Re: [PATCH v2 2/2] mm: proc: smaps_rollup: do not stall write attempts on mmap_lock

[Michel Lespinasse]

On Wed, Aug 12, 2020 at 7:13 PM Chinwen Chang
<chinwen.chang@xxxxxxxxxxxx> wrote:
> smaps_rollup will try to grab mmap_lock and go through the whole vma
> list until it finishes the iterating. When encountering large processes,
> the mmap_lock will be held for a longer time, which may block other
> write requests like mmap and munmap from progressing smoothly.
>
> There are upcoming mmap_lock optimizations like range-based locks, but
> the lock applied to smaps_rollup would be the coarse type, which doesn't
> avoid the occurrence of unpleasant contention.
>
> To solve aforementioned issue, we add a check which detects whether
> anyone wants to grab mmap_lock for write attempts.

I think your retry mechanism still doesn't handle all cases. When you
get back the mmap lock, the address where you stopped last time could
now be in the middle of a vma. I think the consistent thing to do in
that case would be to retry scanning from the address you stopped at,
even if it's not on a vma boundary anymore. You may have to change
smap_gather_stats to support that, though.