Re: [RFC][BUGFIX][PATCH] memcg: fix underflow of mapped_file stat

[KAMEZAWA Hiroyuki]

On Wed, 14 Apr 2010 09:54:08 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:

> On Tue, 13 Apr 2010 15:14:00 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > On Tue, 13 Apr 2010 13:42:07 +0900
> > Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> > 
> > > Hi.
> > > 
> > > When I was testing page migration, I found underflow problem of "mapped_file" field
> > > in memory.stat. This is a fix for the problem.
> > > 
> > > This patch is based on mmotm-2010-04-05-16-09, and IIUC it conflicts with Mel's
> > > compaction patches, so I send it as RFC for now. After next mmotm, which will
> > > include those patches, I'll update and resend this patch.
> > > 
> > > ===
> > > From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> > > 
> > > page_add_file_rmap(), which can be called from remove_migration_ptes(), is
> > > assumed to increment memcg's stat of mapped file. But on success of page
> > > migration, the newpage(mapped file) has not been charged yet, so the stat will
> > > not be incremented. This behavior leads to underflow of memcg's stat because
> > > when the newpage is unmapped afterwards, page_remove_rmap() decrements the stat.
> > > This problem doesn't happen on failure path of page migration, because the old
> > > page(mapped file) hasn't been uncharge at the point of remove_migration_ptes().
> > > This patch fixes this problem by calling commit_charge(mem_cgroup_end_migration)
> > > before remove_migration_ptes().
> > > 
> > > Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> > 
> > Nice catch. but...I want to make all kind of complicated things under
> > prepare/end migration. (And I want to avoid changes in migrate.c...)
> > 
> hmm, I want to call mem_cgroup_update_file_mapped() only where we update
> NR_FILE_MAPPED, but, okey, I see your concern.
> 

Thank you. 


> > Considering some racy condistions, I wonder memcg_update_file_mapped() itself
> > still need fixes..
> > 
> > So, how about this ? We already added FILE_MAPPED flags, then, make use of it.
> > ==
> > 
> > 
> > At migrating mapped file, events happens in following sequence.
> > 
> >  1. allocate a new page.
> >  2. get memcg of an old page.
> >  3. charge ageinst new page, before migration. But at this point
> >     no changes to page_cgroup, no commit-charge.
> >  4. page migration replaces radix-tree, old-page and new-page.
> >  5. page migration remaps the new page if the old page was mapped.
> >  6. memcg commits the charge for newpage.
> > 
> > Because "commit" happens after page-remap, we lose file_mapped
> > accounting information at migration.
> > 
> > This patch fixes it by accounting file_mapped information at
> > commiting charge.
> > 
> > Reported-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > ---
> >  mm/memcontrol.c |   15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> > 
> > Index: mmotm-temp/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-temp.orig/mm/memcontrol.c
> > +++ mmotm-temp/mm/memcontrol.c
> > @@ -1435,11 +1435,13 @@ void mem_cgroup_update_file_mapped(struc
> >  
> >  	/*
> >  	 * Preemption is already disabled. We can use __this_cpu_xxx
> > +	 * We have no lock per page at inc/dec mapcount of pages. We have to do
> > +	 * check by ourselves under lock_page_cgroup().
> >  	 */
> > -	if (val > 0) {
> > +	if (val > 0 && !PageCgroupFileMapped(pc)) {
> >  		__this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> >  		SetPageCgroupFileMapped(pc);
> > -	} else {
> > +	} else if (PageCgroupFileMapped(pc)) {
> >  		__this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> >  		ClearPageCgroupFileMapped(pc);
> >  	}
> Adding likely() is better ? IIUC, these conditions are usually met except for
> the case of page migration. And, can you add a comment about it ?
> 
Sure.


> > @@ -2563,6 +2565,15 @@ void mem_cgroup_end_migration(struct mem
> >  	 */
> >  	if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
> >  		mem_cgroup_uncharge_page(target);
> > +	else {
> > +		/*
> > +		 * When a migrated file cache is remapped, it's not charged.
> > +		 * Verify it. Because we're under lock_page(), there are
> > +		 * no race with uncharge.
> > +		 */
> > +		if (page_mapped(target))
> > +			mem_cgroup_update_file_mapped(mem, target, 1);
> > +	}
> We cannot rely on page lock, because when we succeeded in page migration,
> "target" = "newpage" has already unlocked in move_to_new_page(). So the "target"
> can be removed from the radix-tree theoretically(it's not related to this
> underflow problem, though).
> Shouldn't we call lock_page(target) and check "if (!target->mapping)" to handle
> this case(maybe in another patch) ?
> 
Sounds reasonable. I think about that.

Thanks,
-Kame

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