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Subject[PATCH 2/2] mm: proc: smaps_rollup: do not stall write attempts on mmap_lock
Date
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)
+ continue;
+
+ /* Current vma is not available, just break */
+ break;
+ }
}

show_vma_header_prefix(m, priv->mm->mmap->vm_start,
--
1.9.1
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