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

[T.J. Mercier]

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?

>
>         /* 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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