Re: [PATCH v4 2/4] mm/huge_memory: copy file THPs at fork like copy_present_ptes() does

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Oct 21, 2026 at 10:36:01PM +0200, Konstantin Khorenko wrote:
> This fixes no bug: it makes fork copy a PMD the way copy_present_ptes()
> copies a PTE. Until now copy_huge_pmd() skipped every folio PMD of a
> non-anonymous VMA and left the child to fault them in again, while the
> PTEs of the same VMA are copied.
> 
> Treat file folios as the PTE path does: take a reference, duplicate the
> rmap and account them. fork then pays a reference and an rmap update
> per PMD, and saves the child a fault per PMD it touches. The next patch
> sends PFN mappings through the same tail.
> 
> That covers the VMAs vma_needs_copy() sends here:
> 
> - private file mappings with an anon_vma;
> - VM_COPY_ON_FORK ones: uffd-wp and uffd-rwp, VM_MIXEDMAP (fuse DAX),
>   and VM_MAYBE_GUARD, which shared shmem, file and DAX mappings get
>   from a guard region.
> 
> The DAX huge zero PMD is copied too, with a reference taken by
> mm_get_huge_zero_folio() as for anonymous VMAs. A migration entry of a
> file folio is copied and accounted like copy_nonpresent_pte() does, and
> made read-only in both processes only in a CoW mapping.
> 
> The copy then gets the same tail as __copy_present_ptes(): write
> protect both sides in a CoW mapping, mark the child's copy clean in a
> shared one, and drop the uffd-wp bit where the child is not armed. An
> anonymous THP is now write protected only when its PMD is writable, as
> a PTE is.
> 
> A page table is deposited under the copy where the fault paths deposit
> one: in anonymous VMAs, and wherever the architecture needs one.
> 
> Suggested-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>

Okay, I see benefit of aligning it to copy_present_ptes().

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov