Re: WARNING in memory_failure() at include/linux/huge_mm.h:635 triggered

[Zi Yan <ziy@nvidia.com>]

On 4 Feb 2026, at 14:18, David Hildenbrand (arm) wrote:

> On 2/4/26 18:41, Zi Yan wrote:
>> On 4 Feb 2026, at 12:34, Zi Yan wrote:
>>
>>> On 4 Feb 2026, at 12:23, Zi Yan wrote:
>>>
>>>>
>>>>
>>>>  From the C repro above, syzbot opened a dev "/dev/sg#" and did mmap on it.
>>>> Is it a device driver issue?
>>>
>>> OK, it is CONFIG_CHR_DEV_SG. And the driver allocates a compound page at[1].
>>> Since we initialize folio fields in prep_compound_page(), it becomes a folio
>>> when it is inserted into a VMA.
>>
>> More details:
>> later at sg_vma_fault(), the driver just handles a page fault by supplying
>> a subpage from a pre-allocated compound page[3]. We then get a large folio
>> without !CONFIG_TRANSPARENT_HUGEPAGE.
>
> We can identify such non-folio (but compound) things by looking at PG_large_rmappable IIRC.

What do you mean? Changing memory failure code to only handle large_rmappable?
large_rmappable is a folio flag, memory failure code should see such
non-folio but compound things to begin with, IMHO.

I think we need to be able to tell between raw page (compound or not),
mappable page (compound or not, especially for those used with vm_insert_*),
and folio.

Best Regards,
Yan, Zi