On Thu, Nov 06, 2025 at 10:46:12AM +0000, 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 lorenzo.stoakes@oracle.com
Documentation/filesystems/proc.rst | 1 + fs/proc/task_mmu.c | 1 + include/linux/mm.h | 3 +++ include/trace/events/mmflags.h | 1 + mm/memory.c | 4 ++++ tools/testing/vma/vma_internal.h | 3 +++ 6 files changed, 13 insertions(+)
diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst index 0b86a8022fa1..b8a423ca590a 100644 --- a/Documentation/filesystems/proc.rst +++ b/Documentation/filesystems/proc.rst @@ -591,6 +591,7 @@ encoded manner. The codes are the following: sl sealed lf lock on fault pages dp always lazily freeable mapping
- gu maybe contains guard regions (if not set, definitely doesn't) == =======================================
The nittiest of nits: =============================================================
Note that there is no guarantee that every flag and associated mnemonic will diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 8a9894aefbca..a420dcf9ffbb 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1147,6 +1147,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma) [ilog2(VM_MAYSHARE)] = "ms", [ilog2(VM_GROWSDOWN)] = "gd", [ilog2(VM_PFNMAP)] = "pf",
[ilog2(VM_MAYBE_GUARD)] = "gu",diff --git a/include/linux/mm.h b/include/linux/mm.h index 6e5ca5287e21..2a5516bff75a 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -271,6 +271,8 @@ extern struct rw_semaphore nommu_region_sem; extern unsigned int kobjsize(const void *objp); #endif +#define VM_MAYBE_GUARD_BIT 11
/*
- vm_flags in vm_area_struct, see mm_types.h.
- When changing, update also include/trace/events/mmflags.h
@@ -296,6 +298,7 @@ extern unsigned int kobjsize(const void *objp); #define VM_UFFD_MISSING 0 #endif /* CONFIG_MMU */ #define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */ +#define VM_MAYBE_GUARD BIT(VM_MAYBE_GUARD_BIT) /* The VMA maybe contains guard regions. */
Don't we also need an adjustment on the rust side for this BIT()? Like we for f04aad36a07c ("mm/ksm: fix flag-dropping behavior in ksm_madvise").
In any case: Reviewed-by: Pedro Falcato pfalcato@suse.de