[PATCH v2 2/2] mm: proc: smaps_rollup: do not stall write attempts on mmap_lock

Chinwen Chang
2020-08-13T02:14:10Z

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@arm.com/

Signed-off-by: Chinwen Chang <chinwen.chang@mediatek.com>
---
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