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From:   Pavel Tatashin <pasha.tatashin@soleen.com>
Date:   Tue, 15 Dec 2020 00:20:39 -0500
Message-ID: <CA+CK2bA9u_ZUosha0JvW6ezs-h95UBHZztoFojMyFbC791ximw@mail.gmail.com>
Subject: Re: [PATCH v3 3/6] mm: apply per-task gfp constraints in fast path
To:     Michal Hocko <mhocko@suse.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        Oscar Salvador <osalvador@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Sasha Levin <sashal@kernel.org>,
        Tyler Hicks <tyhicks@linux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, mike.kravetz@oracle.com,
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        Ingo Molnar <mingo@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        David Rientjes <rientjes@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>
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> 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 have studied some other paths, and they are also convoluted.
Therefore, I am worried about performing this optimization in this
series.