Message-ID: <cover.1761756437.git.lorenzo.stoakes@oracle.com>
Date: Wed, 29 Oct 2025 16:50:30 +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 0/3] introduce VM_MAYBE_GUARD and make it sticky

Currently, guard regions are not visible to users except through
/proc/$pid/pagemap, with no explicit visibility at the VMA level.

This makes the feature less useful, as it isn't entirely apparent which
VMAs may have these entries present, especially when performing actions
which walk through memory regions such as those performed by CRIU.

This series addresses this issue by introducing the VM_MAYBE_GUARD flag
which fulfils this role, updating the smaps logic to display an entry for
these.

The semantics of this flag are that a guard region MAY be present if set
(we cannot be sure, as we can't efficiently track whether an
MADV_GUARD_REMOVE finally removes all the guard regions in a VMA) - but if
not set the VMA definitely does NOT have any guard regions present.

It's problematic to establish this flag without further action, because
that means that VMAs with guard regions in them become non-mergeable with
adjacent VMAs for no especially good reason.

To work around this, this series also introduces the concept of 'sticky'
VMA flags - that is flags which:

a. if set in one VMA and not in another still permit those VMAs to be
   merged (if otherwise compatible).

b. When they are merged, the resultant VMA must have the flag set.

The VMA logic is updated to propagate these flags correctly.

Additionally, VM_MAYBE_GUARD being an explicit VMA flag allows us to solve
an issue with file-backed guard regions - previously these established an
anon_vma object for file-backed mappings solely to have vma_needs_copy()
correctly propagate guard region mappings to child processes.

We introduce a new flag alias VM_COPY_ON_FORK (which currently only
specifies VM_MAYBE_GUARD) and update vma_needs_copy() to check explicitly
for this flag and to copy page tables if it is present, which resolves this
issue.

Finally we introduce extensive VMA userland tests to assert that the sticky
VMA logic behaves correctly as well as guard region self tests to assert
that smaps visibility is correctly implemented.

Lorenzo Stoakes (3):
  mm: introduce VM_MAYBE_GUARD and make visible for guard regions
  mm: implement sticky, copy on fork VMA flags
  selftests/mm/guard-regions: add smaps visibility test

 Documentation/filesystems/proc.rst         |   1 +
 fs/proc/task_mmu.c                         |   1 +
 include/linux/mm.h                         |  33 ++++++
 include/trace/events/mmflags.h             |   1 +
 mm/madvise.c                               |  22 ++--
 mm/memory.c                                |   3 +
 mm/vma.c                                   |  22 ++--
 tools/testing/selftests/mm/guard-regions.c | 120 +++++++++++++++++++++
 tools/testing/selftests/mm/vm_util.c       |   5 +
 tools/testing/selftests/mm/vm_util.h       |   1 +
 tools/testing/vma/vma.c                    |  89 +++++++++++++--
 tools/testing/vma/vma_internal.h           |  33 ++++++
 12 files changed, 303 insertions(+), 28 deletions(-)

--
2.51.0

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