Re: [PATCH -v2 -mm] memcg: do not trigger OOM fromadd_to_page_cache_locked

[Michal Hocko]

On Wed 28-11-12 10:26:31, Johannes Weiner wrote:
> On Tue, Nov 27, 2012 at 09:59:44PM +0100, Michal Hocko wrote:
> > @@ -3863,7 +3862,7 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
> >  		return 0;
> >  
> >  	if (!PageSwapCache(page))
> > -		ret = mem_cgroup_charge_common(page, mm, gfp_mask, type);
> > +		ret = mem_cgroup_charge_common(page, mm, gfp_mask, type, oom);
> >  	else { /* page is swapcache/shmem */
> >  		ret = __mem_cgroup_try_charge_swapin(mm, page,
> >  						     gfp_mask, &memcg);
> 
> I think you need to pass it down the swapcache path too, as that is
> what happens when the shmem page written to is in swap and has been
> read into swapcache by the time of charging.

You are right, of course. I shouldn't send patches late in the evening
after staring to a crashdump for a good part of the day. /me ashamed.

> > @@ -1152,8 +1152,16 @@ repeat:
> >  				goto failed;
> >  		}
> >  
> > +		 /*
> > +                  * Cannot trigger OOM even if gfp_mask would allow that
> > +                  * normally because we might be called from a locked
> > +                  * context (i_mutex held) if this is a write lock or
> > +                  * fallocate and that could lead to deadlocks if the
> > +                  * killed process is waiting for the same lock.
> > +		  */
> 
> Indentation broken?

c&p

> >  		error = mem_cgroup_cache_charge(page, current->mm,
> > -						gfp & GFP_RECLAIM_MASK);
> > +						gfp & GFP_RECLAIM_MASK,
> > +						sgp < SGP_WRITE);
> 
> The code tests for read-only paths a bunch of times using
> 
> 	sgp != SGP_WRITE && sgp != SGP_FALLOC
> 
> Would probably be more consistent and more robust to use this here as
> well?

Yes my laziness. I was considering that but it was really long so I've
chosen the simpler way. But you are right that consistency is probably
better here

> > @@ -1209,7 +1217,8 @@ repeat:
> >  		SetPageSwapBacked(page);
> >  		__set_page_locked(page);
> >  		error = mem_cgroup_cache_charge(page, current->mm,
> > -						gfp & GFP_RECLAIM_MASK);
> > +						gfp & GFP_RECLAIM_MASK,
> > +						sgp < SGP_WRITE);
> 
> Same.
> 
> Otherwise, the patch looks good to me, thanks for persisting :)

Thanks for the throughout review.
Here we go with the fixed version.
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