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

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Message-ID: <94eb837b-26c7-4f44-b261-7703ab776099@lucifer.local>
Date: Mon, 17 Nov 2025 11:34:36 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        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 1/2] mm: propagate VM_SOFTDIRTY on merge

On Mon, Nov 17, 2025 at 10:09:37AM +0530, Anshuman Khandual wrote:
>
>
> On 14/11/25 11:23 PM, Lorenzo Stoakes 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.
> >
> > Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > ---
> >  include/linux/mm.h               | 23 +++++++++++------------
> >  tools/testing/vma/vma_internal.h | 23 +++++++++++------------
> >  2 files changed, 22 insertions(+), 24 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 43eec43da66a..fd9eeff07eb5 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -532,29 +532,28 @@ extern unsigned int kobjsize(const void *objp);
> >   * 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 - 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.
> > + * 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.
> >   */
> > -#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
> > +#define VM_IGNORE_MERGE VM_STICKY
>
> Logically VM_STICKY should be the only flag qualifying for VM_IGNORE_MERGE. In that
> case should not VM_IGNORE_MERGE flag be dropped all together ?

I intentionally kept it to be explicit. This is self-documenting as-is.