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From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH 2/2] mm,thp,compaction,cma: allow THP migration for CMA
 allocations
To:     Rik van Riel <riel@surriel.com>, linux-kernel@vger.kernel.org
Cc:     kernel-team@fb.com, akpm@linux-foundation.org, linux-mm@kvack.org,
        mhocko@kernel.org, mgorman@techsingularity.net,
        rientjes@google.com, aarcange@redhat.com
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Date:   Wed, 26 Feb 2020 10:48:37 +0100
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On 2/25/20 7:44 PM, Rik van Riel wrote:
>> Also PageTransHuge() is basically just a PageHead() so for each
>> non-hugetlbfs compound page this will assume it's a THP, while
>> correctly
>> it should reach the __PageMovable() || PageLRU(page) tests below.
>> 
>> So probably this should do something like.
>> 
>> if (PageHuge(page) || PageTransCompound(page)) {
>> ...
>>    if (PageHuge(page) && !hpage_migration_supported)) return page.
> 
> So far so good.
> 
>>    if (!PageLRU(head) && !__PageMovable(head)) return page
> 
> I don't get this one, though. What about a THP that has
> not made it onto the LRU list yet for some reason?

Uh, is it any different from base pages which have to pass the same check? I
guess the caller could do e.g. lru_add_drain_all() first.

> I don't think anonymous pages are marked __PageMovable,
> are they? It looks like they only have the PAGE_MAPPING_ANON
> flag set, not the PAGE_MAPPING_MOVABLE one.
> 
> What am I missing?

My point is that we should not accept compound pages that are neither a
migratable hugetlbfs page nor a THP, as movable. And your PageTransHuge() test
and my PageTransCompound() is really just a test for all compound pages, not
"hugetlbfs or THP only". I should have perhaps suggested PageCompound() instead
of the PageTransCompound() wrapper, to make it more obvious.

So we should test non-hugetlbfs pages first whether they are the kind of
compound pages that are migratable. THP's should pass this test by PageLRU(),
other compound movable pages by __PageMovable(head).

> 
>> ...
>> 
>> >  			struct page *head = compound_head(page);
>> >  			unsigned int skip_pages;
>> >  
>> > -			if
>> > (!hugepage_migration_supported(page_hstate(head)))
>> > +			if (PageHuge(page) &&
>> > +			    !hugepage_migration_supported(page_hstate(h
>> > ead)))
>> >  				return page;
>> >  
>> >  			skip_pages = compound_nr(head) - (page - head);
>> > 
>> 
>> 
>