Commit ff9f47f6 authored by Chinwen Chang's avatar Chinwen Chang Committed by Linus Torvalds
Browse files

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.

Signed-off-by: default avatarChinwen Chang <chinwen.chang@mediatek.com>
Signed-off-by: default avatarAndrew 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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 03b4b114

fs/proc/task_mmu.c

+65 −1
Original line number Diff line number Diff line
@@ -865,9 +865,73 @@ 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, 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,