Re: [RFC 00/12] mm: PUD (1GB) THP implementation

[Usama Arif <usamaarif642@gmail.com>]

On 02/02/2026 01:06, David Hildenbrand (arm) wrote:
> On 2/2/26 05:00, Matthew Wilcox wrote:
>> On Sun, Feb 01, 2026 at 04:50:17PM -0800, Usama Arif wrote:
>>> This is an RFC series to implement 1GB PUD-level THPs, allowing
>>> applications to benefit from reduced TLB pressure without requiring
>>> hugetlbfs. The patches are based on top of
>>> f9b74c13b773b7c7e4920d7bc214ea3d5f37b422 from mm-stable (6.19-rc6).
>>
>> I suggest this has not had enough testing.  There are dozens of places
>> in the MM which assume that if a folio is at leaast PMD size then it is
>> exactly PMD size.  Everywhere that calls folio_test_pmd_mappable() needs
>> to be audited to make sure that it will work properly if the folio is
>> larger than PMD size.
> 
> I think the hack (ehm trick) in this patch set is to do it just like dax PUDs: only map through a PUD or through PTEs, not through PMDs.
> 
> That also avoids dealing with mapcounts until I sorted that out.
> 


Hello!

Thanks for the review! So its as David said, currently for PUD THP case, we
won't run into those paths.
PUD is split via TTU_SPLIT_HUGE_PUD which calls __split_huge_pud_locked().
This splits PUD to PTE directly (not PMD), so we never have a PUD folio
going through do_set_pmd(). The anonymous fault path uses
do_huge_pud_anonymous_page() so we won't go to finish_fault()

When I started working on this, I was really hoping that we could split PUDs to PMDs,
but very quickly realised thats a separate and much more complicated mapcount problem
(which is probably why David is dealing with it as he mentioned in the reply :P)
and should not be dealt with in this series.

In terms of more testing, I would definitely like to add more.
I have added selftests for allocation, memory integrity, fork, partial munmap, mprotect,
reclaim and migration, and am running them with DEBUG_VM to make sure we dont get the VM
bugs/warnings, but I am sure I am missing paths. I will try to think of more
but please let me know if there are more cases we can come up with.