Re: [PATCH v3 0/4] mm: clarify nofail memory allocation

[Michal Hocko <mhocko@suse.com>]

On Fri 30-08-24 10:31:14, Barry Song wrote:
> > > > Patch 4/4: We will move the order > 1 check from the current fast path
> > > > to the slow path and extend
> > > >                  the check of gfp_direct_reclaim flag also in the slow path.
> > >
> > > OK, let's have that go in now as well.
> 
> Hi Michal and Vlastimil,
> Could you please review the changes below before I send v4 for patch 4/4?
> 
> 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.I don't think we 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 I'd rather use WARN_ON_ONCE than WARN_ON_ONCE_GFP.
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c81ee5662cc7..0d3dd679d0ab 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);
> @@ -4174,6 +4168,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  						struct alloc_context *ac)
>  {
>  	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
> +	bool nofail = gfp_mask & __GFP_DIRECT_RECLAIM;
>  	bool can_compact = gfp_compaction_allowed(gfp_mask);
>  	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
>  	struct page *page = NULL;
> @@ -4187,6 +4182,25 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	unsigned int zonelist_iter_cookie;
>  	int reserve_flags;
>  
> +	if (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);
> +	}

Yes, this makes sense. Any reason you have not put that int the nofail
branch below?

> +
>  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 (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
> 
> > >
> > > --
> > > Michal Hocko
> > > SUSE Labs
> 
> Thanks
> Barry

-- 
Michal Hocko
SUSE Labs