linux-kernel - [RFC 2/6] mm: keep dirty bit on anonymous page migration

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Date:	Wed,  3 Jun 2015 15:15:41 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Hugh Dickins <hughd@...gle.com>, Rik van Riel <riel@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>
Subject: [RFC 2/6] mm: keep dirty bit on anonymous page migration

Currently, If VM migrates anonymous page, we lose dirty bit of
page table entry. Instead, VM translates dirty bit of page table
as PG_dirty of page flags. It was okay because dirty bit of
page table for anonymous page was no matter to swap out.
Instead, VM took care of PG_dirty.

However, with introducing MADV_FREE, it's important to keep
page table's dirty bit because It could make MADV_FREE handling
logics more straighforward without taking care of PG_dirty.

This patch aims for preparing to remove PG_dirty check for MADV_FREE.

Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 mm/migrate.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 236ee25e79d9..add30c3aaaa9 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -151,6 +151,10 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
 	if (is_write_migration_entry(entry))
 		pte = maybe_mkwrite(pte, vma);
 
+	/* MADV_FREE relies on pte_dirty. */
+	if (PageAnon(new))
+		pte = pte_mkdirty(pte);
+
 #ifdef CONFIG_HUGETLB_PAGE
 	if (PageHuge(new)) {
 		pte = pte_mkhuge(pte);
-- 
1.9.1

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