[PATCH v12] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages

John Hubbard
2019-12-19T00:41:34Z

An upcoming patch changes and complicates the refcounting and
especially the "put page" aspects of it. In order to keep
everything clean, refactor the devmap page release routines:

* Rename put_devmap_managed_page() to page_is_devmap_managed(),
and limit the functionality to "read only": return a bool,
with no side effects.

* Add a new routine, put_devmap_managed_page(), to handle
decrementing the refcount for ZONE_DEVICE pages.

* Change callers (just release_pages() and put_page()) to check
page_is_devmap_managed() before calling the new
put_devmap_managed_page() routine. This is a performance
point: put_page() is a hot path, so we need to avoid non-
inline function calls where possible.

* Rename __put_devmap_managed_page() to free_devmap_managed_page(),
and limit the functionality to unconditionally freeing a devmap
page.

This is originally based on a separate patch by Ira Weiny, which
applied to an early version of the put_user_page() experiments.
Since then, Jérôme Glisse suggested the refactoring described above.

Cc: Christoph Hellwig <hch@lst.de>
Suggested-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
include/linux/mm.h | 18 +++++++++++++-----
mm/memremap.c | 16 ++--------------
mm/swap.c | 27 ++++++++++++++++++++++++++-
3 files changed, 41 insertions(+), 20 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c97ea3b694e6..87b54126e46d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -952,9 +952,10 @@ static inline bool is_zone_device_page(const struct page *page)
#endif

#ifdef CONFIG_DEV_PAGEMAP_OPS
-void __put_devmap_managed_page(struct page *page);
+void free_devmap_managed_page(struct page *page);
DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
-static inline bool put_devmap_managed_page(struct page *page)
+
+static inline bool page_is_devmap_managed(struct page *page)
{
if (!static_branch_unlikely(&devmap_managed_key))
return false;
@@ -963,7 +964,6 @@ static inline bool put_devmap_managed_page(struct page *page)
switch (page->pgmap->type) {
case MEMORY_DEVICE_PRIVATE:
case MEMORY_DEVICE_FS_DAX:
- __put_devmap_managed_page(page);
return true;
default:
break;
@@ -971,11 +971,17 @@ static inline bool put_devmap_managed_page(struct page *page)
return false;
}

+void put_devmap_managed_page(struct page *page);
+
#else /* CONFIG_DEV_PAGEMAP_OPS */
-static inline bool put_devmap_managed_page(struct page *page)
+static inline bool page_is_devmap_managed(struct page *page)
{
return false;
}
+
+static inline void put_devmap_managed_page(struct page *page)
+{
+}
#endif /* CONFIG_DEV_PAGEMAP_OPS */

static inline bool is_device_private_page(const struct page *page)
@@ -1028,8 +1034,10 @@ static inline void put_page(struct page *page)
* need to inform the device driver through callback. See
* include/linux/memremap.h and HMM for details.
*/
- if (put_devmap_managed_page(page))
+ if (page_is_devmap_managed(page)) {
+ put_devmap_managed_page(page);
return;
+ }

if (put_page_testzero(page))
__put_page(page);
diff --git a/mm/memremap.c b/mm/memremap.c
index e899fa876a62..2ba773859031 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -411,20 +411,8 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
EXPORT_SYMBOL_GPL(get_dev_pagemap);

#ifdef CONFIG_DEV_PAGEMAP_OPS
-void __put_devmap_managed_page(struct page *page)
+void free_devmap_managed_page(struct page *page)
{
- int count = page_ref_dec_return(page);
-
- /* still busy */
- if (count > 1)
- return;
-
- /* only triggered by the dev_pagemap shutdown path */
- if (count == 0) {
- __put_page(page);
- return;
- }
-
/* notify page idle for dax */
if (!is_device_private_page(page)) {
wake_up_var(&page->_refcount);
@@ -461,5 +449,5 @@ void __put_devmap_managed_page(struct page *page)
page->mapping = NULL;
page->pgmap->ops->page_free(page);
}
-EXPORT_SYMBOL(__put_devmap_managed_page);
+EXPORT_SYMBOL(free_devmap_managed_page);
#endif /* CONFIG_DEV_PAGEMAP_OPS */
diff --git a/mm/swap.c b/mm/swap.c
index 5341ae93861f..cf39d24ada2a 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -813,8 +813,10 @@ void release_pages(struct page **pages, int nr)
* processing, and instead, expect a call to
* put_page_testzero().
*/
- if (put_devmap_managed_page(page))
+ if (page_is_devmap_managed(page)) {
+ put_devmap_managed_page(page);
continue;
+ }
}

page = compound_head(page);
@@ -1102,3 +1104,26 @@ void __init swap_setup(void)
* _really_ don't want to cluster much more
*/
}
+
+#ifdef CONFIG_DEV_PAGEMAP_OPS
+void put_devmap_managed_page(struct page *page)
+{
+ int count;
+
+ if (WARN_ON_ONCE(!page_is_devmap_managed(page)))
+ return;
+
+ count = page_ref_dec_return(page);
+
+ /*
+ * devmap page refcounts are 1-based, rather than 0-based: if
+ * refcount is 1, then the page is free and the refcount is
+ * stable because nobody holds a reference on the page.
+ */
+ if (count == 1)
+ free_devmap_managed_page(page);
+ else if (!count)
+ __put_page(page);
+}
+EXPORT_SYMBOL(put_devmap_managed_page);
+#endif
--
2.24.1