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

Steven Price
2020-08-17T09:08:19Z

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

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