Re: [PATCH 1/2] fs/proc/task_mmu: add guard region bit to pagemap

[Kalesh Singh]

On Fri, Feb 21, 2025 at 4:05 AM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> Currently there is no means by which users can determine whether a given
> page in memory is in fact a guard region, that is having had the
> MADV_GUARD_INSTALL madvise() flag applied to it.
>
> This is intentional, as to provide this information in VMA metadata would
> contradict the intent of the feature (providing a means to change fault
> behaviour at a page table level rather than a VMA level), and would require
> VMA metadata operations to scan page tables, which is unacceptable.
>
> In many cases, users have no need to reflect and determine what regions
> have been designated guard regions, as it is the user who has established
> them in the first place.
>
> But in some instances, such as monitoring software, or software that relies
> upon being able to ascertain the nature of mappings within a remote process
> for instance, it becomes useful to be able to determine which pages have
> the guard region marker applied.
>
> This patch makes use of an unused pagemap bit (58) to provide this
> information.
>
> This patch updates the documentation at the same time as making the change
> such that the implementation of the feature and the documentation of it are
> tied together.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/mm/pagemap.rst | 3 ++-
>  fs/proc/task_mmu.c                       | 6 +++++-
>  2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index caba0f52dd36..a297e824f990 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -21,7 +21,8 @@ There are four components to pagemap:
>      * Bit  56    page exclusively mapped (since 4.2)
>      * Bit  57    pte is uffd-wp write-protected (since 5.13) (see
>        Documentation/admin-guide/mm/userfaultfd.rst)
> -    * Bits 58-60 zero
> +    * Bit  58    pte is a guard region (since 6.15) (see madvise (2) man page)

Should this be 6.14 ?

Other than that: Reviewed-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>

Thanks,
Kalesh

> +    * Bits 59-60 zero
>      * Bit  61    page is file-page or shared-anon (since 3.5)
>      * Bit  62    page swapped
>      * Bit  63    page present
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index f02cd362309a..c17615e21a5d 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1632,6 +1632,7 @@ struct pagemapread {
>  #define PM_SOFT_DIRTY          BIT_ULL(55)
>  #define PM_MMAP_EXCLUSIVE      BIT_ULL(56)
>  #define PM_UFFD_WP             BIT_ULL(57)
> +#define PM_GUARD_REGION                BIT_ULL(58)
>  #define PM_FILE                        BIT_ULL(61)
>  #define PM_SWAP                        BIT_ULL(62)
>  #define PM_PRESENT             BIT_ULL(63)
> @@ -1732,6 +1733,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>                         page = pfn_swap_entry_to_page(entry);
>                 if (pte_marker_entry_uffd_wp(entry))
>                         flags |= PM_UFFD_WP;
> +               if (is_guard_swp_entry(entry))
> +                       flags |=  PM_GUARD_REGION;
>         }
>
>         if (page) {
> @@ -1931,7 +1934,8 @@ static const struct mm_walk_ops pagemap_ops = {
>   * Bit  55    pte is soft-dirty (see Documentation/admin-guide/mm/soft-dirty.rst)
>   * Bit  56    page exclusively mapped
>   * Bit  57    pte is uffd-wp write-protected
> - * Bits 58-60 zero
> + * Bit  58    pte is a guard region
> + * Bits 59-60 zero
>   * Bit  61    page is file-page or shared-anon
>   * Bit  62    page swapped
>   * Bit  63    page present
> --
> 2.48.1
>