Re: [PATCH] mm: proc/task_mmu: Use walk_page_range_vma() for smaps

[Kalesh Singh]

On Wed, Jul 22, 2026 at 1:30 AM T.J. Mercier <tjmercier@xxxxxxxxxx> wrote:
>
> On Mon, Jul 20, 2026 at 10:19 PM kaleshsingh via kernel-team
> <kernel-team@xxxxxxxxxxx> wrote:
> >
> > From: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> >
> > Modify smap_gather_stats() to use walk_page_range_vma() instead of
> > walk_page_range() when walking page tables.
> >
> > walk_page_range() internally calls find_vma(), which asserts that
> > mmap_lock is held. This assertion fails when reading /proc/pid/smaps
> > under per-VMA locks (where mmap_lock is not held).
> >
> > walk_page_range_vma() walks a range within a single VMA and does not
> > use find_vma(). It relies on the VMA lock instead (when configured
> > with PGWALK_VMA_RDLOCK_VERIFY), making it compatible with per-VMA
> > locks.
> >
> > Fixes: ea3085dd7a4e ("fs/proc/task_mmu: read proc/pid/{smaps|numa_maps} under per-vma lock")
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Cc: Liam R. Howlett <liam@xxxxxxxxxxxxx>
> > Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> > Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> > ---
> >  fs/proc/task_mmu.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index d32408f7cd5e..5d301bdd05fc 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -1369,9 +1369,12 @@ static void smap_gather_stats(struct proc_maps_private *priv,
> >                               struct mem_size_stats *mss, unsigned long start)
> >  {
> >         const struct mm_walk_ops *ops = get_smaps_walk_ops(priv);
> > +       unsigned long end = vma->vm_end;
> > +
> > +       start = start ?: vma->vm_start;
>
> Doesn't overwriting start here break the !start && (!shmem_swapped ||
> ... check below when smap_gather_stats is called with start = 0 as in
> show_smaps_rollup?

Hi Tj,

You are right, I should have updated it to start == vm_start. I will
fix this in v2.

Thanks,
Kalesh


>
> >
> >         /* Invalid start */
> > -       if (start >= vma->vm_end)
> > +       if (start >= end)
> >                 return;
> >
> >         if (vma == get_gate_vma(priv->lock_ctx.mm))
> > @@ -1401,10 +1404,7 @@ static void smap_gather_stats(struct proc_maps_private *priv,
> >                 }
> >         }
> >
> > -       if (!start)
> > -               walk_page_vma(vma, ops, mss);
> > -       else
> > -               walk_page_range(vma->vm_mm, start, vma->vm_end, ops, mss);
> > +       walk_page_range_vma(vma, start, end, ops, mss);
> >
> >         reacquire_rcu(priv);
> >  }
> >
> > base-commit: b95f03f04d475aa6719d15a636ddf32222d55657
> > --
> > 2.55.0.229.g6434b31f56-goog
> >
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> >