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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Qi Zheng , Hugh Dickins , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm/khugepaged: free unused page table when setting a huge PMD fails Message-ID: References: <20261105-khugepaged_set_pmd-v1-1-cb79ab117199@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261105-khugepaged_set_pmd-v1-1-cb79ab117199@kernel.org> X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2E19F10000E X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-Stat-Signature: zk1b3dtspdmiys85wwtu6h7q5mawf33b X-HE-Tag: 1793883731-910744 X-HE-Meta: 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 Vih2Z3Pn QoC9rDlIgbTv1AHpPXNu7rP7ftJRh0QyUAbNJR1DoTFULdjBV//uZyKtveaKVgZuJiDlvN74PsjR5CEAIp5FzpLISROkm47H/kQpD4fn/c15o9clABr4Y1iwxjROkNjfuTuaBoGO6T47/blSCwaHG5jB5E66ug4pzOEVVMJ+a9Z3qBgm9a+zuLo87yegwEl+YuQBlkINTTx+pLTjGw/De3QCBG+dS92xKhZZj4+/GzlpVJPn2hcCJp4DBh72F6RoKhBeuWEGjSi+prjWxPlsIMqPBSA== 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 01:16:05PM +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. > > Free vmf.prealloc_pte on failure, as do_fault() already does. Successful > PMD installation consumes the table and clears the pointer. > > Having to do this cleanup outside of do_set_pmd() is rather nasty, but > other callers depend on it already having been allocated. This needs a > cleanup at some point. I hate this being outside of do_set_pmd(). What about making do_set_pmd() freeing ->prealloc_pte if it was allocated inside? Nothing changes for caller. bool allocated = false; ... if (arch_needs_pgtable_deposit() && !vmf->prealloc_pte) { vmf->prealloc_pte = pte_alloc_one(vma->vm_mm); if (!vmf->prealloc_pte) return VM_FAULT_OOM; allocated = true; } ... out: spin_unlock(vmf->ptl); if (ret && allocated) { pte_free(vma->vm_mm, vmf->prealloc_pte); vmf->prealloc_pte = NULL; } return ret; -- Kiryl Shutsemau / Kirill A. Shutemov