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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-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261020110759.960089-1-shawnq1@163.com> 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