Re: [RFC PATCH 3/4] xfs: map KM_MAYFAIL to __GFP_RETRY_MAYFAIL

[Michal Hocko]

On Tue 07-03-17 09:05:19, Darrick J. Wong wrote:
> On Tue, Mar 07, 2017 at 04:48:42PM +0100, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@xxxxxxxx>
> > 
> > KM_MAYFAIL didn't have any suitable GFP_FOO counterpart until recently
> > so it relied on the default page allocator behavior for the given set
> > of flags. This means that small allocations actually never failed.
> > 
> > Now that we have __GFP_RETRY_MAYFAIL flag which works independently on the
> > allocation request size we can map KM_MAYFAIL to it. The allocator will
> > try as hard as it can to fulfill the request but fails eventually if
> > the progress cannot be made.
> > 
> > Cc: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> > ---
> >  fs/xfs/kmem.h | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/fs/xfs/kmem.h b/fs/xfs/kmem.h
> > index ae08cfd9552a..ac80a4855c83 100644
> > --- a/fs/xfs/kmem.h
> > +++ b/fs/xfs/kmem.h
> > @@ -54,6 +54,16 @@ kmem_flags_convert(xfs_km_flags_t flags)
> >  			lflags &= ~__GFP_FS;
> >  	}
> >  
> > +	/*
> > +	 * Default page/slab allocator behavior is to retry for ever
> > +	 * for small allocations. We can override this behavior by using
> > +	 * __GFP_RETRY_MAYFAIL which will tell the allocator to retry as long
> > +	 * as it is feasible but rather fail than retry for ever for all
> 
> s/for ever/forever/

fixed

> 
> > +	 * request sizes.
> > +	 */
> > +	if (flags & KM_MAYFAIL)
> > +		lflags |= __GFP_RETRY_MAYFAIL;
> 
> But otherwise seems ok from a quick grep -B5 MAYFAIL through the XFS code.
> 
> (Has this been tested anywhere?)

not yet, this is more for a discussion at this stage. I plan to run it
through xfstests once we agree on the proper semantic. I have to confess
I rely on the proper KM_MAYFAIL annotations here, though.

Thanks!
-- 
Michal Hocko
SUSE Labs