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

[Michal Hocko <mhocko@suse.com>]

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.
-- 
Michal Hocko
SUSE Labs