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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: X-Stat-Signature: kp15t1bk8coqewpk6cpuagqozdbhyaeh X-Rspam-User: 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-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 8E1DA4000B X-HE-Tag: 1793883860-770902 X-HE-Meta: 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 qcF9ljyE tb5wCY1xcWfYb68txwbvV4M0pFAUTPOqGqrcDPdKTYcvSCqpmnVeMmYAL7bwm8ZAgLSF6PKwgsZ7qzVf7IEGgqBo3wJsFE9I4AaUzBllXK+PBzyemfguRodOWtcO/r7JPLXle8UkJkuV3AJCzaR6K9D928AcbsDBeNBnb7ztN2WgptVT72x+gLfSoIcdd62/ynoWQ9vEDgvS1XqPF9UUcmJa+McXuEc791UMfCtK3Y/2ba1GHgHMPzjeFyl4dOLDLBMw2UrFh3WqN7UrCvxQL37gn0A== 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 01:02:08PM +0000, Kiryl Shutsemau wrote: > 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. Or if we want to keep in set_huge_pmd() then set_huge_pmd() has to allocate it too. At least, it makes some sense. -- Kiryl Shutsemau / Kirill A. Shutemov