Messages in this thread | | | Subject | Re: [PATCH 2/2] mm: proc: smaps_rollup: do not stall write attempts on mmap_lock | From | Steven Price <> | Date | Wed, 12 Aug 2020 09:39:09 +0100 |
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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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