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

[Barry Song <21cnbao@gmail.com>]

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.

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.

>

Thanks
Barry