Re: [PATCH 6.1] ext4: honor NULL returns from __filemap_get_folio()

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

On 11/2/2026 10:39 PM, Matthew Wilcox wrote:
> On Mon, Nov 02, 2026 at 04:33:41PM +0000, Naman Jain wrote:
>> The bitmap and buddy folio conversions assume that
>> __filemap_get_folio() returns error pointers. In Linux 6.1, however,
>> it returns NULL both when a non-creating lookup misses and when folio
>> creation fails.
>>
>> A cache miss therefore passes the IS_ERR() check and reaches
>> folio_test_uptodate(NULL) in ext4_mb_load_buddy_gfp(). Creating-lookup
>> failures can also dereference NULL in its slow paths and in
>> ext4_mb_get_buddy_page_lock().
>>
>> Restore NULL checks at all six lookup sites and return -ENOMEM when a
>> creating lookup fails. Preserve the folio locking and migration checks
>> without changing the global filemap API.
>>
>> Fixes: d34e4fd4e73f ("ext4: convert bd_bitmap_page to bd_bitmap_folio")
>> Fixes: 862483175d3d ("ext4: convert bd_buddy_page to bd_buddy_folio")
>> Cc: stable@vger.kernel.org # 6.1.x
>> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
> 
> Elliot beat you to it:
> 
> https://lore.kernel.org/linux-ext4/20261102-ext4-ntfs3-null-folio-v1-0-4fd91ee23a4a@elliotjb.com/
> 
> Care to drop a R-b on his series?

Done.
I actually checked LKML in the morning for this :)

Regards,
Naman