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AJvYcCVMDxGap61WquuCdSTcqixFURmm0P5OYQIUZEzQhq+rgMb7C61cJFNjaIoY6WDzROdV365afiUfEA==@kvack.org X-Gm-Message-State: AOJu0YxFsjk02xgC3Wta1NM/Vd9dMjO7Yw54/W+DMgOwRtaaE69yBRzR M5RVYQ1Czt4xj1tI02dTTkmwwDMeQxNP+qvFhD1fk01z9DErTWJVCLBM3OnqGKCqCith3eVBoZo 8vRMu9oNoc6O/P/iGbbv508PrCCnYHRKq7/F9Ghzv X-Gm-Gg: ASbGncuSejuxhxbeGE2GRCrBrq9P5yZzscoiZLbhZIy1WimNcuc4As/8o0CV3eTcMnc uPcHX0gzieL+TEC0HvpZIMK73jdBg8+2URjhTWwdSjcQk8wgOez/C6ditgU9+iB5kyxIQN7DjWf tJLhqaImL5AtjdbJrSPPJjH0nPE5nAOVp53OSVKCgGLOMwKdfNcb5cmqdtkF4MDNz/ZeVc2nwQT v95+3pVFEh+R/Cxqfs/IYQSHdjAHuewL3B6YGLPeHbfua3GKfnyUY0beOg= X-Google-Smtp-Source: AGHT+IEXE/t7QfqvUm8ZaTuQf2esMJpzDPElyDqBGV0RGdjV360thSADyS4N9hyRQjOYV3uhrbmzYlXY5QHiRxHN85o= X-Received: by 2002:ac8:59cd:0:b0:4e8:85ac:f7a7 with SMTP id d75a77b69052e-4ed23bf1436mr1717491cf.9.1761786365282; Wed, 29 Oct 2025 18:06:05 -0700 (PDT) MIME-Version: 1.0 References: <7de40603015dee82970f5d37332a6d5af7532063.1761756437.git.lorenzo.stoakes@oracle.com> In-Reply-To: <7de40603015dee82970f5d37332a6d5af7532063.1761756437.git.lorenzo.stoakes@oracle.com> From: Suren Baghdasaryan Date: Wed, 29 Oct 2025 18:05:54 -0700 X-Gm-Features: AWmQ_blocebJr4y0y6DBwQJ-I85ZwTlE3oPMzhX8CLpjz_qVxCW5kze5g7B-HN8 Message-ID: Subject: Re: [PATCH 1/3] mm: introduce VM_MAYBE_GUARD and make visible for guard regions To: Lorenzo Stoakes Cc: Andrew Morton , Jonathan Corbet , David Hildenbrand , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Michal Hocko , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jann Horn , Pedro Falcato , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Andrei Vagin Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam01 X-Stat-Signature: 9qy6bx3uybjza54fcwaygdts3zy6jsb8 X-Rspam-User: X-Rspamd-Queue-Id: BE2D8C0009 X-HE-Tag: 1761786366-591663 X-HE-Meta: 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 440uaOhb D7tikldHSmLnkSOyIIch+bILx8YH1H/1mcvTEp0yrCiDqIPyPRqea87Fqv8hVCxK+NDd1ACohGaHW+jcTNu8UJKYMyBVAJ4vAvaGwghlLVCLYKGnletEA9QNA2LQ03tEVx1GQGECu7oSeF4mcpnoYC27Kv49/eYSrKDo7xTczY8Sg+3EJ+RjNGGuXpwzzo5QnlGaiwdqVFmtlqyNqy/V0iSkdWF6BGcuG9rZaFq7JCJfZIIyW7WIdjyCaVA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Oct 29, 2025 at 9:51=E2=80=AFAM 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). nit: I know I suck at naming but I think VM_MAY_HAVE_GUARDS would better represent the meaning. > > We utilise 0x800 for this flag which makes it available to 32-bit > architectures also, a flag that was previously used by VM_DENYWRITE, whic= h > was removed in commit 8d0920bde5eb ("mm: remove VM_DENYWRITE") and hasn't > bee reused yet. s/bee/been but I'm not even sure the above paragraph has to be included in the changelog. It's a technical detail IMHO. > > The MADV_GUARD_INSTALL madvise() operation now must take an mmap write > lock (and also VMA write lock) whereas previously it did not, but this > seems a reasonable overhead. I guess this is because it is modifying vm_flags now? > > 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. > > For anonymous mappings this is inherent, however since commit f807123d578= d > ("mm: allow guard regions in file-backed and read-only mappings") which > allowed file-backed guard regions, we have unfortunately had to enforce > this behaviour by settings vma->anon_vma to force page table copying. > > The existence of this flag removes the need for this, so we simply update > vma_needs_copy() to check for this flag instead. > > Signed-off-by: Lorenzo Stoakes Overall, makes sense to me and I think we could use it. Reviewed-by: Suren Baghdasaryan It would be nice to have a way for userspace to reset this flag if it confirms that the VMA does not really