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Wed, 21 Oct 2026 05:11:23 -0400 (EDT) Date: Wed, 21 Oct 2026 10:11:23 +0100 From: Kiryl Shutsemau To: shaos Cc: akpm@linux-foundation.org, Mike Rapoport , David Hildenbrand , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [BUG] mm: userfaultfd RWP drops a populated shmem PMD Message-ID: References: <20261020110759.960089-1-shawnq1@163.com> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261020110759.960089-1-shawnq1@163.com> X-Rspamd-Queue-Id: 48E68C0005 X-Rspam-User: X-Stat-Signature: ym95embxtca6e1txaadubijb9yc7pduh X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,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: rspam03 X-HE-Tag: 1792573886-897905 X-HE-Meta: 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 Ip3Udsa9 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 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 Tue, Oct 20, 2026 at 11:07:59AM +0000, shaos wrote: > Hello, > > UFFDIO_RWPROTECT_MODE_RWP can return success for a present shmem THP > mapping while dropping the PMD mapping instead of leaving the > populated page resident and write-protected. The next read refaults > the page and completes without a UFFD_PAGEFAULT_FLAG_RWP event. RWP only traps accesses to present PTEs. Arming a PMD-mapped shmem THP splits the PMD, and splitting a file PMD unmaps it, so nothing is left to protect. Populating the page table would not help: the PTEs are empty and there is no RWP marker to put in them. I think the right way to deal with it is for userspace to register the range with UFFDIO_REGISTER_MODE_MINOR as well, and count the MINOR fault that follows as an access. -- Kiryl Shutsemau / Kirill A. Shutemov