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 initial each pages 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.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/compression.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 1ab56a734e70..17c27edd804b 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c

@@ -115,10 +115,14 @@ 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 happens when compression races with remount 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;
        }
 }
--

2.28.0