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] [1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@xxxxxxx/ Signed-off-by: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx> --- fs/proc/task_mmu.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 55 insertions(+), 1 deletion(-) diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index dbda449..23b3a447 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -853,9 +853,63 @@ 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) { + for (vma = priv->mm->mmap; vma;) { 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; + } + + /* + * After dropping the lock, there are three 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. + */ + 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 2 above */ + vma = vma->vm_next; } show_vma_header_prefix(m, priv->mm->mmap->vm_start, -- 1.9.1
