btrfs: Preallocate chunks in cow_file_range_async

This commit changes the implementation of cow_file_range_async in order
to get rid of the BUG_ON in the middle of the loop. Additionally it
reworks the inner loop in the hopes of making it more understandable.

The idea is to make async_cow be a top-level structured, shared amongst
all chunks being sent for compression. This allows to perform one memory
allocation at the beginning and gracefully fail the IO if there isn't
enough memory. Now, each chunk is going to be described by an
async_chunk struct. It's the responsibility of the final chunk
to actually free the memory.
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -355,7 +355,7 @@ struct async_extent {
 	struct list_head list;
 };
 
-struct async_cow {
+struct async_chunk {
 	struct inode *inode;
 	struct btrfs_fs_info *fs_info;
 	struct page *locked_page;
@@ -364,9 +364,16 @@ struct async_cow {
 	unsigned int write_flags;
 	struct list_head extents;
 	struct btrfs_work work;
+	atomic_t *pending;
+};
+
+struct async_cow {
+	/* Number of chunks in flight; must be first in the structure */
+	atomic_t num_chunks;
+	struct async_chunk chunks[];
 };
 
-static noinline int add_async_extent(struct async_cow *cow,
+static noinline int add_async_extent(struct async_chunk *cow,
 				     u64 start, u64 ram_size,
 				     u64 compressed_size,
 				     struct page **pages,
@@ -436,7 +443,7 @@ static inline void inode_should_defrag(struct btrfs_inode *inode,
 static noinline void compress_file_range(struct inode *inode,
 					struct page *locked_page,
 					u64 start, u64 end,
-					struct async_cow *async_cow,
+					struct async_chunk *async_cow,
 					int *num_added)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
@@ -702,7 +709,7 @@ static void free_async_extent_pages(struct async_extent *async_extent)
  * queued.  We walk all the async extents created by compress_file_range
  * and send them down to the disk.
  */
-static noinline void submit_compressed_extents(struct async_cow *async_cow)
+static noinline void submit_compressed_extents(struct async_chunk *async_cow)
 {
 	struct inode *inode = async_cow->inode;
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
@@ -1121,9 +1128,10 @@ static noinline int cow_file_range(struct inode *inode,
  */
 static noinline void async_cow_start(struct btrfs_work *work)
 {
-	struct async_cow *async_cow;
+	struct async_chunk *async_cow;
 	int num_added = 0;
-	async_cow = container_of(work, struct async_cow, work);
+
+	async_cow = container_of(work, struct async_chunk, work);
 
 	compress_file_range(async_cow->inode, async_cow->locked_page,
 			    async_cow->start, async_cow->end, async_cow,
@@ -1140,10 +1148,10 @@ static noinline void async_cow_start(struct btrfs_work *work)
 static noinline void async_cow_submit(struct btrfs_work *work)
 {
 	struct btrfs_fs_info *fs_info;
-	struct async_cow *async_cow;
+	struct async_chunk *async_cow;
 	unsigned long nr_pages;
 
-	async_cow = container_of(work, struct async_cow, work);
+	async_cow = container_of(work, struct async_chunk, work);
 
 	fs_info = async_cow->fs_info;
 	nr_pages = (async_cow->end - async_cow->start + PAGE_SIZE) >>
@@ -1166,11 +1174,17 @@ static noinline void async_cow_submit(struct btrfs_work *work)
 
 static noinline void async_cow_free(struct btrfs_work *work)
 {
-	struct async_cow *async_cow;
-	async_cow = container_of(work, struct async_cow, work);
+	struct async_chunk *async_cow;
+
+	async_cow = container_of(work, struct async_chunk, work);
 	if (async_cow->inode)
 		btrfs_add_delayed_iput(async_cow->inode);
-	kfree(async_cow);
+	/*
+	 * Since the pointer to 'pending' is at the beginning of the array of
+	 * async_cow's, freeing it ensures the whole array has been freed.
+	 */
+	if (atomic_dec_and_test(async_cow->pending))
+		kfree(async_cow->pending);
 }
 
