Re: [PATCH RFC] mm: warn potential return NULL for kmalloc_array and kvmalloc_array with __GFP_NOFAIL

[Michal Hocko <mhocko@suse.com>]

On Mon 22-07-24 20:09:37, Barry Song wrote:
> On Mon, Jul 22, 2024 at 7:26 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Sun 21-07-24 10:14:03, Barry Song wrote:
> > > On Fri, Jul 19, 2024 at 7:53 PM Christoph Hellwig <hch@infradead.org> wrote:
> > > >
> > > > On Fri, Jul 19, 2024 at 07:43:38PM +1200, Barry Song wrote:
> > > > > I doubt this is going to work as users can use a variant to save gfp_flags.
> > > > > On the other hand, isn't it necessarily a bug of vdpa, why can't it be mm?
> > > > >
> > > > > if mm disallows GFP_NOFAIL,  there must be a doc to say that; if it allows,
> > > > > we should never return NULL.
> > > >
> > > > Yeah.  Maybe the right answer is to have separate _nofail variants that
> > > > don't take any flags and make GFP_NOFAIL an entirely mm-private internal
> > > > flags that is rejected by all external interfaces.  That should also
> > > > really help with auditing the users.
> >
> > This would require duplicating many of our allocations APIs.
> >
> > > Just like Michal has consistently asserted that using GFP_NOFAIL with
> > > non-wait is against the rules, I think we should enforce this policy by:
> > >
> > > diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
> > > index 313be4ad79fd..a5c09f9590f2 100644
> > > --- a/include/linux/gfp_types.h
> > > +++ b/include/linux/gfp_types.h
> > > @@ -258,7 +258,7 @@ enum {
> > >  #define __GFP_KSWAPD_RECLAIM   ((__force gfp_t)___GFP_KSWAPD_RECLAIM)
> > > /* kswapd can wake */
> > >  #define __GFP_RECLAIM ((__force
> > > gfp_t)(___GFP_DIRECT_RECLAIM|___GFP_KSWAPD_RECLAIM))
> > >  #define __GFP_RETRY_MAYFAIL    ((__force gfp_t)___GFP_RETRY_MAYFAIL)
> > > -#define __GFP_NOFAIL   ((__force gfp_t)___GFP_NOFAIL)
> > > +#define __GFP_NOFAIL   ((__force gfp_t)(___GFP_NOFAIL | ___GFP_DIRECT_RECLAIM))
> > >  #define __GFP_NORETRY  ((__force gfp_t)___GFP_NORETRY)
> > >
> > > Anyone misusing GFP_ATOMIC | __GFP_NOFAIL in an atomic context
> > > risks experiencing a crash due to sleep in atomic. This is a common
> > > consequence, as all instances of sleep in atomic should result in the
> > > same issue.
> >
> > I really dislike any of __GFP_$FOO to have side effects like this.
> > Please let's not overdo this.
> 
> Okay, but my point is that if GFP_NOFAIL is inevitably blockable, why
> not enforce this and let users understand that it is definitively
> blockable?  ust like when we call alloc_pages(GFP_KERNEL), we know
> it might sleep.

__GFP_$FOO are usually low level. GFP_$FOO are high level and they
combine several subflags to have a specific meaning. So this would need
to be GFP_NOFAIL. Btw. the same applies to __GFP_NORETRY and
__GFP_RETRY_MAYFAIL.

Whether changing existing users of those flags is worth is a different
question.
-- 
Michal Hocko
SUSE Labs