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

[Hugh Dickins]

On Mon, 7 Jul 2014, Johannes Weiner wrote:

> 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.
> 
> Catch pages that have already been migrated and abort migration
> gracefully.
> 
> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  mm/memcontrol.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 1e3b27f8dc2f..e4afdbdda0a7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6653,7 +6653,10 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
>  	if (!PageCgroupUsed(pc))
>  		return;
>  
> -	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM), oldpage);
> +	/* Already migrated */
> +	if (!(pc->flags & PCG_MEM))
> +		return;
> +

I am curious why you chose to fix the BUG in this way, instead of
-	pc->flags &= ~(PCG_MEM | PCG_MEMSW);
+	pc->flags = 0;
a few lines further down.

The page that gets left behind with just PCG_USED is anomalous (for an
LRU page, maybe not for a kmem page), isn'it it?  And liable to cause
other problems.

For example, won't it go the wrong way in the "Surreptitiously" test
in mem_cgroup_page_lruvec(): the page no longer has a hold on any
memcg, so is in a danger of being placed on a gone-memcg's LRU?

Hugh

>  	VM_BUG_ON_PAGE(do_swap_account && !(pc->flags & PCG_MEMSW), oldpage);
>  	pc->flags &= ~(PCG_MEM | PCG_MEMSW);
>  
> -- 
> 2.0.0
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