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

Vladimir Davydov
2015-01-14T07:54:56Z

On Tue, Jan 13, 2015 at 03:56:39PM -0800, Andrew Morton wrote:
> 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?

Yeah, sounds reasonable. Thank you!

>
> --- 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);
> _
>