Re: [PATCH v3 3/3] mm: proc: smaps_rollup: do not stall write attempts on mmap_lock

Chinwen Chang
2020-08-17T09:38:28Z

On Mon, 2020-08-17 at 09:38 +0100, Steven Price wrote:
> On 15/08/2020 07:20, 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.
> >
> > Change since v1:
> > - If current VMA is freed after dropping the lock, it will return
> > - incomplete result. To fix this issue, refine the code flow as
> > - suggested by Steve. [1]
> >
> > Change since v2:
> > - When getting back the mmap lock, the address where you stopped last
> > - time could now be in the middle of a vma. Add one more check to handle
> > - this case as suggested by Michel. [2]
> >
> > [1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@arm.com/
> > [2] https://lore.kernel.org/lkml/CANN689FtCsC71cjAjs0GPspOhgo_HRj+diWsoU1wr98YPktgWg@mail.gmail.com/
> >
> > Signed-off-by: Chinwen Chang <chinwen.chang@mediatek.com>
> > CC: Steven Price <steven.price@arm.com>
> > CC: Michel Lespinasse <walken@google.com>
>
> Reviewed-by: Steven Price <steven.price@arm.com>
>
> > ---
> > fs/proc/task_mmu.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++---
> > 1 file changed, 70 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 76e623a..945904e 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -846,7 +846,7 @@ static int show_smaps_rollup(struct seq_file *m, void *v)
> > struct mem_size_stats mss;
> > struct mm_struct *mm;
> > struct vm_area_struct *vma;
> > - unsigned long last_vma_end = 0;
> > + unsigned long last_vma_end = 0, last_stopped = 0;
> > int ret = 0;
> >
> > priv->task = get_proc_task(priv->inode);
> > @@ -867,9 +867,76 @@ static int show_smaps_rollup(struct seq_file *m, void *v)
> >
> > hold_task_mempolicy(priv);
> >
> > - for (vma = priv->mm->mmap; vma; vma = vma->vm_next) {
> > - smap_gather_stats(vma, &mss, 0);
> > + for (vma = priv->mm->mmap; vma;) {
> > + smap_gather_stats(vma, &mss, last_stopped);
> > + last_stopped = 0;
> > 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;
> > + }
> > +
> > + /*
> > + * After dropping the lock, there are four cases to
> > + * consider. See the following example for explanation.
> > + *
> > + * +------+------+-----------+
> > + * | VMA1 | VMA2 | VMA3 |
> > + * +------+------+-----------+
> > + * | | | |
> > + * 4k 8k 16k 400k
> > + *
> > + * Suppose we drop the lock after reading VMA2 due to
> > + * contention, then we get:
> > + *
> > + * last_vma_end = 16k
> > + *
> > + * 1) VMA2 is freed, but VMA3 exists:
> > + *
> > + * find_vma(mm, 16k - 1) will return VMA3.
> > + * In this case, just continue from VMA3.
> > + *
> > + * 2) VMA2 still exists:
> > + *
> > + * find_vma(mm, 16k - 1) will return VMA2.
> > + * Iterate the loop like the original one.
> > + *
> > + * 3) No more VMAs can be found:
> > + *
> > + * find_vma(mm, 16k - 1) will return NULL.
> > + * No more things to do, just break.
> > + *
> > + * 4) (last_vma_end - 1) is the middle of a vma (VMA'):
> > + *
> > + * find_vma(mm, 16k - 1) will return VMA' whose range
> > + * contains last_vma_end.
> > + * Iterate VMA' from last_vma_end.
> > + */
> > + vma = find_vma(mm, last_vma_end - 1);
> > + /* Case 3 above */
> > + if (!vma)
> > + break;
> > +
> > + /* Case 1 above */
> > + if (vma->vm_start >= last_vma_end)
> > + continue;
> > +
> > + /* Case 4 above */
> > + if (vma->vm_end > last_vma_end) {
> > + last_stopped = last_vma_end;
> > + continue;
>
> Note that instead of having last_stopped, you could replace the above
> with a direct call:
>
> smap_gather_stats(vma, &mss, last_vma_end);
>
> I'm not sure which is cleaner though. last_stopped is a bit messy (it's
> easily confused with last_vma_end), but having just the one call site
> for smap_gather_stats() is nice too.
>
> Steve
>

Hi Steve,

I think your idea is better. Let me try refactoring for further reviews.
Thanks for your kind suggestion:)

Chinwen
> > + }
> > + }
> > + /* Case 2 above */
> > + vma = vma->vm_next;
> > }
> >
> > show_vma_header_prefix(m, priv->mm->mmap->vm_start,
> >
>