linux-kernel - Re: [PATCH 1/7] mm/page_alloc: Move gfp_allowed_mask enforcement to prepare_alloc

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Date:   Fri, 19 Mar 2021 17:11:39 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-Net <netdev@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-NFS <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH 1/7] mm/page_alloc: Move gfp_allowed_mask enforcement to
 prepare_alloc_pages

On 3/12/21 4:43 PM, Mel Gorman wrote:
> __alloc_pages updates GFP flags to enforce what flags are allowed
> during a global context such as booting or suspend. This patch moves the
> enforcement from __alloc_pages to prepare_alloc_pages so the code can be
> shared between the single page allocator and a new bulk page allocator.
> 
> When moving, it is obvious that __alloc_pages() and __alloc_pages
> use different names for the same variable. This is an unnecessary
> complication so rename gfp_mask to gfp in prepare_alloc_pages() so the
> name is consistent.
> 
> No functional change.

Hm, I have some doubts.

> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
>  mm/page_alloc.c | 25 +++++++++++++------------
>  1 file changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 00b67c47ad87..f0c1d74ead6f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4914,15 +4914,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	return page;
>  }
>  
> -static inline bool prepare_alloc_pages(gfp_t gfp_mask, unsigned int order,
> +static inline bool prepare_alloc_pages(gfp_t gfp, unsigned int order,
>  		int preferred_nid, nodemask_t *nodemask,
>  		struct alloc_context *ac, gfp_t *alloc_gfp,
>  		unsigned int *alloc_flags)
>  {
> -	ac->highest_zoneidx = gfp_zone(gfp_mask);
> -	ac->zonelist = node_zonelist(preferred_nid, gfp_mask);
> +	gfp &= gfp_allowed_mask;
> +	*alloc_gfp = gfp;
> +

...

> @@ -4980,8 +4983,6 @@ struct page *__alloc_pages(gfp_t gfp, unsigned int order, int preferred_nid,
>  		return NULL;
>  	}
>  
> -	gfp &= gfp_allowed_mask;
> -	alloc_gfp = gfp;
>  	if (!prepare_alloc_pages(gfp, order, preferred_nid, nodemask, &ac,
>  			&alloc_gfp, &alloc_flags))
>  		return NULL;

As a result, "gfp" doesn't have the restrictions by gfp_allowed_mask applied,
only alloc_gfp does. But in case we go to slowpath, before
going there we throw away the current alloc_gfp:

    alloc_gfp = current_gfp_context(gfp);
    ...
    page = __alloc_pages_slowpath(alloc_gfp, ...);

So we lost the gfp_allowed_mask restrictions here?