Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[KOSAKI Motohiro]

> mm,migration: Do not try to migrate unmapped anonymous pages
> 
> rmap_walk_anon() was triggering errors in memory compaction that look like
> use-after-free errors. The problem is that between the page being isolated
> from the LRU and rcu_read_lock() being taken, the mapcount of the page
> dropped to 0 and the anon_vma gets freed. This can happen during memory
> compaction if pages being migrated belong to a process that exits before
> migration completes. Hence, the use-after-free race looks like
> 
>  1. Page isolated for migration
>  2. Process exits
>  3. page_mapcount(page) drops to zero so anon_vma was no longer reliable
>  4. unmap_and_move() takes the rcu_lock but the anon_vma is already garbage
>  4. call try_to_unmap, looks up tha anon_vma and "locks" it but the lock
>     is garbage.
> 
> This patch checks the mapcount after the rcu lock is taken. If the
> mapcount is zero, the anon_vma is assumed to be freed and no further
> action is taken.
> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
>  mm/migrate.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 98eaaf2..6eb1efe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	 */
>  	if (PageAnon(page)) {
>  		rcu_read_lock();
> +
> +		/*
> +		 * If the page has no mappings any more, just bail. An
> +		 * unmapped anon page is likely to be freed soon but worse,
> +		 * it's possible its anon_vma disappeared between when
> +		 * the page was isolated and when we reached here while
> +		 * the RCU lock was not held
> +		 */
> +		if (!page_mapcount(page)) {
> +			rcu_read_unlock();
> +			goto uncharge;
> +		}

I haven't understand what prevent this check. Why don't we need following scenario?

 1. Page isolated for migration
 2. Passed this if (!page_mapcount(page)) check
 3. Process exits
 4. page_mapcount(page) drops to zero so anon_vma was no longer reliable


Traditionally, page migration logic is, it can touch garbarge of anon_vma, but
SLAB_DESTROY_BY_RCU prevent any disaster. Is this broken concept?



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