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Message-ID: <fe38b1a43364f72d1ce7a6217e53a33c9c0bb0c5.1762422915.git.lorenzo.stoakes@oracle.com>
Date: Thu, 6 Nov 2025 10:46:12 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Jonathan Corbet <corbet@....net>, David Hildenbrand <david@...hat.com>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
Steven Rostedt <rostedt@...dmis.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-doc@...r.kernel.org, linux-mm@...ck.org,
linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
Andrei Vagin <avagin@...il.com>
Subject: [PATCH v2 1/5] mm: introduce VM_MAYBE_GUARD and make visible in /proc/$pid/smaps
Currently, if a user needs to determine if guard regions are present in a
range, they have to scan all VMAs (or have knowledge of which ones might
have guard regions).
Since commit 8e2f2aeb8b48 ("fs/proc/task_mmu: add guard region bit to
pagemap") and the related commit a516403787e0 ("fs/proc: extend the
PAGEMAP_SCAN ioctl to report guard regions"), users can use either
/proc/$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@...cle.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)
== =======================================
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",
[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 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. */
#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/memory.c b/mm/memory.c
index 046579a6ec2f..334732ab6733 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1480,6 +1480,10 @@ 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 c68d382dac81..ddf58a5e1add 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -46,6 +46,8 @@ extern unsigned long dac_mmap_min_addr;
#define MMF_HAS_MDWE 28
+#define VM_MAYBE_GUARD_BIT 11
+
#define VM_NONE 0x00000000
#define VM_READ 0x00000001
#define VM_WRITE 0x00000002
@@ -56,6 +58,7 @@ extern unsigned long dac_mmap_min_addr;
#define VM_MAYEXEC 0x00000040
#define VM_GROWSDOWN 0x00000100
#define VM_PFNMAP 0x00000400
+#define VM_MAYBE_GUARD BIT(VM_MAYBE_GUARD_BIT) /* The VMA maybe contains guard regions. */
#define VM_LOCKED 0x00002000
#define VM_IO 0x00004000
#define VM_SEQ_READ 0x00008000 /* App will access data sequentially */
--
2.51.0
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