[LSF/MM TOPIC] few MM topics

[Dave Chinner]

On Wed, Jan 31, 2018 at 09:24:38PM +0100, Michal Hocko wrote:
> On Wed 31-01-18 11:21:04, Darrick J. Wong wrote:
> > On Wed, Jan 24, 2018 at 10:26:49AM +0100, Michal Hocko wrote:
> [...]
> > > - I would also love to talk to some FS people and convince them to move
> > >   away from GFP_NOFS in favor of the new scope API. I know this just
> > >   means to send patches but the existing code is quite complex and it
> > >   really requires somebody familiar with the specific FS to do that
> > >   work.
> > 
> > Hm, are you talking about setting PF_MEMALLOC_NOFS instead of passing
> > *_NOFS to allocation functions and whatnot?
> 
> yes memalloc_nofs_{save,restore}
> 
> > Right now XFS will set it
> > on any thread which has a transaction open, but that doesn't help for
> > fs operations that don't have transactions (e.g. reading metadata,
> > opening files).  I suppose we could just set the flag any time someone
> > stumbles into the fs code from userspace, though you're right that seems
> > daunting.
> 
> I would really love to see the code to take the nofs scope
> (memalloc_nofs_save) at the point where the FS "critical" section starts
> (from the reclaim recursion POV).

We already do that - the transaction context in XFS is the critical
context, and we set PF_MEMALLOC_NOFS when we allocate a transaction
handle and remove it when we commit the transaction.

> This would both document the context
> and also limit NOFS allocations to bare minumum.

Yup, most of XFS already uses implicit GFP_NOFS allocation calls via
the transaction context process flag manipulation.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com