Re: [PATCH] mm: shmem: enable thp migration (Re: [PATCH v1] mm: consider non-anonymous thp as unmovable page)

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Sun, Apr 22, 2018 at 08:03:49PM -0700, Matthew Wilcox wrote:
> On Fri, Apr 06, 2018 at 03:07:11AM +0000, Naoya Horiguchi wrote:
> > Subject: [PATCH] mm: enable thp migration for shmem thp
> 
> This patch is buggy, but not in a significant way:
> 
> > @@ -524,13 +524,26 @@ int migrate_page_move_mapping(struct address_space *mapping,
> >  	}
> >  
> >  	radix_tree_replace_slot(&mapping->i_pages, pslot, newpage);
> 
> ^^^ this line should have been deleted
> 
> > +	if (PageTransHuge(page)) {
> > +		int i;
> > +		int index = page_index(page);
> > +
> > +		for (i = 0; i < HPAGE_PMD_NR; i++) {
> ^^^ or this iteration should start at 1
> > +			pslot = radix_tree_lookup_slot(&mapping->i_pages,
> > +						       index + i);
> > +			radix_tree_replace_slot(&mapping->i_pages, pslot,
> > +						newpage + i);
> > +		}
> > +	} else {
> > +		radix_tree_replace_slot(&mapping->i_pages, pslot, newpage);
> ^^^ and if the second option, then we don't need this line
> > +	}
> 
> So either this:
> 
> -	radix_tree_replace_slot(&mapping->i_pages, pslot, newpage);
> +	if (PageTransHuge(page)) {
> +		int i;
> +		int index = page_index(page);
> +
> +		for (i = 0; i < HPAGE_PMD_NR; i++) {
> +			pslot = radix_tree_lookup_slot(&mapping->i_pages,
> +						       index + i);
> +			radix_tree_replace_slot(&mapping->i_pages, pslot,
> +						newpage + i);
> +		}
> +	} else {
> +		radix_tree_replace_slot(&mapping->i_pages, pslot, newpage);
> +	}
> 
> Or this:
> 
>  	radix_tree_replace_slot(&mapping->i_pages, pslot, newpage);
> +	if (PageTransHuge(page)) {
> +		int i;
> +		int index = page_index(page);
> +
> +		for (i = 1; i < HPAGE_PMD_NR; i++) {
> +			pslot = radix_tree_lookup_slot(&mapping->i_pages,
> +						       index + i);
> +			radix_tree_replace_slot(&mapping->i_pages, pslot,
> +						newpage + i);
> +		}
> +	}
> 
> The second one is shorter and involves fewer lookups ...

Hi Matthew,

Thank you for poinitng out, I like the second one.
The original patch is now in upsteam, so I wrote a patch on it.

Thanks,
Naoya

--------
From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Date: Sun, 22 Apr 2018 20:03:49 -0700
Subject: [PATCH] mm: migrate: fix double call of radix_tree_replace_slot()

radix_tree_replace_slot() is called twice for head page, it's
obviously a bug. Let's fix it.

Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 mm/migrate.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 568433023831..8c0af0f7cab1 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -528,14 +528,12 @@ int migrate_page_move_mapping(struct address_space *mapping,
 		int i;
 		int index = page_index(page);
 
-		for (i = 0; i < HPAGE_PMD_NR; i++) {
+		for (i = 1; i < HPAGE_PMD_NR; i++) {
 			pslot = radix_tree_lookup_slot(&mapping->i_pages,
 						       index + i);
 			radix_tree_replace_slot(&mapping->i_pages, pslot,
 						newpage + i);
 		}
-	} else {
-		radix_tree_replace_slot(&mapping->i_pages, pslot, newpage);
 	}
 
 	/*
-- 
2.7.4