Re: [PATCH 4/7] mm: introduce memalloc_nofs_{save,restore} API

[Michal Hocko]

On Mon 06-03-17 13:22:14, Andrew Morton wrote:
> On Mon,  6 Mar 2017 14:14:05 +0100 Michal Hocko <mhocko@xxxxxxxxxx> wrote:
[...]
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -210,8 +210,16 @@ struct vm_area_struct;
> >   *
> >   * GFP_NOIO will use direct reclaim to discard clean pages or slab pages
> >   *   that do not require the starting of any physical IO.
> > + *   Please try to avoid using this flag directly and instead use
> > + *   memalloc_noio_{save,restore} to mark the whole scope which cannot
> > + *   perform any IO with a short explanation why. All allocation requests
> > + *   will inherit GFP_NOIO implicitly.
> >   *
> >   * GFP_NOFS will use direct reclaim but will not use any filesystem interfaces.
> > + *   Please try to avoid using this flag directly and instead use
> > + *   memalloc_nofs_{save,restore} to mark the whole scope which cannot/shouldn't
> > + *   recurse into the FS layer with a short explanation why. All allocation
> > + *   requests will inherit GFP_NOFS implicitly.
> 
> I wonder if these are worth a checkpatch rule.

I am not really sure, to be honest. This may easilly end up people
replacing

do_alloc(GFP_NOFS)

with

memalloc_nofs_save()
do_alloc(GFP_KERNEL)
memalloc_nofs_restore()

which doesn't make any sense of course. From my experience, people tend
to do stupid things just to silent checkpatch warnings very often.
Moreover I believe we need to do the transition to the new api first
before we can push back on the explicit GFP_NOFS usage. Maybe then we
can think about the a checkpatch warning.

-- 
Michal Hocko
SUSE Labs