Re: [PATCH 5/9] mm: Turn folio_test_hugetlb into a PageType

[David Hildenbrand <david@redhat.com>]

On 22.03.24 11:19, Vlastimil Babka wrote:
> On 3/21/24 15:24, Matthew Wilcox (Oracle) wrote:
>> The current folio_test_hugetlb() can be fooled by a concurrent folio split
>> into returning true for a folio which has never belonged to hugetlbfs.
>> This can't happen if the caller holds a refcount on it, but we have a
>> few places (memory-failure, compaction, procfs) which do not and should
>> not take a speculative reference.
> 
> Should we add metadata wrt closing the bug report from Luis?
> 
> https://lore.kernel.org/all/8fa1c95c-4749-33dd-42ba-243e492ab109@suse.cz/
> 
> I assume this wouldn't be fun wrt stable...
> 
>> Since hugetlb pages do not use individual page mapcounts (they are always
>> fully mapped and use the entire_mapcount field to record the number
> 
> Wasn't there some discussions to allow partial mappings of hugetlb? What
> would be the implications?

If we ever go that path, we really should avoid messing with any 
subpages right from the start. We should make it work using a single 
total mapcount per folio.

Anyhow, that's stuff for the future.

-- 
Cheers,

David / dhildenb