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 cmarzY5+ 36ZiCyk86uq6hRYnaGtQipG9ifwEVH6fv/wi4JV9eJ0kcnR/+qspav8bsBmm91Tpcj2xUjyz1oCqWMaGktjpGShNsYJEWQk6CHsM3XnSGhXw5Vj0tkD16sQ1Wx+JX8DXJIO0f X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Nov 06, 2025 at 12:12:10PM +0100, Vlastimil Babka wrote: > On 11/6/25 11:46, Lorenzo Stoakes wrote: > > Currently, if a user needs to determine if guard regions are present in a > > range, they have to scan all VMAs (or have knowledge of which ones might > > have guard regions). > > > > Since commit 8e2f2aeb8b48 ("fs/proc/task_mmu: add guard region bit to > > pagemap") and the related commit a516403787e0 ("fs/proc: extend the > > PAGEMAP_SCAN ioctl to report guard regions"), users can use either > > /proc/$pid/pagemap or the PAGEMAP_SCAN functionality to perform this > > operation at a virtual address level. > > > > This is not ideal, and it gives no visibility at a /proc/$pid/smaps level > > that guard regions exist in ranges. > > > > This patch remedies the situation by establishing a new VMA flag, > > VM_MAYBE_GUARD, to indicate that a VMA may contain guard regions (it is > > uncertain because we cannot reasonably determine whether a > > MADV_GUARD_REMOVE call has removed all of the guard regions in a VMA, and > > additionally VMAs may change across merge/split). > > > > We utilise 0x800 for this flag which makes it available to 32-bit > > architectures also, a flag that was previously used by VM_DENYWRITE, which > > was removed in commit 8d0920bde5eb ("mm: remove VM_DENYWRITE") and hasn't > > bee reused yet. > > > > We also update the smaps logic and documentation to identify these VMAs. > > > > Another major use of this functionality is that we can use it to identify > > that we ought to copy page tables on fork. > > > > We do not actually implement usage of this flag in mm/madvise.c yet as we > > need to allow some VMA flags to be applied atomically under mmap/VMA read > > lock in order to avoid the need to acquire a write lock for this purpose. > > > > Signed-off-by: Lorenzo Stoakes > > Reviewed-by: Vlastimil Babka > Thanks