Re: [BUGFIX][PATCH] memcg: charge swapcache to proper memcg

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Tue, 10 Mar 2009 14:04:16 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Tue, 10 Mar 2009 13:47:57 +0900
> Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> 
> > On Tue, 10 Mar 2009 13:33:16 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > > On Tue, 10 Mar 2009 10:07:07 +0900
> > > Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> > > 
> > > > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > > > 
> > > > memcg_test.txt says at 4.1:
> > > > 
> > > > 	This swap-in is one of the most complicated work. In do_swap_page(),
> > > > 	following events occur when pte is unchanged.
> > > > 
> > > > 	(1) the page (SwapCache) is looked up.
> > > > 	(2) lock_page()
> > > > 	(3) try_charge_swapin()
> > > > 	(4) reuse_swap_page() (may call delete_swap_cache())
> > > > 	(5) commit_charge_swapin()
> > > > 	(6) swap_free().
> > > > 
> > > > 	Considering following situation for example.
> > > > 
> > > > 	(A) The page has not been charged before (2) and reuse_swap_page()
> > > > 	    doesn't call delete_from_swap_cache().
> > > > 	(B) The page has not been charged before (2) and reuse_swap_page()
> > > > 	    calls delete_from_swap_cache().
> > > > 	(C) The page has been charged before (2) and reuse_swap_page() doesn't
> > > > 	    call delete_from_swap_cache().
> > > > 	(D) The page has been charged before (2) and reuse_swap_page() calls
> > > > 	    delete_from_swap_cache().
> > > > 
> > > > 	    memory.usage/memsw.usage changes to this page/swp_entry will be
> > > > 	 Case          (A)      (B)       (C)     (D)
> > > >          Event
> > > >        Before (2)     0/ 1     0/ 1      1/ 1    1/ 1
> > > >           ===========================================
> > > >           (3)        +1/+1    +1/+1     +1/+1   +1/+1
> > > >           (4)          -       0/ 0       -     -1/ 0
> > > >           (5)         0/-1     0/ 0     -1/-1    0/ 0
> > > >           (6)          -       0/-1       -      0/-1
> > > >           ===========================================
> > > >        Result         1/ 1     1/ 1      1/ 1    1/ 1
> > > > 
> > > >        In any cases, charges to this page should be 1/ 1.
> > > > 
> > > > In case of (D), mem_cgroup_try_get_from_swapcache() returns NULL
> > > > (because lookup_swap_cgroup() returns NULL), so "+1/+1" at (3) means
> > > > charges to the memcg("foo") to which the "current" belongs.
> > > > OTOH, "-1/0" at (4) and "0/-1" at (6) means uncharges from the memcg("baa")
> > > > to which the page has been charged.
> > > > 
> > > > So, if the "foo" and "baa" is different(for example because of task move),
> > > > this charge will be moved from "baa" to "foo".
> > > > 
> > > > I think this is an unexpected behavior.
> > > > 
> > > > This patch fixes this by modifying mem_cgroup_try_get_from_swapcache()
> > > > to return the memcg to which the swapcache has been charged if PCG_USED bit
> > > > is set.
> > > > IIUC, checking PCG_USED bit of swapcache is safe under page lock.
> > > > 
> > > > 
> > > > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > > > ---
> > > >  mm/memcontrol.c |   15 +++++++++++++--
> > > >  1 files changed, 13 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > index 73c51c8..f2efbc0 100644
> > > > --- a/mm/memcontrol.c
> > > > +++ b/mm/memcontrol.c
> > > > @@ -909,13 +909,24 @@ nomem:
> > > >  static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
> > > >  {
> > > >  	struct mem_cgroup *mem;
> > > > +	struct page_cgroup *pc;
> > > >  	swp_entry_t ent;
> > > >  
> > > > +	VM_BUG_ON(!PageLocked(page));
> > > > +
> > > >  	if (!PageSwapCache(page))
> > > >  		return NULL;
> > > >  
> > > > -	ent.val = page_private(page);
> > > > -	mem = lookup_swap_cgroup(ent);
> > > > +	pc = lookup_page_cgroup(page);
> > > > +	/*
> > > > +	 * Used bit of swapcache is solid under page lock.
> > > > +	 */
> > > > +	if (PageCgroupUsed(pc))
> > > > +		mem = pc->mem_cgroup;
> > > 
> > > I've already acked but how about returning NULL here ?
> > > 
> > Returning NULL here means try_charge_swapin charges "current" memcg("foo"
> > in the patch description above).
> > So, it doesn't change current behavior at all.
> > 
> 
> ok, maybe try_charge_swapin() should check Used bit...and set ptr=NULL 
> before reaching here.
> 
> Can't we move this 
> + pc = lookup_page_cgroup(page);
> + if (PageCgroupUsed(pc))
> 
> check to try_charge_swapin() ? (I think this is safe because the page is locked.)
> 
> By this, we can avoid more works in commit_charge().
> 
hmm, I thought the same thing at first, but this means:

     Case       (D)
     Event
   Before (2)   1/ 1
      ===============
      (3)       0/ 0
      (4)      -1/ 0
      (5)       0/ 0
      (6)       0/-1
      ===============
   Result       0/ 0

Instead of handling this case properly, I selected
the current derection(changed memcg codes only).


Thanks,
Daisuke Nishimura.

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