[Devel] Re: [PATCH -mmotm 3/3] memcg: dirty pages instrumentation

[Andrea Righi]

On Tue, Mar 02, 2010 at 10:05:29AM -0500, Vivek Goyal wrote:
> On Mon, Mar 01, 2010 at 11:18:31PM +0100, Andrea Righi wrote:
> > On Mon, Mar 01, 2010 at 05:02:08PM -0500, Vivek Goyal wrote:
> > > > @@ -686,10 +699,14 @@ void throttle_vm_writeout(gfp_t gfp_mask)
> > > >                   */
> > > >                  dirty_thresh += dirty_thresh / 10;      /* wheeee... */
> > > >  
> > > > -                if (global_page_state(NR_UNSTABLE_NFS) +
> > > > -			global_page_state(NR_WRITEBACK) <= dirty_thresh)
> > > > -                        	break;
> > > > -                congestion_wait(BLK_RW_ASYNC, HZ/10);
> > > > +
> > > > +		dirty = mem_cgroup_page_stat(MEMCG_NR_DIRTY_WRITEBACK_PAGES);
> > > > +		if (dirty < 0)
> > > > +			dirty = global_page_state(NR_UNSTABLE_NFS) +
> > > > +				global_page_state(NR_WRITEBACK);
> > > 
> > > dirty is unsigned long. As mentioned last time, above will never be true?
> > > In general these patches look ok to me. I will do some testing with these.
> > 
> > Re-introduced the same bug. My bad. :(
> > 
> > The value returned from mem_cgroup_page_stat() can be negative, i.e.
> > when memory cgroup is disabled. We could simply use a long for dirty,
> > the unit is in # of pages so s64 should be enough. Or cast dirty to long
> > only for the check (see below).
> > 
> > Thanks!
> > -Andrea
> > 
> > Signed-off-by: Andrea Righi <arighi at develer.com>
> > ---
> >  mm/page-writeback.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index d83f41c..dbee976 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -701,7 +701,7 @@ void throttle_vm_writeout(gfp_t gfp_mask)
> >  
> >  
> >  		dirty = mem_cgroup_page_stat(MEMCG_NR_DIRTY_WRITEBACK_PAGES);
> > -		if (dirty < 0)
> > +		if ((long)dirty < 0)
> 
> This will also be problematic as on 32bit systems, your uppper limit of
> dirty memory will be 2G?
> 
> I guess, I will prefer one of the two.
> 
> - return the error code from function and pass a pointer to store stats
>   in as function argument.
> 
> - Or Peter's suggestion of checking mem_cgroup_has_dirty_limit() and if
>   per cgroup dirty control is enabled, then use per cgroup stats. In that
>   case you don't have to return negative values.
> 
>   Only tricky part will be careful accouting so that none of the stats go
>   negative in corner cases of migration etc.

What do you think about Peter's suggestion + the locking stuff? (see the
previous email). Otherwise, I'll choose the other solution, passing a
pointer and always return the error code is not bad.

Thanks,
-Andrea
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