btrfs: extract the max compression chunk size into a macro

We have two locations using open-coded 512K size, as the async chunk
size.

For compression we have not only the max size a compressed extent can
represent (128K), but also how large an async chunk can be (512K).

Although we have a macro for the maximum compressed extent size, we do
not have any macro for the async chunk size.

Add such a macro and replace the two open-coded SZ_512K.

Reviewed-by: Filipe Manana <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: adam900710

fs/btrfs/compression.h

@@ -36,6 +36,9 @@ struct btrfs_ordered_extent;
 #define BTRFS_MAX_COMPRESSED_PAGES	(BTRFS_MAX_COMPRESSED / PAGE_SIZE)
 static_assert((BTRFS_MAX_COMPRESSED % PAGE_SIZE) == 0);
 
+/* The max size for a single worker to compress. */
+#define BTRFS_COMPRESSION_CHUNK_SIZE	(SZ_512K)
+
 /* Maximum size of data before compression */
 #define BTRFS_MAX_UNCOMPRESSED		(SZ_128K)
 

fs/btrfs/inode.c

@@ -1691,7 +1691,7 @@ static bool run_delalloc_compressed(struct btrfs_inode *inode,
 	struct async_cow *ctx;
 	struct async_chunk *async_chunk;
 	unsigned long nr_pages;
-	u64 num_chunks = DIV_ROUND_UP(end - start, SZ_512K);
+	u64 num_chunks = DIV_ROUND_UP(end - start, BTRFS_COMPRESSION_CHUNK_SIZE);
 	int i;
 	unsigned nofs_flag;
 	const blk_opf_t write_flags = wbc_to_write_flags(wbc);
@@ -1708,7 +1708,7 @@ static bool run_delalloc_compressed(struct btrfs_inode *inode,
 	atomic_set(&ctx->num_chunks, num_chunks);
 
 	for (i = 0; i < num_chunks; i++) {
-		u64 cur_end = min(end, start + SZ_512K - 1);
+		u64 cur_end = min(end, start + BTRFS_COMPRESSION_CHUNK_SIZE - 1);
 
 		/*
 		 * igrab is called higher up in the call chain, take only the