Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS allocations

[Michal Hocko]

On Thu 02-07-15 10:25:51, Theodore Ts'o wrote:
> On Wed, Jul 01, 2015 at 03:37:15PM +0200, Michal Hocko wrote:
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 37e90db1520b..6c44d424968e 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -995,7 +995,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  				goto keep_locked;
> >  
> >  			/* Case 3 above */
> > -			} else {
> > +			} else if (sc->gfp_mask & __GFP_FS) {
> >  				wait_on_page_writeback(page);
> >  			}
> >  		}
> 
> Um, I've just taken a closer look at this code now that I'm back from
> vacation, and I'm not sure this is right.  This Case 3 code occurs
> inside an
> 
> 	if (PageWriteback(page)) {
> 	    ...
> 	}
> 
> conditional, and if I'm not mistaken, if the flow of control exits
> this conditional, it is assumed that the page is *not* under writeback.
> This patch will assume the page has been cleaned if __GFP_FS is set,
> which could lead to a dirty page getting dropped, so I believe this is
> a bug.  No?

Yes you are right! My bad. I should have noticed that. Sorry about that.

> It would seem to me that a better fix would be to change the Case 2
> handling:
> 
> 			/* Case 2 above */
> 			} else if (global_reclaim(sc) ||
> -			    !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
> +			    !PageReclaim(page) || !(sc->gfp_mask & __GFP_FS)) {

OK, this should work because the loopback path should clear both
__GFP_IO and __GFP_FS. I would be tempted to use may_enter_fs here
as the original patch which introduced wait_on_page_writeback did
but this sounds more clear.

> 				/*
> 				 * This is slightly racy - end_page_writeback()
> 				 * might have just cleared PageReclaim, then
> 				 * setting PageReclaim here end up interpreted
> 				 * as PageReadahead - but that does not matter
> 				 * enough to care.  What we do want is for this
> 				 * page to have PageReclaim set next time memcg
> 				 * reclaim reaches the tests above, so it will
> 				 * then wait_on_page_writeback() to avoid OOM;
> 				 * and it's also appropriate in global reclaim.
> 				 */
> 				SetPageReclaim(page);
> 				nr_writeback++;
> 
> 				goto keep_locked;
> 
> 
> Am I missing something?

You are not missing anything and thanks for the double checking. This
was wery well spotted!
The updated patch with the full changelog:
---