linux-kernel - Re: [PATCH v2 1/2] mm: propagate VM

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <0019e0b8-ee1e-4359-b5ee-94225cbe5588@lucifer.local>
Date: Wed, 19 Nov 2025 18:04:19 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...nel.org>,
        "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>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: propagate VM_SOFTDIRTY on merge

On Mon, Nov 17, 2025 at 02:54:13PM -0800, Andrew Morton wrote:
> On Mon, 17 Nov 2025 17:33:38 +0000 Lorenzo Stoakes <lorenzo.stoakes@...cle.com> wrote:
>
> > Currently we set VM_SOFTDIRTY when a new mapping is set up (whether by
> > establishing a new VMA, or via merge) as implemented in __mmap_complete()
> > and do_brk_flags().
> >
> > However, when performing a merge of existing mappings such as when
> > performing mprotect(), we may lose the VM_SOFTDIRTY flag.
> >
> > This is because currently we simply ignore VM_SOFTDIRTY for the purposes
> > of merge, so one VMA may possess the flag and another not, and whichever
> > happens to be the target VMA will be the one upon which the merge is
> > performed which may or may not have VM_SOFTDIRTY set.
> >
> > Now we have the concept of 'sticky' VMA flags, let's make VM_SOFTDIRTY one
> > which solves this issue.
> >
> > Additionally update VMA userland tests to propagate changes.
> >
>
> Oh.  This patch messes with the comments which
> mm-implement-sticky-vma-flags-fix-2.patch just altered.  Not sure what
> you intend here, so I left it as below - please advise.

Ah sorry, missed this!

Yes a couple things to fixup below, sorry to be a pain!

>
> (also, I fixed a typo: s/most/must/, both files)
>
> --- a/include/linux/mm.h~mm-propagate-vm_softdirty-on-merge
> +++ a/include/linux/mm.h
> @@ -532,28 +532,28 @@ extern unsigned int kobjsize(const void
>   * possesses it but the other does not, the merged VMA should nonetheless have
>   * applied to it:
>   *
> + *   VM_SOFTDIRTY - if a VMA is marked soft-dirty, that is has not had its
> + *                  references cleared via /proc/$pid/clear_refs, any merged VMA
> + *                  should be considered soft-dirty also as it operates at a VMA
> + *                  granularity.
> + *
>   * VM_MAYBE_GUARD - If a VMA may have guard regions in place it implies that
>   *                  mapped page tables may contain metadata not described by the
>   *                  VMA and thus any merged VMA may also contain this metadata,
>   *                  and thus we must make this flag sticky.
>   */
> -#define VM_STICKY VM_MAYBE_GUARD
> +#define VM_STICKY (VM_SOFTDIRTY | VM_MAYBE_GUARD)
>
>  /*
>   * VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
>   * of these flags and the other not does not preclude a merge.
>   *
> - * VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
> - *                dirty bit -- the caller should mark merged VMA as dirty. If
> - *                dirty bit won't be excluded from comparison, we increase
> - *                pressure on the memory system forcing the kernel to generate
> - *                new VMAs when old one could be extended instead.
> - *
> - *    VM_STICKY - When merging VMAs, VMA flags must match, unless they are
> - *                'sticky'. If any sticky flags exist in either VMA, we simply
> - *                set all of them on the merged VMA.
> + * VM_STICKY - If one VMA has flags which must be 'sticky', that is ones
> + *             which should propagate to all VMAs, but the other does not,
> + *             the merge should still proceed with the merge logic applying
> + *             sticky flags to the final VMA.

Could we fix this up actually? Sorry to be a pain but better to have the new wording:

+ *    VM_STICKY - When merging VMAs, VMA flags must match, unless they are
+ *                'sticky'. If any sticky flags exist in either VMA, we simply
+ *                set all of them on the merged VMA.

>   */
> -#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
> +#define VM_IGNORE_MERGE VM_STICKY
>
>  /*
>   * Flags which should result in page tables being copied on fork. These are
> --- a/tools/testing/vma/vma_internal.h~mm-propagate-vm_softdirty-on-merge
> +++ a/tools/testing/vma/vma_internal.h
> @@ -122,28 +122,23 @@ extern unsigned long dac_mmap_min_addr;
>   * possesses it but the other does not, the merged VMA should nonetheless have
>   * applied to it:
>   *
> - * VM_MAYBE_GUARD - If a VMA may have guard regions in place it implies that
> - *                  mapped page tables may contain metadata not described by the
> - *                  VMA and thus any merged VMA may also contain this metadata,
> - *                  and thus we must make this flag sticky.
> + *   VM_SOFTDIRTY - if a VMA is marked soft-dirty, that is has not had its
> + *                  references cleared via /proc/$pid/clear_refs, any merged VMA
> + *                  should be considered soft-dirty also as it operates at a VMA
> + *                  granularity.
>   */
> -#define VM_STICKY VM_MAYBE_GUARD
> +#define VM_STICKY (VM_SOFTDIRTY | VM_MAYBE_GUARD)
>
>  /*
>   * VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
>   * of these flags and the other not does not preclude a merge.
>   *
> - * VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
> - *                dirty bit -- the caller should mark merged VMA as dirty. If
> - *                dirty bit won't be excluded from comparison, we increase
> - *                pressure on the memory system forcing the kernel to generate
> - *                new VMAs when old one could be extended instead.
> - *
> - *    VM_STICKY - When merging VMAs, VMA flags must match, unless they are
> - *                'sticky'. If any sticky flags exist in either VMA, we simply
> - *                set all of them on the merged VMA.
> + * VM_STICKY - If one VMA has flags which most be 'sticky', that is ones
> + *             which should propagate to all VMAs, but the other does not,
> + *             the merge should still proceed with the merge logic applying
> + *             sticky flags to the final VMA.

Also could we do the same here?

+ *    VM_STICKY - When merging VMAs, VMA flags must match, unless they are
+ *                'sticky'. If any sticky flags exist in either VMA, we simply
+ *                set all of them on the merged VMA.

>   */
> -#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
> +#define VM_IGNORE_MERGE VM_STICKY
>
>  /*
>   * Flags which should result in page tables being copied on fork. These are
> _
>

Thanks, Lorenzo