Re: [PATCH 1/4] memcg: simplify page cache charging.

Andrew Morton
2011-12-20T21:58:20Z

On Mon, 19 Dec 2011 09:01:22 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Fri, 16 Dec 2011 14:28:14 -0800
> Andrew Morton <akpm@linux-foundation.org> wrote:
>
> > On Wed, 14 Dec 2011 16:49:22 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >
> > > Because of commit ef6a3c6311, FUSE uses replace_page_cache() instead
> > > of add_to_page_cache(). Then, mem_cgroup_cache_charge() is not
> > > called against FUSE's pages from splice.
> >
> > Speaking of ef6a3c6311 ("mm: add replace_page_cache_page() function"),
> > may I pathetically remind people that it's rather inefficient?
> >
> > http://lkml.indiana.edu/hypermail/linux/kernel/1109.1/00375.html
> >
>
> IIRC, people says inefficient because it uses memcg codes for page-migration
> for fixing up accounting. Now, We added replace-page-cache for memcg in
> memcg-add-mem_cgroup_replace_page_cache-to-fix-lru-issue.patch
>
> So, I think the problem originally mentioned is fixed.
>

No, the inefficiency in replace_page_cache_page() is still there. Two
identical walks down the radix tree, a pointless decrement then
increment of mapping->nrpages, two writes to page->mapping, an often
pointless decrement then increment of NR_FILE_PAGES, and probably other things.