The patch titled
Subject: mm: proc: smaps_rollup: do not stall write attempts on mmap_lock
has been added to the -mm tree. Its filename is
mm-proc-smaps_rollup-do-not-stall-write-attempts-on-mmap_lock.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-proc-smaps_rollup-do-not-stall-write-attempts-on-mmap_lock.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-proc-smaps_rollup-do-not-stall-write-attempts-on-mmap_lock.patch
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------------------------------------------------------
From: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx>
Subject: mm: proc: smaps_rollup: do not stall write attempts on mmap_lock
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.
Link: http://lkml.kernel.org/r/1597715898-3854-4-git-send-email-chinwen.chang@xxxxxxxxxxxx
Signed-off-by: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx>
Cc: Steven Price <steven.price@xxxxxxx>
Cc: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
Cc: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Song Liu <songliubraving@xxxxxx>
Cc: Jimmy Assarsson <jimmyassarsson@xxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: Daniel Kiss <daniel.kiss@xxxxxxx>
Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/proc/task_mmu.c | 66 ++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 65 insertions(+), 1 deletion(-)
--- a/fs/proc/task_mmu.c~mm-proc-smaps_rollup-do-not-stall-write-attempts-on-mmap_lock
+++ a/fs/proc/task_mmu.c
@@ -867,9 +867,73 @@ static int show_smaps_rollup(struct seq_
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, 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)
+ smap_gather_stats(vma, &mss, last_vma_end);
+ }
+ /* Case 2 above */
+ vma = vma->vm_next;
}
show_vma_header_prefix(m, priv->mm->mmap->vm_start,
_
Patches currently in -mm which might be from chinwen.chang@xxxxxxxxxxxx are
mmap-locking-api-add-mmap_lock_is_contended.patch
mm-smaps-extend-smap_gather_stats-to-support-specified-beginning.patch
mm-proc-smaps_rollup-do-not-stall-write-attempts-on-mmap_lock.patch
