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Fri, 1 May 2026 06:41:35 -0400 (EDT) Date: Fri, 1 May 2026 11:41:30 +0100 From: Kiryl Shutsemau To: akpm@linux-foundation.org, rppt@kernel.org, peterx@redhat.com, david@kernel.org Cc: ljs@kernel.org, surenb@google.com, vbabka@kernel.org, Liam.Howlett@oracle.com, ziy@nvidia.com, corbet@lwn.net, skhan@linuxfoundation.org, seanjc@google.com, pbonzini@redhat.com, jthoughton@google.com, aarcange@redhat.com, sj@kernel.org, usama.arif@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, kvm@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH 10/14] mm/pagemap: add PAGE_IS_ACCESSED for RWP tracking Message-ID: References: <20260427114607.4068647-1-kas@kernel.org> <20260427114607.4068647-11-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260427114607.4068647-11-kas@kernel.org> sashiko.dev -- https://sashiko.dev/#/patchset/20260427114607.4068647-1-kas@kernel.org -- wrote: > commit 6d6b8f986b2959f8ae0932deccc5afa4ec485813 > Author: Kiryl Shutsemau (Meta) > Subject: mm/pagemap: add PAGE_IS_ACCESSED for RWP tracking [ ... ] > Does this new PAGE_IS_ACCESSED tracking correctly support the atomic > read-and-reset workflow (PM_SCAN_WP_MATCHING) for RWP VMAs? [ ... ] > Since WP and RWP are mutually exclusive per VMA, userfaultfd_wp_async() and > userfaultfd_wp_use_markers() will return false for RWP VMAs, causing > pagemap_scan_test_walk() to return 1 and skip the VMA. [ ... ] > Because these loops hardcode a check against PAGE_IS_WRITTEN, and RWP VMAs > now populate PAGE_IS_ACCESSED instead, this check will always evaluate to > true for RWP. As a result, it looks like make_uffd_wp_pte() and > make_uffd_wp_pmd() will be silently skipped, preventing the atomic > reset of the tracking bits. > Would it make sense to update both the wp_allowed check and the hardcoded > PAGE_IS_WRITTEN checks to accommodate PAGE_IS_ACCESSED for RWP VMAs? PM_SCAN_WP_MATCHING is the atomic read-and-reset flavour and is implemented for the WP marker only. The silent-skip on PM_SCAN_WP_MATCHING + RWP VMA is not intended Will fold this fixup if (userfaultfd_rwp(vma) && (p->arg.flags & PM_SCAN_WP_MATCHING)) return -EINVAL; We can add similar operation for RWP later if there's a use-case. -- Kiryl Shutsemau / Kirill A. Shutemov