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SubjectRe: [PATCH 2/2] mm: proc: smaps_rollup: do not stall write attempts on mmap_lock
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On 11/08/2020 05:42, Chinwen Chang 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.
>
> Signed-off-by: Chinwen Chang <chinwen.chang@mediatek.com>
> ---
> fs/proc/task_mmu.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index dbda449..4b51f25 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -856,6 +856,27 @@ static int show_smaps_rollup(struct seq_file *m, void *v)
> for (vma = priv->mm->mmap; vma; vma = vma->vm_next) {
> smap_gather_stats(vma, &mss);
> last_vma_end = vma->vm_end;
> +
> + /*
> + * Release mmap_lock temporarily if someone wants to
> + * access it for write request.
> + */
> + if (mmap_lock_is_contended(mm)) {
> + mmap_read_unlock(mm);
> + ret = mmap_read_lock_killable(mm);
> + if (ret) {
> + release_task_mempolicy(priv);
> + goto out_put_mm;
> + }
> +
> + /* Check whether current vma is available */
> + vma = find_vma(mm, last_vma_end - 1);
> + if (vma && vma->vm_start < last_vma_end)

I may be wrong, but this looks like it could return incorrect results.
For example if we start reading with the following VMAs:

+------+------+-----------+
| VMA1 | VMA2 | VMA3 |
+------+------+-----------+
| | | |
4k 8k 16k 400k

Then after reading VMA2 we drop the lock due to contention. So:

last_vma_end = 16k

Then if VMA2 is freed while the lock is dropped, so we have:

+------+ +-----------+
| VMA1 | | VMA3 |
+------+ +-----------+
| | | |
4k 8k 16k 400k

find_vma(mm, 16k-1) will then return VMA3 and the condition vm_start <
last_vma_end will be false.

> + continue;
> +
> + /* Current vma is not available, just break */
> + break;

Which means we break out here and report an incomplete output (the
numbers will be much smaller than reality).

Would it be better to have a loop like:

for (vma = priv->mm->mmap; vma;) {
smap_gather_stats(vma, &mss);
last_vma_end = vma->vm_end;

if (contended) {
/* drop/acquire lock */

vma = find_vma(mm, last_vma_end - 1);
if (!vma)
break;
if (vma->vm_start >= last_vma_end)
continue;
}
vma = vma->vm_next;
}

that way if the VMA is removed while the lock is dropped the loop can
just continue from the next VMA.

Or perhaps I missed something obvious? I haven't actually tested
anything above.

Steve

> + }
> }
>
> show_vma_header_prefix(m, priv->mm->mmap->vm_start,
>

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