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

[Matthew Wilcox <willy@infradead.org>]

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.

zap_pmd_range() for example.  Or finish_fault():

                page = vmf->page;
(can be any page within the folio)
        folio = page_folio(page);
        if (pmd_none(*vmf->pmd)) {
                if (!needs_fallback && folio_test_pmd_mappable(folio)) {
                        ret = do_set_pmd(vmf, folio, page);

then do_set_pmd() does:

        if (folio_order(folio) != HPAGE_PMD_ORDER)
                return ret;
        page = &folio->page;

so that check needs to be changed, and then we need to select the
appropriate page within the folio rather than just the first page
of the folio.  And then after the call:

        entry = folio_mk_pmd(folio, vma->vm_page_prot);

we need to adjust entry to point to the appropriate PMD-sized range
within the folio.