linux-kernel - Re: [RFC PATCH 1/5] mm/mmap: Correctly position vma_iterator in __split

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Message-ID: <mg2orzdqididfjexb7mj7gep3guehtdzamjn2v5a5vce7cd2ch@uvjjc2xzv7ao>
Date: Fri, 7 Jun 2024 10:25:29 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>, sidhartha.kumar@...cle.com,
        Matthew Wilcox <willy@...radead.org>,
        Lorenzo Stoakes <lstoakes@...il.com>, linux-fsdevel@...r.kernel.org,
        bpf@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/5] mm/mmap: Correctly position vma_iterator in
 __split_vma()

* Suren Baghdasaryan <surenb@...gle.com> [240605 20:51]:
> On Fri, May 31, 2024 at 9:33 AM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
> >
> > The vma iterator may be left pointing to the newly created vma.  This
> > happens when inserting the new vma at the end of the old vma
> > (!new_below).
> >
> > The incorrect position in the vma iterator is not exposed currently
> > since the vma iterator is repositioned in the munmap path and is not
> > reused in any of the other paths.
> >
> > This has limited impact in the current code, but is required for future
> > changes.
> >
> > Fixes: b2b3b886738f ("mm: don't use __vma_adjust() in __split_vma()")
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > ---
> >  mm/mmap.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 83b4682ec85c..31d464e6a656 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2442,6 +2442,9 @@ static int __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >         /* Success. */
> >         if (new_below)
> >                 vma_next(vmi);
> > +       else
> > +               vma_prev(vmi);
> > +
> 
> IIUC the goal is to always point vmi to the old (original) vma? If so,
> then change LGTM.

Yes, we need the iterator to keep pointing to the original VMAs, I think
this makes sense.  I will update the function comment in the next
revision to state as much.

> 
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
> 
> >         return 0;
> >
> >  out_free_mpol:
> > --
> > 2.43.0
> >