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Thu, 12 Nov 2026 14:25:30 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id w9GzDFnM9WoAPwAAD6G6ig:T2 (envelope-from ); Thu, 12 Nov 2026 14:25:30 +0000 From: =?UTF-8?q?Carlos=20L=C3=B3pez?= To: kvm@vger.kernel.org, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, linux-riscv@lists.infradead.org, seanjc@google.com, pbonzini@redhat.com, akpm@linux-foundation.org, david@kernel.org, peterx@redhat.com, ljs@kernel.org, imbrenda@linux.ibm.com, borntraeger@linux.ibm.com, frankja@linux.ibm.com, =?UTF-8?q?Carlos=20L=C3=B3pez?= , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Peter Feiner Subject: [PATCH v3 1/9] mm: fix eager write-bit restore for write-notified shared mappings Date: Thu, 12 Nov 2026 15:20:30 +0100 Message-ID: <20261112142039.907090-3-clopez@suse.de> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20261112142039.907090-2-clopez@suse.de> References: <20261112142039.907090-2-clopez@suse.de> 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=UTF-8 Content-Transfer-Encoding: 8bit X-Spamd-Result: default: False [-12.30 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; MID_CONTAINS_FROM(1.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[20]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email,imap1.dmz-prg2.suse.org:helo] X-Spam-Score: -12.30 X-Spam-Level: X-Spam-Flag: NO change_protection() applies vma->vm_page_prot, which lacks the write bit for CoW and write-notified mappings. Callers set MM_CP_TRY_CHANGE_WRITABLE when vma_wants_manual_pte_write_upgrade() returns true, so that entries that can be written without a fault, such as dirty PTEs in a shared mapping, are made writable right away. For shared mappings, that function passes vma->vm_page_prot to vma_wants_writenotify(). Once writenotify is enabled, this protection lacks the write bit, while the vma_pgprot_modify() result it is compared against adds it back. The two never match, so vma_wants_writenotify() returns false before checking for soft-dirty, uffd-wp and writeback. As a result, write-notified shmem mappings never get MM_CP_TRY_CHANGE_WRITABLE. For example, commit 931298e103c2 ("mm/userfaultfd: rely on vma->vm_page_prot in uffd_wp_range()") made uffd-wp unprotect set the flag so that dirty shmem PTEs become writable again, but this never takes effect. Unprotecting N dirty shmem pages still leads to N faults that only set the write bit. This dates back to the commit in the Fixes tag, which moved the equivalent check in mprotect_fixup() before vma_set_page_prot(). Fix this by passing the protection recomputed from the VMA flags, as vma_set_page_prot() does. Note: this only affects performance, as the next write fault restores the write bit anyway. Fixes: 64e455079e1b ("mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared") Signed-off-by: Carlos López --- mm/vma.h | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-) diff --git a/mm/vma.h b/mm/vma.h index 3fc3b47a954b..e83700e248e4 100644 --- a/mm/vma.h +++ b/mm/vma.h @@ -589,6 +589,14 @@ int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma, unsigned long addr, int new_below); #endif +#ifdef CONFIG_MMU +static inline pgprot_t vma_pgprot_modify(pgprot_t oldprot, vma_flags_t vma_flags) +{ + const pgprot_t prot = vma_flags_to_page_prot(vma_flags); + + return pgprot_modify(oldprot, prot); +} + static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma) { /* @@ -596,19 +604,16 @@ static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma * if we can't do that automatically for all PTEs in a mapping. For * private mappings, that's always the case when we have write * permissions as we properly have to handle COW. + * + * vma->vm_page_prot lacks the write bit if writenotify is already + * enabled, so pass it with the bits restored, as vma_set_page_prot() + * does. */ if (vma->vm_flags & VM_SHARED) - return vma_wants_writenotify(vma, vma->vm_page_prot); + return vma_wants_writenotify(vma, + vma_pgprot_modify(vma->vm_page_prot, vma->flags)); return !!(vma->vm_flags & VM_WRITE); } - -#ifdef CONFIG_MMU -static inline pgprot_t vma_pgprot_modify(pgprot_t oldprot, vma_flags_t vma_flags) -{ - const pgprot_t prot = vma_flags_to_page_prot(vma_flags); - - return pgprot_modify(oldprot, prot); -} #endif static inline struct vm_area_struct *vma_prev_limit(struct vma_iterator *vmi, -- 2.51.0