have any guards (using say PAGEMAP_SCAN) but I think such an API can be abused. > --- > Documentation/filesystems/proc.rst | 1 + > fs/proc/task_mmu.c | 1 + > include/linux/mm.h | 1 + > include/trace/events/mmflags.h | 1 + > mm/madvise.c | 22 ++++++++++++++-------- > mm/memory.c | 4 ++++ > tools/testing/vma/vma_internal.h | 1 + > 7 files changed, 23 insertions(+), 8 deletions(-) > > diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesyste= ms/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) > =3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > > 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 fc35a0543f01..db16ed91c269 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1146,6 +1146,7 @@ static void show_smap_vma_flags(struct seq_file *m,= struct vm_area_struct *vma) > [ilog2(VM_MAYSHARE)] =3D "ms", > [ilog2(VM_GROWSDOWN)] =3D "gd", > [ilog2(VM_PFNMAP)] =3D "pf", > + [ilog2(VM_MAYBE_GUARD)] =3D "gu", > [ilog2(VM_LOCKED)] =3D "lo", > [ilog2(VM_IO)] =3D "io", > [ilog2(VM_SEQ_READ)] =3D "sr", > diff --git a/include/linux/mm.h b/include/linux/mm.h > index aada935c4950..f963afa1b9de 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -296,6 +296,7 @@ extern unsigned int kobjsize(const void *objp); > #define VM_UFFD_MISSING 0 > #endif /* CONFIG_MMU */ > #define VM_PFNMAP 0x00000400 /* Page-ranges managed without "s= truct page", just pure PFN */ > +#define VM_MAYBE_GUARD 0x00000800 /* The VMA maybe contains guard r= egions. */ > #define VM_UFFD_WP 0x00001000 /* wrprotect pages tracking */ > > #define VM_LOCKED 0x00002000 > diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflag= s.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/madvise.c b/mm/madvise.c > index fb1c86e630b6..216ae6ed344e 100644 > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -1141,15 +1141,22 @@ static long madvise_guard_install(struct madvise_= behavior *madv_behavior) > return -EINVAL; > > /* > - * If we install guard markers, then the range is no longer > - * empty from a page table perspective and therefore it's > - * appropriate to have an anon_vma. > + * It would be confusing for anonymous mappings to have page tabl= e > + * entries but no anon_vma established, so ensure that it is. > + */ > + if (vma_is_anonymous(vma)) > + anon_vma_prepare(vma); > + > + /* > + * Indicate that the VMA may contain guard regions, making it vis= ible to > + * the user that a VMA may contain these, narrowing down the rang= e which > + * must be scanned in order to detect them. > * > - * This ensures that on fork, we copy page tables correctly. > + * This additionally causes page tables to be copied on fork rega= rdless > + * of whether the VMA is anonymous or not, correctly preserving t= he > + * guard region page table entries. > */ > - err =3D anon_vma_prepare(vma); > - if (err) > - return err; > + vm_flags_set(vma, VM_MAYBE_GUARD); > > /* > * Optimistically try to install the guard marker pages first. If= any > @@ -1709,7 +1716,6 @@ static enum madvise_lock_mode get_lock_mode(struct = madvise_behavior *madv_behavi > case MADV_POPULATE_READ: > case MADV_POPULATE_WRITE: > case MADV_COLLAPSE: > - case MADV_GUARD_INSTALL: > case MADV_GUARD_REMOVE: > return MADVISE_MMAP_READ_LOCK; > case MADV_DONTNEED: > diff --git a/mm/memory.c b/mm/memory.c > index 4c3a7e09a159..a2c79ee43d68 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -1478,6 +1478,10 @@ vma_needs_copy(struct vm_area_struct *dst_vma, str= uct 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 read= only > diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_int= ernal.h > index d873667704e8..e40c93edc5a7 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 sequentia= lly */ > -- > 2.51.0 >