On 15/08/2020 07:20, 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.
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]
Change since v2:
- When getting back the mmap lock, the address where you stopped last
- time could now be in the middle of a vma. Add one more check to handle
- this case as suggested by Michel. [2]
[1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@xxxxxxx/
[2] https://lore.kernel.org/lkml/CANN689FtCsC71cjAjs0GPspOhgo_HRj+diWsoU1wr98YPktgWg@xxxxxxxxxxxxxx/
Signed-off-by: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx>
CC: Steven Price <steven.price@xxxxxxx>
CC: Michel Lespinasse <walken@xxxxxxxxxx>
Reviewed-by: Steven Price <steven.price@xxxxxxx>
---
fs/proc/task_mmu.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 70 insertions(+), 3 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 76e623a..945904e 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -846,7 +846,7 @@ static int show_smaps_rollup(struct seq_file *m, void *v)
struct mem_size_stats mss;
struct mm_struct *mm;
struct vm_area_struct *vma;
- unsigned long last_vma_end = 0;
+ unsigned long last_vma_end = 0, last_stopped = 0;
int ret = 0;
priv->task = get_proc_task(priv->inode);
@@ -867,9 +867,76 @@ 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) {
- smap_gather_stats(vma, &mss, 0);
+ for (vma = priv->mm->mmap; vma;) {
+ smap_gather_stats(vma, &mss, last_stopped);
+ last_stopped = 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) {
+ last_stopped = last_vma_end;
+ continue;
Note that instead of having last_stopped, you could replace the above
with a direct call:
smap_gather_stats(vma, &mss, last_vma_end);
I'm not sure which is cleaner though. last_stopped is a bit messy (it's
easily confused with last_vma_end), but having just the one call site
for smap_gather_stats() is nice too.
Steve
+ }
+ }
+ /* Case 2 above */
+ vma = vma->vm_next;
}
show_vma_header_prefix(m, priv->mm->mmap->vm_start,
