[PATCH v2 2/3] mm: introduce put_user_page[s](), placeholder versions

[john.hubbard@gmail.com]

From: John Hubbard <jhubbard@nvidia.com>

Introduces put_user_page(), which simply calls put_page().
This provides a way to update all get_user_pages*() callers,
so that they call put_user_page(), instead of put_page().

Also introduces put_user_pages(), and a few dirty/locked variations,
as a replacement for release_pages(), for the same reasons.
These may be used for subsequent performance improvements,
via batching of pages to be released.

This prepares for eventually fixing the problem described
in [1], and is following a plan listed in [2], [3], [4].

[1] https://lwn.net/Articles/753027/ : "The Trouble with get_user_pages()"

[2] https://lkml.kernel.org/r/20180709080554.21931-1-jhubbard@nvidia.com
    Proposed steps for fixing get_user_pages() + DMA problems.

[3]https://lkml.kernel.org/r/20180710082100.mkdwngdv5kkrcz6n@quack2.suse.cz
    Bounce buffers (otherwise [2] is not really viable).

[4] https://lkml.kernel.org/r/20181003162115.GG24030@quack2.suse.cz
    Follow-up discussions.

CC: Matthew Wilcox <willy@infradead.org>
CC: Michal Hocko <mhocko@kernel.org>
CC: Christopher Lameter <cl@linux.com>
CC: Jason Gunthorpe <jgg@ziepe.ca>
CC: Dan Williams <dan.j.williams@intel.com>
CC: Jan Kara <jack@suse.cz>
CC: Al Viro <viro@zeniv.linux.org.uk>
CC: Jerome Glisse <jglisse@redhat.com>
CC: Christoph Hellwig <hch@infradead.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h | 42 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 40 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a61ebe8ad4ca..1a9aae7c659f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -137,6 +137,8 @@ extern int overcommit_ratio_handler(struct ctl_table *, int, void __user *,
 				    size_t *, loff_t *);
 extern int overcommit_kbytes_handler(struct ctl_table *, int, void __user *,
 				    size_t *, loff_t *);
+int set_page_dirty(struct page *page);
+int set_page_dirty_lock(struct page *page);
 
 #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
 
@@ -943,6 +945,44 @@ static inline void put_page(struct page *page)
 		__put_page(page);
 }
 
+/* Placeholder version, until all get_user_pages*() callers are updated. */
+static inline void put_user_page(struct page *page)
+{
+	put_page(page);
+}
+
+/* For get_user_pages*()-pinned pages, use these variants instead of
+ * release_pages():
+ */
+static inline void put_user_pages_dirty(struct page **pages,
+					unsigned long npages)
+{
+	while (npages) {
+		set_page_dirty(pages[npages]);
+		put_user_page(pages[npages]);
+		--npages;
+	}
+}
+
+static inline void put_user_pages_dirty_lock(struct page **pages,
+					     unsigned long npages)
+{
+	while (npages) {
+		set_page_dirty_lock(pages[npages]);
+		put_user_page(pages[npages]);
+		--npages;
+	}
+}
+
+static inline void put_user_pages(struct page **pages,
+				  unsigned long npages)
+{
+	while (npages) {
+		put_user_page(pages[npages]);
+		--npages;
+	}
+}
+
 #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
 #define SECTION_IN_PAGE_FLAGS
 #endif
@@ -1534,8 +1574,6 @@ int redirty_page_for_writepage(struct writeback_control *wbc,
 void account_page_dirtied(struct page *page, struct address_space *mapping);
 void account_page_cleaned(struct page *page, struct address_space *mapping,
 			  struct bdi_writeback *wb);
-int set_page_dirty(struct page *page);
-int set_page_dirty_lock(struct page *page);
 void __cancel_dirty_page(struct page *page);
 static inline void cancel_dirty_page(struct page *page)
 {
-- 
2.19.0