The patch titled
Subject: fs/proc/task_mmu: add guard region bit to pagemap
has been added to the -mm mm-unstable branch. Its filename is
fs-proc-task_mmu-add-guard-region-bit-to-pagemap.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fs-proc-task_mmu-add-guard-region-bit-to-pagemap.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Subject: fs/proc/task_mmu: add guard region bit to pagemap
Date: Fri, 21 Feb 2025 12:05:22 +0000
Patch series "fs/proc/task_mmu: add guard region bit to pagemap".
Currently there is no means of determining whether a given page in a
mapping range is designated a guard region (as installed via madvise()
using the MADV_GUARD_INSTALL flag).
This is generally not an issue, but in some instances users may wish to
determine whether this is the case.
This series adds this ability via /proc/$pid/pagemap, updates the
documentation and adds a self test to assert that this functions
correctly.
This patch (of 2):
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.
Link: https://lkml.kernel.org/r/cover.1740139449.git.lorenzo.stoakes@xxxxxxxxxx
Link: https://lkml.kernel.org/r/521d99c08b975fb06a1e7201e971cc24d68196d1.1740139449.git.lorenzo.stoakes@xxxxxxxxxx
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
Cc: Liam Howlett <liam.howlett@xxxxxxxxxx>
Cc: Matthew Wilcow (Oracle) <willy@xxxxxxxxxxxxx>
Cc: "Paul E . McKenney" <paulmck@xxxxxxxxxx>
Cc: Shuah Khan (Samsung OSG) <shuah@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/mm/pagemap.rst | 3 ++-
fs/proc/task_mmu.c | 6 +++++-
2 files changed, 7 insertions(+), 2 deletions(-)
--- a/Documentation/admin-guide/mm/pagemap.rst~fs-proc-task_mmu-add-guard-region-bit-to-pagemap
+++ a/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)
+ * Bits 59-60 zero
* Bit 61 page is file-page or shared-anon (since 3.5)
* Bit 62 page swapped
* Bit 63 page present
--- a/fs/proc/task_mmu.c~fs-proc-task_mmu-add-guard-region-bit-to-pagemap
+++ a/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_en
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_
* 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
_
Patches currently in -mm which might be from lorenzo.stoakes@xxxxxxxxxx are
mm-abort-vma_modify-on-merge-out-of-memory-failure.patch
mm-simplify-vma-merge-structure-and-expand-comments.patch
mm-further-refactor-commit_merge.patch
mm-eliminate-adj_start-parameter-from-commit_merge.patch
mm-make-vmg-target-consistent-and-further-simplify-commit_merge.patch
mm-completely-abstract-unnecessary-adj_start-calculation.patch
mm-madvise-split-out-mmap-locking-operations-for-madvise-fix.patch
mm-use-read-write_once-for-vma-vm_flags-on-migrate-mprotect.patch
mm-refactor-rmap_walk_file-to-separate-out-traversal-logic.patch
mm-provide-mapping_wrprotect_range-function.patch
fb_defio-do-not-use-deprecated-page-mapping-index-fields.patch
fb_defio-do-not-use-deprecated-page-mapping-index-fields-fix.patch
mm-allow-guard-regions-in-file-backed-and-read-only-mappings.patch
selftests-mm-rename-guard-pages-to-guard-regions.patch
tools-selftests-expand-all-guard-region-tests-to-file-backed.patch
tools-selftests-add-file-shmem-backed-mapping-guard-region-tests.patch
fs-proc-task_mmu-add-guard-region-bit-to-pagemap.patch
tools-selftests-add-guard-region-test-for-proc-pid-pagemap.patch
tools-selftests-add-guard-region-test-for-proc-pid-pagemap-fix.patch
