btrfs: handle remount to no compress during compression

[BUG]
When running btrfs/071 with inode_need_compress() removed from
compress_file_range(), we got the following crash:

  BUG: kernel NULL pointer dereference, address: 0000000000000018
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  Workqueue: btrfs-delalloc btrfs_work_helper [btrfs]
  RIP: 0010:compress_file_range+0x476/0x7b0 [btrfs]
  Call Trace:
   ? submit_compressed_extents+0x450/0x450 [btrfs]
   async_cow_start+0x16/0x40 [btrfs]
   btrfs_work_helper+0xf2/0x3e0 [btrfs]
   process_one_work+0x278/0x5e0
   worker_thread+0x55/0x400
   ? process_one_work+0x5e0/0x5e0
   kthread+0x168/0x190
   ? kthread_create_worker_on_cpu+0x70/0x70
   ret_from_fork+0x22/0x30
  ---[ end trace 65faf4eae941fa7d ]---

This is already after the patch "btrfs: inode: fix NULL pointer
dereference if inode doesn't need compression."

[CAUSE]
@pages is firstly created by kcalloc() in compress_file_extent():
                pages = kcalloc(nr_pages, sizeof(struct page *), GFP_NOFS);

Then passed to btrfs_compress_pages() to be utilized there:

                ret = btrfs_compress_pages(...
                                           pages,
                                           &nr_pages,
                                           ...);

btrfs_compress_pages() will initialize each page as output, in
zlib_compress_pages() we have:

                        pages[nr_pages] = out_page;
                        nr_pages++;

Normally this is completely fine, but there is a special case which
is in btrfs_compress_pages() itself:

        switch (type) {
        default:
                return -E2BIG;
        }

In this case, we didn't modify @pages nor @out_pages, leaving them
untouched, then when we cleanup pages, the we can hit NULL pointer
dereference again:

        if (pages) {
                for (i = 0; i < nr_pages; i++) {
                        WARN_ON(pages[i]->mapping);
                        put_page(pages[i]);
                }
        ...
        }

Since pages[i] are all initialized to zero, and btrfs_compress_pages()
doesn't change them at all, accessing pages[i]->mapping would lead to
NULL pointer dereference.

This is not possible for current kernel, as we check
inode_need_compress() before doing pages allocation.
But if we're going to remove that inode_need_compress() in
compress_file_extent(), then it's going to be a problem.

[FIX]
When btrfs_compress_pages() hits its default case, modify @out_pages to
0 to prevent such problem from happening.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=212331
CC: stable@vger.kernel.org # 5.10+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.c

@@ -80,10 +80,15 @@ static int compression_compress_pages(int type, struct list_head *ws,
 	case BTRFS_COMPRESS_NONE:
 	default:
 		/*
-		 * This can't happen, the type is validated several times
-		 * before we get here. As a sane fallback, return what the
-		 * callers will understand as 'no compression happened'.
+		 * This can happen when compression races with remount setting
+		 * it to 'no compress', while caller doesn't call
+		 * inode_need_compress() to check if we really need to
+		 * compress.
+		 *
+		 * Not a big deal, just need to inform caller that we
+		 * haven't allocated any pages yet.
 		 */
+		*out_pages = 0;
 		return -E2BIG;
 	}
 }