btrfs: use page alloc/free wrappers for compression pages

This is a preparation for managing compression pages in a cache-like
manner, instead of asking the allocator each time. The common allocation
and free wrappers are introduced and are functionally equivalent to the
current code.

The freeing helpers need to be carefully placed where the last reference
is dropped.  This is either after directly allocating (error handling)
or when there are no other users of the pages (after copying the contents).

It's safe to not use the helper and use put_page() that will handle the
reference count. Not using the helper means there's lower number of
pages that could be reused without passing them back to allocator.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 15 insertions, 1 deletions

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 19b22b4653c8..1cd15d6a9c49 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -163,12 +163,26 @@ static int compression_decompress(int type, struct list_head *ws,
 static void btrfs_free_compressed_pages(struct compressed_bio *cb)
 {
 	for (unsigned int i = 0; i < cb->nr_pages; i++)
-		put_page(cb->compressed_pages[i]);
+		btrfs_free_compr_page(cb->compressed_pages[i]);
 	kfree(cb->compressed_pages);
 }
 
 static int btrfs_decompress_bio(struct compressed_bio *cb);
 
+/*
+ * Common wrappers for page allocation from compression wrappers
+ */
+struct page *btrfs_alloc_compr_page(void)
+{
+	return alloc_page(GFP_NOFS);
+}
+
+void btrfs_free_compr_page(struct page *page)
+{
+	ASSERT(page_ref_count(page) == 1);
+	put_page(page);
+}
+
 static void end_compressed_bio_read(struct btrfs_bio *bbio)
 {
 	struct compressed_bio *cb = to_compressed_bio(bbio);