linux-kernel - [PATCH v5 5/6] page_idle: Drain all LRU pagevec before idle tracking

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Message-Id: <20190807171559.182301-5-joel@joelfernandes.org>
Date:   Wed,  7 Aug 2019 13:15:58 -0400
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Brendan Gregg <bgregg@...flix.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Christian Hansen <chansen3@...co.com>, dancol@...gle.com,
        fmayer@...gle.com, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>, joelaf@...gle.com,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>, kernel-team@...roid.com,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...ux.ibm.com>, minchan@...nel.org,
        namhyung@...gle.com, paulmck@...ux.ibm.com,
        Robin Murphy <robin.murphy@....com>,
        Roman Gushchin <guro@...com>,
        Stephen Rothwell <sfr@...b.auug.org.au>, surenb@...gle.com,
        Thomas Gleixner <tglx@...utronix.de>, tkjos@...gle.com,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>
Subject: [PATCH v5 5/6] page_idle: Drain all LRU pagevec before idle tracking

During idle page tracking, we see that sometimes faulted anon pages are in
pagevec but are not drained to LRU. Idle page tracking only considers pages
on LRU.

I am able to find multiple issues involving this. One issue looks like
idle tracking is completely broken. It shows up in my testing as if a
page that is marked as idle is always "accessed" -- because it was never
marked as idle (due to not draining of pagevec).

The other issue shows up as a failure during swapping (support for which
this series adds), with the following sequence:
 1. Allocate some pages
 2. Write to them
 3. Mark them as idle                                  <--- fails
 4. Introduce some memory pressure to induce swapping.
 5. Check the swap bit I introduced in this series.    <--- fails to set idle
                                                            bit in swap PTE.

To fix this, this patch drains all CPU's pagevec before starting idle tracking.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 mm/page_idle.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/mm/page_idle.c b/mm/page_idle.c
index 2766d4ab348c..26440a497609 100644
--- a/mm/page_idle.c
+++ b/mm/page_idle.c
@@ -180,6 +180,13 @@ static ssize_t page_idle_bitmap_read(struct file *file, struct kobject *kobj,
 	unsigned long pfn, end_pfn;
 	int bit, ret;
 
+	/*
+	 * Idle page tracking currently works only on LRU pages, so drain
+	 * them. This can cause slowness, but in the future we could
+	 * remove this operation if we are tracking non-LRU pages too.
+	 */
+	lru_add_drain_all();
+
 	ret = page_idle_get_frames(pos, count, NULL, &pfn, &end_pfn);
 	if (ret == -ENXIO)
 		return 0;  /* Reads beyond max_pfn do nothing */
@@ -211,6 +218,13 @@ static ssize_t page_idle_bitmap_write(struct file *file, struct kobject *kobj,
 	unsigned long pfn, end_pfn;
 	int bit, ret;
 
+	/*
+	 * Idle page tracking currently works only on LRU pages, so drain
+	 * them. This can cause slowness, but in the future we could
+	 * remove this operation if we are tracking non-LRU pages too.
+	 */
+	lru_add_drain_all();
+
 	ret = page_idle_get_frames(pos, count, NULL, &pfn, &end_pfn);
 	if (ret)
 		return ret;
@@ -428,6 +442,13 @@ ssize_t page_idle_proc_generic(struct file *file, char __user *ubuff,
 	walk.private = &priv;
 	walk.mm = mm;
 
+	/*
+	 * Idle page tracking currently works only on LRU pages, so drain
+	 * them. This can cause slowness, but in the future we could
+	 * remove this operation if we are tracking non-LRU pages too.
+	 */
+	lru_add_drain_all();
+
 	down_read(&mm->mmap_sem);
 
 	/*
-- 
2.22.0.770.g0f2c4a37fd-goog