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Sun, 08 Nov 2026 09:15:04 +0000 X-Mizu-Trace-ID: 4eda76d6e28ebe59 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org Cc: akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, kas@kernel.org, qi.zheng@linux.dev, hughd@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2 1/2] mm: free unused page table allocated by do_set_pmd() Date: Sun, 8 Nov 2026 17:14:55 +0800 Message-ID: <20261108091455.88759-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261106-khugepaged_set_pmd-v2-1-d2457b5e377a@kernel.org> References: <20261106-khugepaged_set_pmd-v2-1-d2457b5e377a@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 0F40A1C0004 X-Rspamd-Server: rspam12 X-Stat-Signature: yzjkogg185q7dfgsd5pcheaipjpw7roz X-Rspam-User: 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: 1794129305-212300 X-HE-Meta: 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 X+nBIdwQ HxH6X4kM07jlVngzoSGFwNshnxDp7cG7/hN2cr+W+W5TLH4gb11oL8D1nivotUtOgink3NgmYGAFn2rwya7PNPUF+K8Awv7pZzCeAy7j/dZYS0hkvq9lwuWXVff5cArmN9XNaSbH+nqeQtsZCvSs3GzZ3tOVNQt0WMMGXLBqUM4pk5TOygT7LJ1gnPglR0RdIe0H4+hHZOiMxJlxgt1NTOLVglq92MYF6OIYUS159Oa0/2sY4SWRInehxtCFBM2ir0rljcypyMBazYuHZv/PlCAy6JPCwRzrs7cf5INS7anCm+rM6N54pxHcnZfeQTj0w6h62rZ9Y001646HErdXVoWUlliv/w/9DJHjDnKpKLyzbTlnqj6fdokO0Dzxoql9bMkobpY/isiXERY2Ge0bBAX1vLxvsYP/Spram6IX8ZRkvcdeRGqu9/QX6cLxdHlgNE86Eldm/Kz+kpYiDh7U3RT2U2gk34n5VJmuu 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 Fri, Nov 06, 2026 at 01:36:12PM +0100, David Hildenbrand (Arm) wrote: >do_set_pmd() can allocate vmf->prealloc_pte on architectures that need a >deposited page table, such as PowerPC with the hash MMU. If the PMD is >no longer empty when it takes the PMD lock, it returns VM_FAULT_FALLBACK >without consuming that table. > >Since collapse_pte_mapped_thp() uses mmap_read_lock(), a concurrent >userfaultfd operation can install a PTE table before taking the folio >lock. If that races with set_huge_pmd(), the unused preallocated table >is lost when its local vm_fault goes out of scope. > >Track whether do_set_pmd() allocated the table and free it after dropping >the PMD lock if it was not consumed. Preserve any preallocated table >supplied by the caller for PTE fallback. Successful PMD installation >consumes the table and clears the pointer. > >Returning without a preallocated table matches the existing behavior on >architectures that do not need a deposited table. All callers >already handle that case. > >Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()") >Cc: stable@vger.kernel.org >Reviewed-by: Kiryl Shutsemau (Meta) >Signed-off-by: David Hildenbrand (Arm) >--- > mm/memory.c | 6 ++++++ > 1 file changed, 6 insertions(+) > >diff --git a/mm/memory.c b/mm/memory.c >index becf53a2a0944..421064aef3cf0 100644 >--- a/mm/memory.c >+++ b/mm/memory.c >@@ -5687,6 +5687,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa > struct vm_area_struct *vma = vmf->vma; > bool write = vmf->flags & FAULT_FLAG_WRITE; > unsigned long haddr = vmf->address & HPAGE_PMD_MASK; >+ bool pgtable_allocated = false; > pmd_t entry; > vm_fault_t ret = VM_FAULT_FALLBACK; > >@@ -5725,6 +5726,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa > vmf->prealloc_pte = pte_alloc_one(vma->vm_mm); > if (!vmf->prealloc_pte) > return VM_FAULT_OOM; >+ pgtable_allocated = true; > } > > vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd); >@@ -5755,6 +5757,10 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa > count_vm_event(THP_FILE_MAPPED); > out: > spin_unlock(vmf->ptl); >+ if (pgtable_allocated && vmf->prealloc_pte) { Took me a sec ... to see why we can't just check vmf->prealloc_pte here ... Emm ... as it turns out, the caller might have supplied it for fallback :D LGTM1 Reviewed-by: Lance Yang >+ pte_free(vma->vm_mm, vmf->prealloc_pte); >+ vmf->prealloc_pte = NULL; >+ } > return ret; > } > #else > >-- >2.43.0 > >