Re: [patch 2/3] mm: memcontrol: rewrite uncharge API fix - double migration

[Johannes Weiner]

On Wed, Jul 16, 2014 at 10:34:56AM +0200, Michal Hocko wrote:
> [Sorry I have missed this thread]
> 
> On Tue 15-07-14 10:45:39, Johannes Weiner wrote:
> [...]
> > From 274b94ad83b38fe7dc1707a8eb4015b3ab1673c5 Mon Sep 17 00:00:00 2001
> > From: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Date: Thu, 10 Jul 2014 01:02:11 +0000
> > Subject: [patch] mm: memcontrol: rewrite uncharge API fix - double migration
> > 
> > Hugh reports:
> > 
> > VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM))
> > mm/memcontrol.c:6680!
> > page had count 1 mapcount 0 mapping anon index 0x196
> > flags locked uptodate reclaim swapbacked, pcflags 1, memcg not root
> > mem_cgroup_migrate < move_to_new_page < migrate_pages < compact_zone <
> > compact_zone_order < try_to_compact_pages < __alloc_pages_direct_compact <
> > __alloc_pages_nodemask < alloc_pages_vma < do_huge_pmd_anonymous_page <
> > handle_mm_fault < __do_page_fault
> > 
> > mem_cgroup_migrate() assumes that a page is only migrated once and
> > then freed immediately after.
> > 
> > However, putting the page back on the LRU list and dropping the
> > isolation refcount is not done atomically.  This allows a PFN-based
> > migrator like compaction to isolate the page, see the expected
> > anonymous page refcount of 1, and migrate the page once more.
> > 
> > Furthermore, once the charges are transferred to the new page, the old
> > page no longer has a pin on the memcg, which might get released before
> > the page itself now.  pc->mem_cgroup is invalid at this point, but
> > PCG_USED suggests otherwise, provoking use-after-free.
> 
> The same applies to to the new page because we are transferring only
> statistics. The old page with PCG_USED would uncharge the res_counter
> and so the new page is not backed by any and so memcg can go away.
> This sounds like a more probable scenario to me because old page should
> go away quite early after successful migration.

No, the charges are carried by PCG_MEM and PCG_MEMSW, not PCG_USED.

> > Properly uncharge the page after it's been migrated, including the
> > clearing of PCG_USED, so that a subsequent charge migration attempt
> > will be able to detect it and bail out.
> > 
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > ---
> >  mm/memcontrol.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 1e3b27f8dc2f..1439537fe7c9 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6655,7 +6655,6 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
> >  
> >  	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM), oldpage);
> >  	VM_BUG_ON_PAGE(do_swap_account && !(pc->flags & PCG_MEMSW), oldpage);
> > -	pc->flags &= ~(PCG_MEM | PCG_MEMSW);
> >  
> >  	if (PageTransHuge(oldpage)) {
> >  		nr_pages <<= compound_order(oldpage);
> > @@ -6663,6 +6662,13 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
> >  		VM_BUG_ON_PAGE(!PageTransHuge(newpage), newpage);
> >  	}
> >  
> > +	pc->flags = 0;
> > +
> > +	local_irq_disable();
> > +	mem_cgroup_charge_statistics(pc->mem_cgroup, oldpage, -nr_pages);
> > +	memcg_check_events(pc->mem_cgroup, oldpage);
> > +	local_irq_enable();
> > +
> >  	commit_charge(newpage, pc->mem_cgroup, nr_pages, lrucare);
> >  }
> 
> Looks good to me. I am just wondering whether we should really
> fiddle with stats and events when actually nothing changed during
> the transition. I would simply extract core of commit_charge into
> __commit_charge which would be called from here.
> 
> The impact is minimal because events are rate limited and stats are
> per-cpu so it is not a big deal it just looks ugly to me.

Agreed.  This is the minimal change to get it functionally right (we
are already at -rc5), it can always be optimized later.  I'll send a
patch soon.
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