linux-kernel - [PATCH 2/2] mm: proc: smaps_rollup: do not stall write attempts on mmap

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Message-ID: <1597120955-16495-3-git-send-email-chinwen.chang@mediatek.com>
Date:   Tue, 11 Aug 2020 12:42:35 +0800
From:   Chinwen Chang <chinwen.chang@...iatek.com>
To:     Matthias Brugger <matthias.bgg@...il.com>,
        Michel Lespinasse <walken@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Steven Price <steven.price@....com>,
        Song Liu <songliubraving@...com>,
        Jimmy Assarsson <jimmyassarsson@...il.com>,
        Huang Ying <ying.huang@...el.com>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>,
        <linux-fsdevel@...r.kernel.org>, <wsd_upstream@...iatek.com>
Subject: [PATCH 2/2] 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: Chinwen Chang <chinwen.chang@...iatek.com>
---
 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)
+				continue;
+
+			/* Current vma is not available, just break */
+			break;
+		}
 	}
 
 	show_vma_header_prefix(m, priv->mm->mmap->vm_start,
-- 
1.9.1