Re: [PATCH -mm] fs: shrinker: always scan at least one object of each type

Andrew Morton
2015-01-13T23:56:41Z

On Mon, 12 Jan 2015 13:20:46 +0300 Vladimir Davydov <vdavydov@parallels.com> wrote:

> In super_cache_scan() we divide the number of objects of particular type
> by the total number of objects in order to distribute pressure among
> different types of fs objects (inodes, dentries, fs-private objects).
> As a result, in some corner cases we can get nr_to_scan=0 even if there
> are some objects to reclaim, e.g. dentries=1, inodes=1, fs_objects=1,
> nr_to_scan=1/3=0.
>
> This is unacceptable for per memcg kmem accounting, because this means
> that some objects may never get reclaimed after memcg death, preventing
> it from being freed.
>
> This patch therefore assures that super_cache_scan() will scan at least
> one object of each type if any.
>
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -92,13 +92,13 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
> * prune the dcache first as the icache is pinned by it, then
> * prune the icache, followed by the filesystem specific caches
> */
> - sc->nr_to_scan = dentries;
> + sc->nr_to_scan = dentries + 1;
> freed = prune_dcache_sb(sb, sc);
> - sc->nr_to_scan = inodes;
> + sc->nr_to_scan = inodes + 1;
> freed += prune_icache_sb(sb, sc);
>
> if (fs_objects) {
> - sc->nr_to_scan = fs_objects;
> + sc->nr_to_scan = fs_objects + 1;
> freed += sb->s_op->free_cached_objects(sb, sc);
> }

A reader of this code will wonder "why is it adding 1 everywhere".
Let's tell them?

--- a/fs/super.c~fs-shrinker-always-scan-at-least-one-object-of-each-type-fix
+++ a/fs/super.c
@@ -91,6 +91,9 @@ static unsigned long super_cache_scan(st
/*
* prune the dcache first as the icache is pinned by it, then
* prune the icache, followed by the filesystem specific caches
+ *
+ * Ensure that we always scan at least one object - memcg kmem
+ * accounting uses this to fully empty the caches.
*/
sc->nr_to_scan = dentries + 1;
freed = prune_dcache_sb(sb, sc);
_