Re: [PATCH v4 3/3] mm: warn about illegal __GFP_NOFAIL usage in a more appropriate location and manner

[Michal Hocko <mhocko@suse.com>]

On Sat 31-08-24 08:28:23, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> Three points for this change:
> 
> 1. We should consolidate all warnings in one place. Currently, the
>    order > 1 warning is in the hotpath, while others are in less
>    likely scenarios. Moving all warnings to the slowpath will reduce
>    the overhead for order > 1 and increase the visibility of other
>    warnings.
> 
> 2. We currently have two warnings for order: one for order > 1 in
>    the hotpath and another for order > costly_order in the laziest
>    path. I suggest standardizing on order > 1 since it’s been in
>    use for a long time.
> 
> 3. We don't need to check for __GFP_NOWARN in this case. __GFP_NOWARN
>    is meant to suppress allocation failure reports, but here we're
>    dealing with bug detection, not allocation failures. So replace
>    WARN_ON_ONCE_GFP by WARN_ON_ONCE.
> 
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>

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

Updating the doc about order > 1 sounds like it would still fall into
the scope of this patch. I don not think we absolutely have to document
each unsupported gfp flags combination for GFP_NOFAIL but the order is a
good addition with a note that kvmalloc should be used instead in such a
case.

> ---
>  mm/page_alloc.c | 50 ++++++++++++++++++++++++-------------------------
>  1 file changed, 25 insertions(+), 25 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c81ee5662cc7..e790b4227322 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3033,12 +3033,6 @@ struct page *rmqueue(struct zone *preferred_zone,
>  {
>  	struct page *page;
>  
> -	/*
> -	 * We most definitely don't want callers attempting to
> -	 * allocate greater than order-1 page units with __GFP_NOFAIL.
> -	 */
> -	WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
> -
>  	if (likely(pcp_allowed_order(order))) {
>  		page = rmqueue_pcplist(preferred_zone, zone, order,
>  				       migratetype, alloc_flags);
> @@ -4175,6 +4169,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  {
>  	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
>  	bool can_compact = gfp_compaction_allowed(gfp_mask);
> +	bool nofail = gfp_mask & __GFP_NOFAIL;
>  	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
>  	struct page *page = NULL;
>  	unsigned int alloc_flags;
> @@ -4187,6 +4182,25 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	unsigned int zonelist_iter_cookie;
>  	int reserve_flags;
>  
> +	if (unlikely(nofail)) {
> +		/*
> +		 * We most definitely don't want callers attempting to
> +		 * allocate greater than order-1 page units with __GFP_NOFAIL.
> +		 */
> +		WARN_ON_ONCE(order > 1);
> +		/*
> +		 * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,
> +		 * otherwise, we may result in lockup.
> +		 */
> +		WARN_ON_ONCE(!can_direct_reclaim);
> +		/*
> +		 * PF_MEMALLOC request from this context is rather bizarre
> +		 * because we cannot reclaim anything and only can loop waiting
> +		 * for somebody to do a work for us.
> +		 */
> +		WARN_ON_ONCE(current->flags & PF_MEMALLOC);
> +	}
> +
>  restart:
>  	compaction_retries = 0;
>  	no_progress_loops = 0;
> @@ -4404,29 +4418,15 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	 * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
>  	 * we always retry
>  	 */
> -	if (gfp_mask & __GFP_NOFAIL) {
> +	if (unlikely(nofail)) {
>  		/*
> -		 * All existing users of the __GFP_NOFAIL are blockable, so warn
> -		 * of any new users that actually require GFP_NOWAIT
> +		 * Lacking direct_reclaim we can't do anything to reclaim memory,
> +		 * we disregard these unreasonable nofail requests and still
> +		 * return NULL
>  		 */
> -		if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
> +		if (!can_direct_reclaim)
>  			goto fail;
>  
> -		/*
> -		 * PF_MEMALLOC request from this context is rather bizarre
> -		 * because we cannot reclaim anything and only can loop waiting
> -		 * for somebody to do a work for us
> -		 */
> -		WARN_ON_ONCE_GFP(current->flags & PF_MEMALLOC, gfp_mask);
> -
> -		/*
> -		 * non failing costly orders are a hard requirement which we
> -		 * are not prepared for much so let's warn about these users
> -		 * so that we can identify them and convert them to something
> -		 * else.
> -		 */
> -		WARN_ON_ONCE_GFP(costly_order, gfp_mask);
> -
>  		/*
>  		 * Help non-failing allocations by giving some access to memory
>  		 * reserves normally used for high priority non-blocking
> -- 
> 2.34.1

-- 
Michal Hocko
SUSE Labs