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Date: Wed, 18 Jun 2014 11:00:52 +0200
From: Jerome Marchand <jmarchan@...hat.com>
To: Chen Yucong <slaoub@...il.com>, akpm@...ux-foundation.org
CC: minchan@...nel.org, mgorman@...e.de, hannes@...xchg.org,
mhocko@...e.cz, riel@...hat.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmscan.c: fix an implementation flaw in proportional
scanning
On 06/17/2014 06:55 AM, Chen Yucong wrote:
> Via https://lkml.org/lkml/2013/4/10/897, we can know that the relative design
> idea is to keep
>
> scan_target[anon] : scan_target[file]
> == really_scanned_num[anon] : really_scanned_num[file]
>
> But we can find the following snippet in shrink_lruvec():
>
> if (nr_file > nr_anon) {
> ...
> } else {
> ...
> }
>
> However, the above code fragment broke the design idea. We can assume:
>
> nr[LRU_ACTIVE_FILE] = 30
> nr[LRU_INACTIVE_FILE] = 30
> nr[LRU_ACTIVE_ANON] = 0
> nr[LRU_INACTIVE_ANON] = 40
>
> When the value of (nr_reclaimed < nr_to_reclaim) become false, there are
> the following results:
>
> nr[LRU_ACTIVE_FILE] = 15
> nr[LRU_INACTIVE_FILE] = 15
> nr[LRU_ACTIVE_ANON] = 0
> nr[LRU_INACTIVE_ANON] = 25
> nr_file = 30
> nr_anon = 25
> file_percent = 30 / 60 = 0.5
> anon_percent = 25 / 40 = 0.65
>
> According to the above design idea, we should scan some pages from ANON,
> but in fact we execute the an error code path due to "if (nr_file > nr_anon)".
> In this way, nr[lru] is likely to be a negative number. Luckily,
> "nr[lru] -= min(nr[lru], nr_scanned)" can help us to filter this situation,
> but it has rebelled against our design idea.
>
> Signed-off-by: Chen Yucong <slaoub@...il.com>
> ---
> mm/vmscan.c | 39 ++++++++++++++++++---------------------
> 1 file changed, 18 insertions(+), 21 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a8ffe4e..2c35e34 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2057,8 +2057,7 @@ out:
> static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> {
> unsigned long nr[NR_LRU_LISTS];
> - unsigned long targets[NR_LRU_LISTS];
> - unsigned long nr_to_scan;
> + unsigned long file_target, anon_target;
> enum lru_list lru;
> unsigned long nr_reclaimed = 0;
> unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> @@ -2067,8 +2066,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>
> get_scan_count(lruvec, sc, nr);
>
> - /* Record the original scan target for proportional adjustments later */
> - memcpy(targets, nr, sizeof(nr));
> + file_target = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
> + anon_target = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
Current code adds 1 to these value to avoid divide by zero error.
>
> /*
> * Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
> @@ -2087,8 +2086,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> blk_start_plug(&plug);
> while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> nr[LRU_INACTIVE_FILE]) {
> - unsigned long nr_anon, nr_file, percentage;
> - unsigned long nr_scanned;
> + unsigned long nr_anon, nr_file, file_percent, anon_percent;
> + unsigned long nr_to_scan, nr_scanned, percentage;
>
> for_each_evictable_lru(lru) {
> if (nr[lru]) {
> @@ -2122,16 +2121,19 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> if (!nr_file || !nr_anon)
> break;
>
> - if (nr_file > nr_anon) {
> - unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
> - targets[LRU_ACTIVE_ANON] + 1;
> + file_percent = nr_file * 100 / file_target;
> + anon_percent = nr_anon * 100 / anon_target;
Here it could happen.
Jerome
> +
> + if (file_percent > anon_percent) {
> lru = LRU_BASE;
> - percentage = nr_anon * 100 / scan_target;
> + nr_scanned = file_target - nr_file;
> + nr_to_scan = file_target * (100 - anon_percent) / 100;
> + percentage = nr[LRU_FILE] * 100 / nr_file;
> } else {
> - unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
> - targets[LRU_ACTIVE_FILE] + 1;
> lru = LRU_FILE;
> - percentage = nr_file * 100 / scan_target;
> + nr_scanned = anon_target - nr_anon;
> + nr_to_scan = anon_target * (100 - file_percent) / 100;
> + percentage = nr[LRU_BASE] * 100 / nr_anon;
> }
>
> /* Stop scanning the smaller of the LRU */
> @@ -2143,14 +2145,9 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> * scan target and the percentage scanning already complete
> */
> lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
> - nr_scanned = targets[lru] - nr[lru];
> - nr[lru] = targets[lru] * (100 - percentage) / 100;
> - nr[lru] -= min(nr[lru], nr_scanned);
> -
> - lru += LRU_ACTIVE;
> - nr_scanned = targets[lru] - nr[lru];
> - nr[lru] = targets[lru] * (100 - percentage) / 100;
> - nr[lru] -= min(nr[lru], nr_scanned);
> + nr_to_scan -= min(nr_to_scan, nr_scanned);
> + nr[lru] = nr_to_scan * percentage / 100;
> + nr[lru + LRU_ACTIVE] = nr_to_scan - nr[lru];
>
> scan_adjusted = true;
> }
>
--
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