Re: [PATCH 6.1.y 0/2] Fix NULL folio dereferences from IS_ERR() checks in 6.1.191

[Naman Jain <namjain@linux.microsoft.com>]

> Elliot Berman <elliotjb@elliotjb.com>
>
> We hit a kernel oops in 6.1.191 in ext4_mb_regular_allocator(), When
> ext4's s_buddy_cache pages are evicted, the next block allocation
> dereferences a NULL folio.
> The cause is a mismatch in these backports first released in 6.1.191:
>     d34e4fd4e73f ("ext4: convert bd_bitmap_page to bd_bitmap_folio")
>     862483175d3d ("ext4: convert bd_buddy_page to bd_buddy_folio")
>
> The ext4 mballoc folio conversions were written for v6.4 and later,
> where __filemap_get_folio() returns ERR_PTR() on failure. 6.1 does
> not have commit 66dabbb65d67 ("mm: return an ERR_PTR from
> __filemap_get_folio") and returns NULL instead. The backports should
> check for NULL and not IS_ERR.
> Since the same mistake could exist elsewhere, I ran a Coccinelle script
> over the tree to flag similar missed backports. Besides ext4, I found
> one more case in ntfs3:
>     95b917a14b15 ("ntfs: Do not overwrite uptodate pages")
>
> That one needs a folio allocation failure while reading a compressed
> file, so it required crafting a fault injection in
> __filemap_add_folio to reproduce. I've fixed this one as well.

I actually sent a similar patch a little later to fix the same issue :)
We can now ignore that. For ntfs3, there is already a patch fixing the
same issue:

https://lore.kernel.org/all/20261029031815.2783998-1-cmllamas@google.com/

For the series:
Reviewed-by: Naman Jain <namjain@linux.microsoft.com>

Regards,
Naman Jain