btrfs: prevent inline data extents read from touching blocks beyond its range

Currently reading an inline data extent will zero out the remaining
range in the page.

This is not yet causing problems even for block size < page size
(subpage) cases because:

1) An inline data extent always starts at file offset 0
Meaning at page read, we always read the inline extent first, before
any other blocks in the page. Then later blocks are properly read out
and re-fill the zeroed out ranges.

2) Currently btrfs will read out the whole page if a buffered write is
not page aligned
So a page is either fully uptodate at buffered write time (covers the
whole page), or we will read out the whole page first.
Meaning there is nothing to lose for such an inline extent read.

But it's still not ideal:

- We're zeroing out the page twice
Once done by read_inline_extent()/uncompress_inline(), once done by
btrfs_do_readpage() for ranges beyond i_size.

- We're touching blocks that don't belong to the inline extent
In the incoming patches, we can have a partial uptodate folio, of
which some dirty blocks can exist while the page is not fully uptodate:

The page size is 16K and block size is 4K:

0 4K 8K 12K 16K
| | |/////////| |

And range [8K, 12K) is dirtied by a buffered write, the remaining
blocks are not uptodate.

If range [0, 4K) contains an inline data extent, and we try to read
the whole page, the current behavior will overwrite range [8K, 12K)
with zero and cause data loss.

So to make the behavior more consistent and in preparation for future
changes, limit the inline data extents read to only zero out the range
inside the first block, not the whole page.

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

fs/btrfs/inode.c

···
 {
 	int ret;
 	struct extent_buffer *leaf = path->nodes[0];
+	const u32 blocksize = leaf->fs_info->sectorsize;
 	char *tmp;
 	size_t max_size;
 	unsigned long inline_size;
···
 
 	read_extent_buffer(leaf, tmp, ptr, inline_size);
 
-	max_size = min_t(unsigned long, PAGE_SIZE, max_size);
+	max_size = min_t(unsigned long, blocksize, max_size);
 	ret = btrfs_decompress(compress_type, tmp, folio, 0, inline_size,
 			       max_size);
 
···
 	 * cover that region here.
 	 */
 
-	if (max_size < PAGE_SIZE)
-		folio_zero_range(folio, max_size, PAGE_SIZE - max_size);
+	if (max_size < blocksize)
+		folio_zero_range(folio, max_size, blocksize - max_size);
 	kfree(tmp);
 	return ret;
 }
 
 static int read_inline_extent(struct btrfs_path *path, struct folio *folio)
 {
+	const u32 blocksize = path->nodes[0]->fs_info->sectorsize;
 	struct btrfs_file_extent_item *fi;
 	void *kaddr;
 	size_t copy_size;
···
 	if (btrfs_file_extent_compression(path->nodes[0], fi) != BTRFS_COMPRESS_NONE)
 		return uncompress_inline(path, folio, fi);
 
-	copy_size = min_t(u64, PAGE_SIZE,
+	copy_size = min_t(u64, blocksize,
 			  btrfs_file_extent_ram_bytes(path->nodes[0], fi));
 	kaddr = kmap_local_folio(folio, 0);
 	read_extent_buffer(path->nodes[0], kaddr,
 			   btrfs_file_extent_inline_start(fi), copy_size);
 	kunmap_local(kaddr);
-	if (copy_size < PAGE_SIZE)
-		folio_zero_range(folio, copy_size, PAGE_SIZE - copy_size);
+	if (copy_size < blocksize)
+		folio_zero_range(folio, copy_size, blocksize - copy_size);
 	return 0;
 }