btrfs: subpage: check if there are compressed extents inside one page

[BUG]
When testing experimental subpage compressed write support, it hits a
NULL pointer dereference inside read path:

 Unable to handle kernel NULL pointer dereference at virtual address 0000000000000018
 pc : __pi_memcmp+0x28/0x1ec
 lr : check_data_csum+0xd0/0x274 [btrfs]
 Call trace:
  __pi_memcmp+0x28/0x1ec
  btrfs_verify_data_csum+0xf4/0x244 [btrfs]
  end_bio_extent_readpage+0x1d0/0x6b0 [btrfs]
  bio_endio+0x15c/0x1dc
  end_workqueue_fn+0x44/0x64 [btrfs]
  btrfs_work_helper+0x74/0x250 [btrfs]
  process_one_work+0x1d4/0x47c
  worker_thread+0x180/0x400
  kthread+0x11c/0x120
  ret_from_fork+0x10/0x30
 Code: 54000261 d100044c d343fd8c f8408403 (f8408424)
 ---[ end trace 9e2c59f33ea40866 ]---

[CAUSE]
When reading two compressed extents inside the same page, like the
following layout, we trigger above crash:

	0	32K	64K
	|-------|\\\\\\\|
	     |	     \- Compressed extent (A)
	     \--------- Compressed extent (B)

For compressed read, we don't need to populate its io_bio->csum, as we
rely on compressed_bio->csum to verify the compressed data, and then
copy the decompressed to inode pages.

Normally btrfs_verify_data_csum() skip such page by checking and
clearing its PageChecked flag

But since that flag is still for the full page, when endio for inode
page range [0, 32K) gets executed, it clears PageChecked flag for the
full page.

Then when endio for inode page range [32K, 64K) gets executed, since the
page no longer has PageChecked flag, it just continues checking, even
though io_bio->csum is NULL.

[FIX]
Thankfully there are only two users of PageChecked bit:

- Cow fixup
  Since subpage has its own way to trace page dirty (dirty_bitmap) and
  ordered bit (ordered_bitmap), it should never trigger cow fixup.

- Compressed read
  We can distinguish such read by just checking io_bio->csum.

So just check io_bio->csum before doing the verification to avoid such
NULL pointer dereference.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -3256,6 +3256,20 @@ unsigned int btrfs_verify_data_csum(struct btrfs_io_bio *io_bio, u32 bio_offset,
 		return 0;
 	}
 
+	/*
+	 * For subpage case, above PageChecked is not safe as it's not subpage
+	 * compatible.
+	 * But for now only cow fixup and compressed read utilize PageChecked
+	 * flag, while in this context we can easily use io_bio->csum to
+	 * determine if we really need to do csum verification.
+	 *
+	 * So for now, just exit if io_bio->csum is NULL, as it means it's
+	 * compressed read, and its compressed data csum has already been
+	 * verified.
+	 */
+	if (io_bio->csum == NULL)
+		return 0;
+
 	if (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)
 		return 0;