[RFC PATCH v2 2/3] btrfs: Use kmap_local_page() on "out_page" in zlib_compress_pages()

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

The use of kmap() is being deprecated in favor of kmap_local_page(). With
kmap_local_page(), the mapping is per thread, CPU local and not globally
visible.

Therefore, use kmap_local_page() / kunmap_local() for "out_page" in
zlib_compress_pages() because in this function the mappings are per thread
and are not visible in other contexts.

Tested with xfstests on QEMU + KVM 32 bits VM with 4GB of RAM and
HIGHMEM64G enabled. This patch passes 26/26 tests of group "compress".

Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
 fs/btrfs/zlib.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/zlib.c b/fs/btrfs/zlib.c
index 770c4c6bbaef..c7c69ce4a1a9 100644
--- a/fs/btrfs/zlib.c
+++ b/fs/btrfs/zlib.c
@@ -97,8 +97,8 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 {
 	struct workspace *workspace = list_entry(ws, struct workspace, list);
 	int ret;
-	char *data_in;
-	char *cpage_out;
+	char *data_in = NULL;
+	char *cpage_out = NULL;
 	int nr_pages = 0;
 	struct page *in_page = NULL;
 	struct page *out_page = NULL;
@@ -126,7 +126,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 		ret = -ENOMEM;
 		goto out;
 	}
-	cpage_out = kmap(out_page);
+	cpage_out = kmap_local_page(out_page);
 	pages[0] = out_page;
 	nr_pages = 1;
 
@@ -196,7 +196,8 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 		 * the stream end if required
 		 */
 		if (workspace->strm.avail_out == 0) {
-			kunmap(out_page);
+			kunmap_local(cpage_out);
+			cpage_out = NULL;
 			if (nr_pages == nr_dest_pages) {
 				out_page = NULL;
 				ret = -E2BIG;
@@ -207,7 +208,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 				ret = -ENOMEM;
 				goto out;
 			}
-			cpage_out = kmap(out_page);
+			cpage_out = kmap_local_page(out_page);
 			pages[nr_pages] = out_page;
 			nr_pages++;
 			workspace->strm.avail_out = PAGE_SIZE;
@@ -234,7 +235,8 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 			goto out;
 		} else if (workspace->strm.avail_out == 0) {
 			/* get another page for the stream end */
-			kunmap(out_page);
+			kunmap_local(cpage_out);
+			cpage_out = NULL;
 			if (nr_pages == nr_dest_pages) {
 				out_page = NULL;
 				ret = -E2BIG;
@@ -245,7 +247,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 				ret = -ENOMEM;
 				goto out;
 			}
-			cpage_out = kmap(out_page);
+			cpage_out = kmap_local_page(out_page);
 			pages[nr_pages] = out_page;
 			nr_pages++;
 			workspace->strm.avail_out = PAGE_SIZE;
@@ -264,8 +266,8 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 	*total_in = workspace->strm.total_in;
 out:
 	*out_pages = nr_pages;
-	if (out_page)
-		kunmap(out_page);
+	if (cpage_out)
+		kunmap_local(cpage_out);
 
 	if (in_page) {
 		kunmap(in_page);
-- 
2.36.1