Re: [PATCH 2/5] mm: Document __GFP_NOFAIL must be blockable

[Michal Hocko <mhocko@suse.com>]

On Wed 24-07-24 20:55:41, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> Non-blocking allocation with __GFP_NOFAIL is not supported and may
> still result in NULL pointers (if we don't return NULL, we result
> in busy-loop within non-sleepable contexts):
> 
> static inline struct page *
> __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> 						struct alloc_context *ac)
> {
> 	...
> 	/*
> 	 * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
> 	 * we always retry
> 	 */
> 	if (gfp_mask & __GFP_NOFAIL) {
> 		/*
> 		 * All existing users of the __GFP_NOFAIL are blockable, so warn
> 		 * of any new users that actually require GFP_NOWAIT
> 		 */
> 		if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
> 			goto fail;
> 		...
> 	}
> 	...
> fail:
> 	warn_alloc(gfp_mask, ac->nodemask,
> 			"page allocation failure: order:%u", order);
> got_pg:
> 	return page;
> }
> 
> Highlight this in the documentation of __GFP_NOFAIL so that non-mm
> subsystems can reject any illegal usage of __GFP_NOFAIL with
> GFP_ATOMIC, GFP_NOWAIT, etc.
> 
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  include/linux/gfp_types.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
> index 313be4ad79fd..0dad2c7914be 100644
> --- a/include/linux/gfp_types.h
> +++ b/include/linux/gfp_types.h
> @@ -246,6 +246,8 @@ enum {
>   * cannot handle allocation failures. The allocation could block
>   * indefinitely but will never return with failure. Testing for
>   * failure is pointless.
> + * It _must_ be blockable and used together with __GFP_DIRECT_RECLAIM.
> + * It should _never_ be used in non-sleepable contexts.
>   * New users should be evaluated carefully (and the flag should be
>   * used only when there is no reasonable failure policy) but it is
>   * definitely preferable to use the flag rather than opencode endless
> -- 
> 2.34.1

Do you think the following addendum should be folded in just for
completness?

diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
index 313be4ad79fd..d024cfd1af8e 100644
--- a/include/linux/gfp_types.h
+++ b/include/linux/gfp_types.h
@@ -215,7 +215,8 @@ enum {
  * the caller still has to check for failures) while costly requests try to be
  * not disruptive and back off even without invoking the OOM killer.
  * The following three modifiers might be used to override some of these
- * implicit rules.
+ * implicit rules. Please note that all of them must be used along with
+ * %__GFP_DIRECT_RECLAIM flag.
  *
  * %__GFP_NORETRY: The VM implementation will try only very lightweight
  * memory direct reclaim to get some memory under memory pressure (thus

-- 
Michal Hocko
SUSE Labs