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Subject[PATCH v3] mm/swap: Fix release_pages() when releasing devmap pages
Date
From: Ira Weiny <ira.weiny@intel.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@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

---
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..6d153ce4cb8c 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;
+ /*
+ * Not all zone-device-pages require special
+ * processing. Those pages return 'false' from
+ * put_devmap_managed_page() expecting a call to
+ * put_page_testzero()
+ */
+ if (put_devmap_managed_page(page))
+ continue;
}

page = compound_head(page);
--
2.20.1
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