On Wed, Oct 29, 2025 at 9:51 AM Lorenzo Stoakes
<[email protected]> 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).
nit: I know I suck at naming but I think VM_MAY_HAVE_GUARDS would
better represent the meaning.
>
> 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.
s/bee/been
but I'm not even sure the above paragraph has to be included in the
changelog. It's a technical detail IMHO.
>
> The MADV_GUARD_INSTALL madvise() operation now must take an mmap write
> lock (and also VMA write lock) whereas previously it did not, but this
> seems a reasonable overhead.
I guess this is because it is modifying vm_flags now?
>
> 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.
>
> For anonymous mappings this is inherent, however since commit f807123d578d
> ("mm: allow guard regions in file-backed and read-only mappings") which
> allowed file-backed guard regions, we have unfortunately had to enforce
> this behaviour by settings vma->anon_vma to force page table copying.
>
> The existence of this flag removes the need for this, so we simply update
> vma_needs_copy() to check for this flag instead.
>
> Signed-off-by: Lorenzo Stoakes <[email protected]>
Overall, makes sense to me and I think we could use it.
Reviewed-by: Suren Baghdasaryan <[email protected]>
It would be nice to have a way for userspace to reset this flag if it
confirms that the VMA does not really have any guards (using say
PAGEMAP_SCAN) but I think such an API can be abused.
> ---
> Documentation/filesystems/proc.rst | 1 +
> fs/proc/task_mmu.c | 1 +
> include/linux/mm.h | 1 +
> include/trace/events/mmflags.h | 1 +
> mm/madvise.c | 22 ++++++++++++++--------
> mm/memory.c | 4 ++++
> tools/testing/vma/vma_internal.h | 1 +
> 7 files changed, 23 insertions(+), 8 deletions(-)
>
> 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)
> == =======================================
>
> 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 fc35a0543f01..db16ed91c269 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1146,6 +1146,7 @@ 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",
> [ilog2(VM_LOCKED)] = "lo",
> [ilog2(VM_IO)] = "io",
> [ilog2(VM_SEQ_READ)] = "sr",
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index aada935c4950..f963afa1b9de 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -296,6 +296,7 @@ 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 0x00000800 /* The VMA maybe contains guard
> regions. */
> #define VM_UFFD_WP 0x00001000 /* wrprotect pages tracking */
>
> #define VM_LOCKED 0x00002000
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index aa441f593e9a..a6e5a44c9b42 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -213,6 +213,7 @@ IF_HAVE_PG_ARCH_3(arch_3)
> {VM_UFFD_MISSING, "uffd_missing" }, \
> IF_HAVE_UFFD_MINOR(VM_UFFD_MINOR, "uffd_minor" ) \
> {VM_PFNMAP, "pfnmap" }, \
> + {VM_MAYBE_GUARD, "maybe_guard" }, \
> {VM_UFFD_WP, "uffd_wp" }, \
> {VM_LOCKED, "locked" }, \
> {VM_IO, "io" }, \
> diff --git a/mm/madvise.c b/mm/madvise.c
> index fb1c86e630b6..216ae6ed344e 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1141,15 +1141,22 @@ static long madvise_guard_install(struct
> madvise_behavior *madv_behavior)
> return -EINVAL;
>
> /*
> - * If we install guard markers, then the range is no longer
> - * empty from a page table perspective and therefore it's
> - * appropriate to have an anon_vma.
> + * It would be confusing for anonymous mappings to have page table
> + * entries but no anon_vma established, so ensure that it is.
> + */
> + if (vma_is_anonymous(vma))
> + anon_vma_prepare(vma);
> +
> + /*
> + * Indicate that the VMA may contain guard regions, making it visible
> to
> + * the user that a VMA may contain these, narrowing down the range
> which
> + * must be scanned in order to detect them.
> *
> - * This ensures that on fork, we copy page tables correctly.
> + * This additionally causes page tables to be copied on fork
> regardless
> + * of whether the VMA is anonymous or not, correctly preserving the
> + * guard region page table entries.
> */
> - err = anon_vma_prepare(vma);
> - if (err)
> - return err;
> + vm_flags_set(vma, VM_MAYBE_GUARD);
>
> /*
> * Optimistically try to install the guard marker pages first. If any
> @@ -1709,7 +1716,6 @@ static enum madvise_lock_mode get_lock_mode(struct
> madvise_behavior *madv_behavi
> case MADV_POPULATE_READ:
> case MADV_POPULATE_WRITE:
> case MADV_COLLAPSE:
> - case MADV_GUARD_INSTALL:
> case MADV_GUARD_REMOVE:
> return MADVISE_MMAP_READ_LOCK;
> case MADV_DONTNEED:
> diff --git a/mm/memory.c b/mm/memory.c
> index 4c3a7e09a159..a2c79ee43d68 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1478,6 +1478,10 @@ vma_needs_copy(struct vm_area_struct *dst_vma, struct
> vm_area_struct *src_vma)
> if (src_vma->anon_vma)
> return true;
>
> + /* Guard regions have momdified page tables that require copying. */
> + if (src_vma->vm_flags & VM_MAYBE_GUARD)
> + return true;
> +
> /*
> * Don't copy ptes where a page fault will fill them correctly. Fork
> * becomes much lighter when there are big shared or private readonly
> diff --git a/tools/testing/vma/vma_internal.h
> b/tools/testing/vma/vma_internal.h
> index d873667704e8..e40c93edc5a7 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -56,6 +56,7 @@ extern unsigned long dac_mmap_min_addr;
> #define VM_MAYEXEC 0x00000040
> #define VM_GROWSDOWN 0x00000100
> #define VM_PFNMAP 0x00000400
> +#define VM_MAYBE_GUARD 0x00000800
> #define VM_LOCKED 0x00002000
> #define VM_IO 0x00004000
> #define VM_SEQ_READ 0x00008000 /* App will access data sequentially
> */
> --
> 2.51.0
>
