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

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