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

[Rik van Riel <riel@surriel.com>]

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On Mon, 2020-02-24 at 16:29 +0100, Vlastimil Babka wrote:
> On 2/21/20 10:53 PM, Rik van Riel wrote:
> > @@ -981,7 +981,9 @@ isolate_migratepages_block(struct
> > compact_control *cc, unsigned long low_pfn,
> >  		if (__isolate_lru_page(page, isolate_mode) != 0)
> >  			goto isolate_fail;
> >  
> > -		VM_BUG_ON_PAGE(PageCompound(page), page);
> > +		/* The whole page is taken off the LRU; skip the tail
> > pages. */
> > +		if (PageCompound(page))
> > +			low_pfn += compound_nr(page) - 1;
> >  
> >  		/* Successfully isolated */
> >  		del_page_from_lru_list(page, lruvec, page_lru(page));
> 
> This continues by:
> inc_node_page_state(page, NR_ISOLATED_ANON +
> page_is_file_cache(page));
> 
> 
> I think it now needs to use mod_node_page_state() with
> hpage_nr_pages(page) otherwise the counter will underflow after the
> migration?

You are absolutely right. I have not observed the
underflow, but the functions doing the decrementing
use hpage_nr_pages, and I need to do that as well
on the incrementing side.

Change made.

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index a36736812596..38c8ddfcecc8 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -8253,14 +8253,16 @@ 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 (PageTransHuge(page)) {
> 
> Hmm, PageTransHuge() has VM_BUG_ON() for tail pages, while this code
> is
> written so that it can encounter a tail page and skip the rest of the
> compound page properly. So I would be worried about this.

Good point, a CMA allocation could start partway into a
compound page. 

> 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?

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?

> ...
> 
> >  			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);
> > 
> 
> 
-- 
All Rights Reversed.

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