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

[Pavel Tatashin]

> 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.