btrfs: don't fail writeback when allocating the compression context fails

If cow_file_range_async fails to allocate the asynchronous writeback
context, it currently returns an error and entirely fails the writeback.
This is not a good idea as a writeback failure is a non-temporary error
condition that will make the file system unusable.  Just fall back to
synchronous uncompressed writeback instead.  This requires us to delay
setting the BTRFS_INODE_HAS_ASYNC_EXTENT flag until we've committed to
the async writeback.

The compression checks INODE_NOCOMPRESS and FORCE_COMPRESS are moved
from cow_file_range_async to the preceding checks btrfs_run_delalloc_range().
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -1720,7 +1720,7 @@ static noinline void async_cow_free(struct btrfs_work *work)
 		kvfree(async_cow);
 }
 
-static int cow_file_range_async(struct btrfs_inode *inode,
+static bool cow_file_range_async(struct btrfs_inode *inode,
 				 struct writeback_control *wbc,
 				 struct page *locked_page,
 				 u64 start, u64 end, int *page_started,
@@ -1731,47 +1731,25 @@ static int cow_file_range_async(struct btrfs_inode *inode,
 	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;
 	unsigned nofs_flag;
 	const blk_opf_t write_flags = wbc_to_write_flags(wbc);
 
-	unlock_extent(&inode->io_tree, start, end, NULL);
-
-	if (inode->flags & BTRFS_INODE_NOCOMPRESS &&
-	    !btrfs_test_opt(fs_info, FORCE_COMPRESS)) {
-		num_chunks = 1;
-		should_compress = false;
-	} else {
-		should_compress = true;
-	}
-
 	nofs_flag = memalloc_nofs_save();
 	ctx = kvmalloc(struct_size(ctx, chunks, num_chunks), GFP_KERNEL);
 	memalloc_nofs_restore(nofs_flag);
+	if (!ctx)
+		return false;
 
-	if (!ctx) {
-		unsigned clear_bits = EXTENT_LOCKED | EXTENT_DELALLOC |
-			EXTENT_DELALLOC_NEW | EXTENT_DEFRAG |
-			EXTENT_DO_ACCOUNTING;
-		unsigned long page_ops = PAGE_UNLOCK | PAGE_START_WRITEBACK |
-					 PAGE_END_WRITEBACK | PAGE_SET_ERROR;
-
-		extent_clear_unlock_delalloc(inode, start, end, locked_page,
-					     clear_bits, page_ops);
-		return -ENOMEM;
-	}
+	unlock_extent(&inode->io_tree, start, end, NULL);
+	set_bit(BTRFS_INODE_HAS_ASYNC_EXTENT, &inode->runtime_flags);
 
 	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;
+		u64 cur_end = min(end, start + SZ_512K - 1);
 
 		/*
 		 * igrab is called higher up in the call chain, take only the
@@ -1832,7 +1810,7 @@ static int cow_file_range_async(struct btrfs_inode *inode,
 		start = cur_end + 1;
 	}
 	*page_started = 1;
-	return 0;
+	return true;
 }
 
 static noinline int run_delalloc_zoned(struct btrfs_inode *inode,
@@ -2413,7 +2391,7 @@ int btrfs_run_delalloc_range(struct btrfs_inode *inode, struct page *locked_page
 		u64 start, u64 end, int *page_started, unsigned long *nr_written,
 		struct writeback_control *wbc)
 {
-	int ret;
+	int ret = 0;
 	const bool zoned = btrfs_is_zoned(inode->root->fs_info);
 
 	/*
@@ -2434,19 +2412,23 @@ int btrfs_run_delalloc_range(struct btrfs_inode *inode, struct page *locked_page
 		ASSERT(!zoned || btrfs_is_data_reloc_root(inode->root));
 		ret = run_delalloc_nocow(inode, locked_page, start, end,
 					 page_started, nr_written);
-	} else if (!btrfs_inode_can_compress(inode) ||
-		   !inode_need_compress(inode, start, end)) {
+		goto out;
+	}
+
+	if (btrfs_inode_can_compress(inode) &&
+	    inode_need_compress(inode, start, end) &&
+	    cow_file_range_async(inode, wbc, locked_page, start,
+				 end, page_started, nr_written))
+		goto out;
+
 	if (zoned)
 		ret = run_delalloc_zoned(inode, locked_page, start, end,
 					 page_started, nr_written);
 	else
 		ret = cow_file_range(inode, locked_page, start, end,
 				     page_started, nr_written, 1, NULL);
-	} else {
-		set_bit(BTRFS_INODE_HAS_ASYNC_EXTENT, &inode->runtime_flags);
-		ret = cow_file_range_async(inode, wbc, locked_page, start, end,
-					   page_started, nr_written);
-	}
+
+out:
 	ASSERT(ret <= 0);
 	if (ret)
 		btrfs_cleanup_ordered_extents(inode, locked_page, start,