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

[Steven Price]

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@xxxxxxxxxxxx>
> ---
>   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,