On 11/08/2020 05:42, 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.
Signed-off-by: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx>
---
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)
I may be wrong, but this looks like it could return incorrect results.
For example if we start reading with the following VMAs:
+------+------+-----------+
| VMA1 | VMA2 | VMA3 |
+------+------+-----------+
| | | |
4k 8k 16k 400k
Then after reading VMA2 we drop the lock due to contention. So:
last_vma_end = 16k
Then if VMA2 is freed while the lock is dropped, so we have:
+------+ +-----------+
| VMA1 | | VMA3 |
+------+ +-----------+
| | | |
4k 8k 16k 400k
find_vma(mm, 16k-1) will then return VMA3 and the condition vm_start <
last_vma_end will be false.
+ continue;
+
+ /* Current vma is not available, just break */
+ break;
Which means we break out here and report an incomplete output (the
numbers will be much smaller than reality).
Would it be better to have a loop like:
for (vma = priv->mm->mmap; vma;) {
smap_gather_stats(vma, &mss);
last_vma_end = vma->vm_end;
if (contended) {
/* drop/acquire lock */
vma = find_vma(mm, last_vma_end - 1);
if (!vma)
break;
if (vma->vm_start >= last_vma_end)
continue;
}
vma = vma->vm_next;
}
that way if the VMA is removed while the lock is dropped the loop can
just continue from the next VMA.
Or perhaps I missed something obvious? I haven't actually tested
anything above.
Steve
+ }
}
show_vma_header_prefix(m, priv->mm->mmap->vm_start,
