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>

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);

fs/btrfs/compression.h

@@ -32,6 +32,8 @@ static_assert((BTRFS_MAX_COMPRESSED % PAGE_SIZE) == 0);
 
 #define	BTRFS_ZLIB_DEFAULT_LEVEL		3
 
+struct page;
+
 struct compressed_bio {
 	/* Number of compressed pages in the array */
 	unsigned int nr_pages;
@@ -96,6 +98,9 @@ void btrfs_submit_compressed_read(struct btrfs_bio *bbio);
 
 unsigned int btrfs_compress_str2level(unsigned int type, const char *str);
 
+struct page *btrfs_alloc_compr_page(void);
+void btrfs_free_compr_page(struct page *page);
+
 enum btrfs_compression_type {
 	BTRFS_COMPRESS_NONE  = 0,
 	BTRFS_COMPRESS_ZLIB  = 1,

fs/btrfs/inode.c

@@ -1037,7 +1037,7 @@ static void compress_file_range(struct btrfs_work *work)
 	if (pages) {
 		for (i = 0; i < nr_pages; i++) {
 			WARN_ON(pages[i]->mapping);
-			put_page(pages[i]);
+			btrfs_free_compr_page(pages[i]);
 		}
 		kfree(pages);
 	}
@@ -1052,7 +1052,7 @@ static void free_async_extent_pages(struct async_extent *async_extent)
 
 	for (i = 0; i < async_extent->nr_pages; i++) {
 		WARN_ON(async_extent->pages[i]->mapping);
-		put_page(async_extent->pages[i]);
+		btrfs_free_compr_page(async_extent->pages[i]);
 	}
 	kfree(async_extent->pages);
 	async_extent->nr_pages = 0;

fs/btrfs/lzo.c

@@ -152,7 +152,7 @@ static int copy_compressed_data_to_page(char *compressed_data,
 	cur_page = out_pages[*cur_out / PAGE_SIZE];
 	/* Allocate a new page */
 	if (!cur_page) {
-		cur_page = alloc_page(GFP_NOFS);
+		cur_page = btrfs_alloc_compr_page();
 		if (!cur_page)
 			return -ENOMEM;
 		out_pages[*cur_out / PAGE_SIZE] = cur_page;
@@ -178,7 +178,7 @@ static int copy_compressed_data_to_page(char *compressed_data,
 		cur_page = out_pages[*cur_out / PAGE_SIZE];
 		/* Allocate a new page */
 		if (!cur_page) {
-			cur_page = alloc_page(GFP_NOFS);
+			cur_page = btrfs_alloc_compr_page();
 			if (!cur_page)
 				return -ENOMEM;
 			out_pages[*cur_out / PAGE_SIZE] = cur_page;

fs/btrfs/zlib.c

@@ -121,7 +121,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 	workspace->strm.total_in = 0;
 	workspace->strm.total_out = 0;
 
-	out_page = alloc_page(GFP_NOFS);
+	out_page = btrfs_alloc_compr_page();
 	if (out_page == NULL) {
 		ret = -ENOMEM;
 		goto out;
@@ -200,7 +200,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 				ret = -E2BIG;
 				goto out;
 			}
-			out_page = alloc_page(GFP_NOFS);
+			out_page = btrfs_alloc_compr_page();
 			if (out_page == NULL) {
 				ret = -ENOMEM;
 				goto out;
@@ -236,7 +236,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 				ret = -E2BIG;
 				goto out;
 			}
-			out_page = alloc_page(GFP_NOFS);
+			out_page = btrfs_alloc_compr_page();
 			if (out_page == NULL) {
 				ret = -ENOMEM;
 				goto out;

fs/btrfs/zstd.c

@@ -410,9 +410,8 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 	workspace->in_buf.pos = 0;
 	workspace->in_buf.size = min_t(size_t, len, PAGE_SIZE);
 
-
 	/* Allocate and map in the output buffer */
-	out_page = alloc_page(GFP_NOFS);
+	out_page = btrfs_alloc_compr_page();
 	if (out_page == NULL) {
 		ret = -ENOMEM;
 		goto out;
@@ -457,7 +456,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 				ret = -E2BIG;
 				goto out;
 			}
-			out_page = alloc_page(GFP_NOFS);
+			out_page = btrfs_alloc_compr_page();
 			if (out_page == NULL) {
 				ret = -ENOMEM;
 				goto out;
@@ -514,7 +513,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
 			ret = -E2BIG;
 			goto out;
 		}
-		out_page = alloc_page(GFP_NOFS);
+		out_page = btrfs_alloc_compr_page();
 		if (out_page == NULL) {
 			ret = -ENOMEM;
 			goto out;