 static int cow_file_range_async(struct inode *inode, struct page *locked_page,
@@ -1179,45 +1193,71 @@ static int cow_file_range_async(struct inode *inode, struct page *locked_page,
 				unsigned int write_flags)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
-	struct async_cow *async_cow;
+	struct async_cow *ctx;
+	struct async_chunk *async_chunk;
 	unsigned long nr_pages;
 	u64 cur_end;
+	u64 num_chunks = DIV_ROUND_UP(end - start, SZ_512K);
+	int i;
+	bool should_compress;
 
 	clear_extent_bit(&BTRFS_I(inode)->io_tree, start, end, EXTENT_LOCKED,
 			 1, 0, NULL);
-	while (start < end) {
-		async_cow = kmalloc(sizeof(*async_cow), GFP_NOFS);
-		BUG_ON(!async_cow); /* -ENOMEM */
+
+	if (BTRFS_I(inode)->flags & BTRFS_INODE_NOCOMPRESS &&
+	    !btrfs_test_opt(fs_info, FORCE_COMPRESS)) {
+		num_chunks = 1;
+		should_compress = false;
+	} else {
+		should_compress = true;
+	}
+
+	ctx = kmalloc(struct_size(ctx, chunks, num_chunks), GFP_NOFS);
+	if (!ctx) {
+		unsigned clear_bits = EXTENT_LOCKED | EXTENT_DELALLOC |
+			EXTENT_DELALLOC_NEW | EXTENT_DEFRAG |
+			EXTENT_DO_ACCOUNTING;
+		unsigned long page_ops = PAGE_UNLOCK | PAGE_CLEAR_DIRTY |
+			PAGE_SET_WRITEBACK | PAGE_END_WRITEBACK |
+			PAGE_SET_ERROR;
+
+		extent_clear_unlock_delalloc(inode, start, end, 0, locked_page,
+					     clear_bits, page_ops);
+		return -ENOMEM;
+	}
+
+	async_chunk = ctx->chunks;
+	atomic_set(&ctx->num_chunks, num_chunks);
+
+	for (i = 0; i < num_chunks; i++) {
+		if (should_compress)
+			cur_end = min(end, start + SZ_512K - 1);
+		else
+			cur_end = end;
+
 		/*
 		 * igrab is called higher up in the call chain, take only the
 		 * lightweight reference for the callback lifetime
 		 */
 		ihold(inode);
-		async_cow->inode = inode;
-		async_cow->fs_info = fs_info;
-		async_cow->locked_page = locked_page;
-		async_cow->start = start;
-		async_cow->write_flags = write_flags;
-
-		if (BTRFS_I(inode)->flags & BTRFS_INODE_NOCOMPRESS &&
-		    !btrfs_test_opt(fs_info, FORCE_COMPRESS))
-			cur_end = end;
-		else
-			cur_end = min(end, start + SZ_512K - 1);
-
-		async_cow->end = cur_end;
-		INIT_LIST_HEAD(&async_cow->extents);
-
-		btrfs_init_work(&async_cow->work,
+		async_chunk[i].pending = &ctx->num_chunks;
+		async_chunk[i].inode = inode;
+		async_chunk[i].start = start;
+		async_chunk[i].end = cur_end;
+		async_chunk[i].fs_info = fs_info;
+		async_chunk[i].locked_page = locked_page;
+		async_chunk[i].write_flags = write_flags;
+		INIT_LIST_HEAD(&async_chunk[i].extents);
+
+		btrfs_init_work(&async_chunk[i].work,
 				btrfs_delalloc_helper,
 				async_cow_start, async_cow_submit,
 				async_cow_free);
 
-		nr_pages = (cur_end - start + PAGE_SIZE) >>
-			PAGE_SHIFT;
+		nr_pages = DIV_ROUND_UP(cur_end - start, PAGE_SIZE);
 		atomic_add(nr_pages, &fs_info->async_delalloc_pages);
 
-		btrfs_queue_work(fs_info->delalloc_workers, &async_cow->work);
+		btrfs_queue_work(fs_info->delalloc_workers, &async_chunk[i].work);
 
 		*nr_written += nr_pages;
 		start = cur_end + 1;