \ >- }) >- >-/* >- * This is the general pud/p4d/pgd version of change_pmd_prepare(). We need to >- * have separate change_pmd_prepare() because pte_alloc() returns 0 on success, >- * while {pmd|pud|p4d}_alloc() returns the valid pointer on success. >+ * Populate the lower-level page table if marker installation requires it. >+ * Return -ENOMEM on allocation failure. > */ > #define change_prepare(vma, high, low, addr, cp_flags) \ > ({ \ >@@ -517,30 +500,21 @@ static inline long change_pmd_range(struct mmu_gather *tlb, > again: > next = pmd_addr_end(addr, end); > >- ret = change_pmd_prepare(vma, pmd, cp_flags); >- if (ret) { >- pages = ret; >- break; >- } >- >- if (pmd_none(*pmd)) >+ if (unlikely(pgtable_populate_needed(vma, cp_flags))) { >+ if (pte_alloc(vma->vm_mm, pmd)) { >+ pages = -ENOMEM; >+ break; >+ } >+ } else if (pmd_none(*pmd)) { > goto next; >+ } Previously, reclaim could clear the PMD after allocation and we'd just skip the range without installing the markers ... Now it's change_pte_range() -> -EAGAIN -> goto again (retry the same range) And yeah, that should do the trick ;) > > _pmd = pmdp_get_lockless(pmd); > if (pmd_is_huge(_pmd)) { > if ((next - addr != HPAGE_PMD_SIZE) || > pgtable_split_needed(vma, cp_flags)) { > __split_huge_pmd(vma, pmd, addr, false); >- /* >- * For file-backed, the pmd could have been >- * cleared; make sure pmd populated if >- * necessary, then fall-through to pte level. >- */ >- ret = change_pmd_prepare(vma, pmd, cp_flags); >- if (ret) { >- pages = ret; >- break; >- } >+ goto again; And, after the split, we just go back to again -> allocate a PTE table if needed -> recheck the PMD. So, no need for that second change_pmd_prepare() ANYMORE :D LGTM! Reviewed-by: Lance Yang Cheers, Lance