On Fri, Feb 21, 2025 at 4:05 AM Lorenzo Stoakes lorenzo.stoakes@oracle.com 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@oracle.com
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@google.com
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