linux-kernel - [PATCH v4] mm/swap: Fix release_pages() when releasing devmap pages

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Date:   Wed,  5 Jun 2019 14:49:22 -0700
From:   ira.weiny@...el.com
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH v4] mm/swap: Fix release_pages() when releasing devmap pages

From: Ira Weiny <ira.weiny@...el.com>

release_pages() is an optimized version of a loop around put_page().
Unfortunately for devmap pages the logic is not entirely correct in
release_pages().  This is because device pages can be more than type
MEMORY_DEVICE_PUBLIC.  There are in fact 4 types, private, public, FS
DAX, and PCI P2PDMA.  Some of these have specific needs to "put" the
page while others do not.

This logic to handle any special needs is contained in
put_devmap_managed_page().  Therefore all devmap pages should be
processed by this function where we can contain the correct logic for a
page put.

Handle all device type pages within release_pages() by calling
put_devmap_managed_page() on all devmap pages.  If
put_devmap_managed_page() returns true the page has been put and we
continue with the next page.  A false return of
put_devmap_managed_page() means the page did not require special
processing and should fall to "normal" processing.

This was found via code inspection while determining if release_pages()
and the new put_user_pages() could be interchangeable.[1]

[1] https://lore.kernel.org/lkml/20190523172852.GA27175@iweiny-DESK2.sc.intel.com/

Cc: Jérôme Glisse <jglisse@...hat.com>
Cc: Michal Hocko <mhocko@...e.com>
Reviewed-by: Dan Williams <dan.j.williams@...el.com>
Reviewed-by: John Hubbard <jhubbard@...dia.com>
Signed-off-by: Ira Weiny <ira.weiny@...el.com>

---
Changes from V3:
	Update comment to the one provided by John

Changes from V2:
	Update changelog for more clarity as requested by Michal
	Update comment WRT "failing" of put_devmap_managed_page()

Changes from V1:
	Add comment clarifying that put_devmap_managed_page() can still
	fail.
	Add Reviewed-by tags.

 mm/swap.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index 7ede3eddc12a..607c48229a1d 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -740,15 +740,20 @@ void release_pages(struct page **pages, int nr)
 		if (is_huge_zero_page(page))
 			continue;
 
-		/* Device public page can not be huge page */
-		if (is_device_public_page(page)) {
+		if (is_zone_device_page(page)) {
 			if (locked_pgdat) {
 				spin_unlock_irqrestore(&locked_pgdat->lru_lock,
 						       flags);
 				locked_pgdat = NULL;
 			}
-			put_devmap_managed_page(page);
-			continue;
+			/*
+			 * ZONE_DEVICE pages that return 'false' from
+			 * put_devmap_managed_page() do not require special
+			 * processing, and instead, expect a call to
+			 * put_page_testzero().
+			 */
+			if (put_devmap_managed_page(page))
+				continue;
 		}
 
 		page = compound_head(page);
-- 
2.20.1