Re: [PATCH] vmscan: add barrier to prevent evictable page in unevictable list

[Minchan Kim <minchan.kim@gmail.com>]

On Wed, Sep 28, 2011 at 10:14:52AM +0200, Johannes Weiner wrote:
> On Wed, Sep 28, 2011 at 10:45:30AM +0900, Minchan Kim wrote:
> > When racing between putback_lru_page and shmem_unlock happens,
> > progrom execution order is as follows, but clear_bit in processor #1
> > could be reordered right before spin_unlock of processor #1.
> > Then, the page would be stranded on the unevictable list.
> > 
> > spin_lock
> > SetPageLRU
> > spin_unlock
> >                                 clear_bit(AS_UNEVICTABLE)
> >                                 spin_lock
> >                                 if PageLRU()
> >                                         if !test_bit(AS_UNEVICTABLE)
> >                                         	move evictable list
> > smp_mb
> > if !test_bit(AS_UNEVICTABLE)
> >         move evictable list
> >                                 spin_unlock
> > 
> > But, pagevec_lookup in scan_mapping_unevictable_pages has rcu_read_[un]lock so
> > it could protect reordering before reaching test_bit(AS_UNEVICTABLE) on processor #1
> > so this problem never happens. But it's a unexpected side effect and we should
> > solve this problem properly.
> > 
> > This patch adds a barrier after mapping_clear_unevictable.
> > 
> > side-note: I didn't meet this problem but just found during review.
> > 
> > Cc: Johannes Weiner <jweiner@redhat.com>
> > Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > Cc: Mel Gorman <mel@csn.ul.ie>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
> > Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
> > ---
> >  mm/shmem.c  |    1 +
> >  mm/vmscan.c |   11 ++++++-----
> >  2 files changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 2d35772..22cb349 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1068,6 +1068,7 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user)
> >  		user_shm_unlock(inode->i_size, user);
> >  		info->flags &= ~VM_LOCKED;
> >  		mapping_clear_unevictable(file->f_mapping);
> > +		smp_mb__after_clear_bit();
> >  		scan_mapping_unevictable_pages(file->f_mapping);
> 
> I always get nervous when I see undocumented barriers.  Maybe add a
> teensy tiny comment here?

Agree. I will try it.

> 
> 	/*
> 	 * Ensure that a racing putback_lru_page() can see
> 	 * the pages of this mapping are evictable when we
> 	 * skip them due to !PageLRU during the scan.
> 	 */
> 
> Or something like that.  Otherwise, nice catch :-)
> 
> Acked-by: Johannes Weiner <jweiner@redhat.com>

Thanks!

-- 
Kind regards,
Minchan Kim

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