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Message-ID: <1c4b521a-f84c-4a3a-99f7-ecb9ace3baf1@oracle.com>
Date: Mon, 17 Nov 2025 17:05:26 -0800
From: jane.chu@...cle.com
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
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>,
Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
Lance Yang <lance.yang@...ux.dev>, 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: Re: [PATCH v3 1/8] mm: introduce VM_MAYBE_GUARD and make visible in
/proc/$pid/smaps
On 11/7/2025 8:11 AM, Lorenzo Stoakes wrote:
> Currently, if a user needs to determine if guard regions are present in a
> range, they have to scan all VMAs (or have knowledge of which ones might
> have guard regions).
>
> Since commit 8e2f2aeb8b48 ("fs/proc/task_mmu: add guard region bit to
> pagemap") and the related commit a516403787e0 ("fs/proc: extend the
> PAGEMAP_SCAN ioctl to report guard regions"), users can use either
> /proc/$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.
>
> Reviewed-by: Pedro Falcato <pfalcato@...e.de>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
> Documentation/filesystems/proc.rst | 5 +++--
> 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 | 1 +
> 6 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 0b86a8022fa1..8256e857e2d7 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -553,7 +553,7 @@ otherwise.
> kernel flags associated with the particular virtual memory area in two letter
> encoded manner. The codes are the following:
>
> - == =======================================
> + == =============================================================
> rd readable
> wr writeable
> ex executable
> @@ -591,7 +591,8 @@ 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
> be present in all further kernel releases. Things get changed, the flags may
> 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. */
Nit: s/momdified/modified.
> + 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..46acb4df45de 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -56,6 +56,7 @@ extern unsigned long dac_mmap_min_addr;
> #define VM_MAYEXEC 0x00000040
> #define VM_GROWSDOWN 0x00000100
> #define VM_PFNMAP 0x00000400
> +#define VM_MAYBE_GUARD 0x00000800
> #define VM_LOCKED 0x00002000
> #define VM_IO 0x00004000
> #define VM_SEQ_READ 0x00008000 /* App will access data sequentially */
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