linux-kernel - Re: [PATCH v2 2/2] mm,thp,compaction,cma: allow THP migration for CMA allocations

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Message-ID: <47198271414db19cecbfa1a6ea685577dad3a72c.camel@surriel.com>
Date:   Fri, 28 Feb 2020 09:32:04 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-kernel@...r.kernel.org
Cc:     kernel-team@...com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        mhocko@...nel.org, mgorman@...hsingularity.net,
        rientjes@...gle.com, aarcange@...hat.com, ziy@...dia.com
Subject: Re: [PATCH v2 2/2] mm,thp,compaction,cma: allow THP migration for
 CMA allocations

On Fri, 2020-02-28 at 09:25 +0100, Vlastimil Babka wrote:
> On 2/28/20 2:21 AM, Rik van Riel wrote:
> > On Thu, 2020-02-27 at 15:41 -0800, Mike Kravetz wrote:
> > > On 2/27/20 1:32 PM, Rik van Riel wrote:
> > > > +++ b/mm/page_alloc.c
> > > > @@ -8253,14 +8253,19 @@ struct page *has_unmovable_pages(struct
> > > > zone *zone, struct page *page,
> > > >  
> > > >  		/*
> > > >  		 * Hugepages are not in LRU lists, but they're
> > > > movable.
> > > > +		 * THPs are on the LRU, but need to be counted
> > > > as
> > > > #small pages.
> > > >  		 * We need not scan over tail pages because we
> > > > don't
> > > >  		 * handle each tail page individually in
> > > > migration.
> > > >  		 */
> > > > -		if (PageHuge(page)) {
> > > > +		if (PageHuge(page) || PageTransCompound(page))
> > > > {
> > > >  			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;
> > > > +
> > > > +			if (!PageLRU(head) &&
> > > > !__PageMovable(head))
> > > 
> > > Pretty sure this is going to be true for hugetlb pages.  So, this
> > > will change
> > > behavior and make all hugetlb pages look unmovable.  Perhaps,
> > > only
> > > check this
> > > condition for THP pages?
> 
> Oh right you are.
> 
> > Does that need to be the following, then?
> > 
> >      if (PageTransHuge(head) && !PageHuge(page) && !PageLRU(head)
> > &&
> > !__PageMovable(head))
> >                  return page;
> 
> I would instead make it an "else if" to the "if (PageHuge(page)...)"
> above.

That was my first thought too, but that could break on
pages that are PageHuge when hugepage_migration_supported
returns true.

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