Re: [patch 8/8] mm: make per-memcg lru lists exclusive

Johannes Weiner
2011-06-02T17:57:21Z

On Fri, Jun 03, 2011 at 12:54:39AM +0900, Hiroyuki Kamezawa wrote:
> 2011/6/2 Johannes Weiner <hannes@cmpxchg.org>:
> > On Thu, Jun 02, 2011 at 10:16:59PM +0900, Hiroyuki Kamezawa wrote:
> >> 2011/6/1 Johannes Weiner <hannes@cmpxchg.org>:
> >> > All lru list walkers have been converted to operate on per-memcg
> >> > lists, the global per-zone lists are no longer required.
> >> >
> >> > This patch makes the per-memcg lists exclusive and removes the global
> >> > lists from memcg-enabled kernels.
> >> >
> >> > The per-memcg lists now string up page descriptors directly, which
> >> > unifies/simplifies the list isolation code of page reclaim as well as
> >> > it saves a full double-linked list head for each page in the system.
> >> >
> >> > At the core of this change is the introduction of the lruvec
> >> > structure, an array of all lru list heads.  It exists for each zone
> >> > globally, and for each zone per memcg.  All lru list operations are
> >> > now done in generic code against lruvecs, with the memcg lru list
> >> > primitives only doing accounting and returning the proper lruvec for
> >> > the currently scanned memcg on isolation, or for the respective page
> >> > on putback.
> >> >
> >> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> >>
> >>
> >> could you divide this into
> >>   - introduce lruvec
> >>   - don't record section? information into pc->flags because we see
> >> "page" on memcg LRU
> >>     and there is no requirement to get page from "pc".
> >>   - remove pc->lru completely
> >
> > Yes, that makes sense.  It shall be fixed in the next version.
> >
>
> BTW, IIUC, Transparent hugepage has a code to link a page to the
> page->lru directly.

[...]

> But it may put a page onto wrong memcgs if we do link a page to
> another page's page->lru
> because 2 pages may be in different cgroup each other.

Yes, I noticed that. If it splits a huge page, it does not just add
the tailpages to the lru head, but it links them next to the head
page.

But I don't see how those pages could ever be in different memcgs?
pages with page->mapping pointing to the same anon_vma are always in
the same memcg, AFAIU. Only since broken COWs get their own, though.

>
> Could you check there are more codes which does link page->lru to nearby page's
> page->lru ? Now, I'm not sure there are other codes....but we need care.

I double check again. It's a tricky area, but I thought I covered all
cases. Never hurts to reassure, though.