linux-kernel - Re: [PATCH v3 3/6] mm: apply per-task gfp constraints in fast path

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Date:   Tue, 15 Dec 2020 09:25:24 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v3 3/6] mm: apply per-task gfp constraints in fast path

On Tue 15-12-20 00:20:39, Pavel Tatashin wrote:
> > Ack to this.
> 
> Thank you.
> 
> >
> > But I do not really understand this. All allocation contexts should have
> > a proper gfp mask so why do we have to call current_gfp_context here?
> > In fact moving the current_gfp_context in the allocator path should have
> > made all this games unnecessary. Memcg reclaim path might need some
> > careful check because gfp mask is used more creative there but the
> > general reclaim paths should be ok.
> >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> >
> > Again, why do we need this when the gfp_mask
> > >       };
> > >
> --
> 
> Hi Michal,
> 
> Beside from __alloc_pages_nodemask(), the current_gfp_context() is
> called from the following six functions:
> 
> try_to_free_pages()
> try_to_free_mem_cgroup_pages()
> __node_reclaim()
> __need_fs_reclaim()
> alloc_contig_range()
> pcpu_alloc()
> 
> As I understand, the idea is that because the allocator now honors
> gfp_context values for all paths, the call can be removed from some of
> the above functions. I think you are correct. But, at least from a
> quick glance, this is not obvious, and is not the case for all of the
> above functions.
> 
> For example:
> 
> alloc_contig_range()
>   __alloc_contig_migrate_range
>    isolate_migratepages_range
>      isolate_migratepages_block
>         /*
>          * Only allow to migrate anonymous pages in GFP_NOFS context
>          * because those do not depend on fs locks.
>          */
>        if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
>           goto isolate_fail;
> 
> If we remove current_gfp_context() from alloc_contig_range(), the
> cc->gfp_mask will not be updated with proper __GFP_FS flag.

I do not think I was proposing to drop current_gfp_context from
alloc_contig_range. ACR needs some work to be properly scoped gfp mask
aware. This should be addressed but I do not think think the code
works properly now so I wouldn't lose sleep over it in this series. At
least __alloc_contig_migrate_range should follow the gfp mask given to
alloc_contig_range.

> I have studied some other paths, and they are also convoluted.
> Therefore, I am worried about performing this optimization in this
> series.

Dropping current_gfp_context from the reclaim context should be done in
a separate patch. I didn't mean to push for this here. All I meant was
to simply not touch gfp/zone_idx in the reclaim path. The changelog
should call out that the page allocator always provides proper gfp mask.

-- 
Michal Hocko
SUSE Labs