Re: [PATCH 2/2] mm: proc: smaps_rollup: do not stall write attempts on mmap_lock
From: Steven Price <steven.price@arm.com>
Date: 2020-08-12 08:39:17
Also in:
linux-fsdevel, linux-mediatek, lkml
On 11/08/2020 05:42, Chinwen Chang wrote:
quoted hunk ↗ jump to hunk
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 <redacted> --- 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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