On Tue, Jul 27, 2021 at 06:29:08PM +0800, Qu Wenruo wrote:

On 2021/7/27 上午9:39, Qu Wenruo wrote:

quoted

Btrfs uses set_page_extent_mapped() to properly setup a page.

That function would set PagePrivate, and populate needed structure for
subpage.
The timing of calling set_page_extent_mapped() happens before reading a
page or dirtying a page.
Thus when we got a page to write back, if it doesn't have PagePrivate,
it is a big problem in code logic.

Calling set_page_extent_mapped() for such page would just mask the
problem.
Furthermore, for subpage case, we call subpage error helper to clear the
page error bit before calling set_page_extent_mapped().
If we really got a page without Private bit, it can call kernel NULL
pointer dereference.

So change the set_page_extent_mapped() call to an ASSERT(), and move the
check before any page status update call.

Signed-off-by: Qu Wenruo <redacted>

Please discard the patch.

Although I haven't hit any problem testing the patch, it's still 
possible that we have a special page that would need cow fixup.

Such page will be:

- Dirty

- Not Private
   Thus no page->private, this could still cause problem for subpage case
   though

- No EXTENT_DELALLOC flags set for any range inside the page
   Thus writepage_delalloc() will not find a delalloc range inside the
   page.

Such page will be caught by btrfs_writepage_cow_fixup(), but it will 
trigger the ASSERT() added by this patch.

Hm yeah the mismatch between dirty/private/delalloc sounds exactly like
work for the cow fixup.