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mm: proc: smaps_rollup: do not stall write attempts on mmap_lock

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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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Chinwen Chang <chinwen.chang@mediatek.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Song Liu <songliubraving@fb.com>
Cc: Jimmy Assarsson <jimmyassarsson@gmail.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Daniel Kiss <daniel.kiss@arm.com>
Cc: Laurent Dufour <ldufour@linux.ibm.com>
Link: http://lkml.kernel.org/r/1597715898-3854-4-git-send-email-chinwen.chang@mediatek.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Chinwen Chang 2020-10-13 16:53:47 -07:00 • committed by Linus Torvalds
parent 03b4b11493
commit ff9f47f6f0
1 changed files with 65 additions and 1 deletions
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66
fs/proc/task_mmu.c
View file

@ -865,9 +865,73 @@ static int show_smaps_rollup(struct seq_file *m, void *v)
hold_task_mempolicy(priv); 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); smap_gather_stats(vma, &mss, 0);
last_vma_end = vma->vm_end; 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, show_vma_header_prefix(m, priv->mm->mmap->vm_start,