Re: [PATCH v2 1/2] mm/huge_memory: take a reference on a folio PUD copied by fork

[Zi Yan]

On Thu Nov 5, 2026 at 12:00 PM EST, Konstantin Khorenko wrote:
> Device DAX maps its memory with vmf_insert_folio_pud(), which takes a
> folio reference, adds a PUD rmap and accounts the pages to the mm.
> zap_huge_pud() drops all three, but copy_huge_pud() copies the PUD into
> the child without taking any of them. Each child's unmap then drops
> what the parent took:
>
> - the folio is a reference short, so with two children its refcount
>   drops to zero and free_zone_device_folio() runs while the parent
>   still maps it;
> - the child's file counter goes negative, "Bad rss-counter state" when
>   its mm is freed;
> - the parent's unmap drives the entire mapcount below zero.
>
> fork copies the page tables of a device DAX mapping that has a guard
> region, and commit f807123d578d ("mm: allow guard regions in file-backed
> and read-only mappings") allowed MADV_GUARD_INSTALL there.
>
> Take the reference, the rmap and the counter for a PUD that
> vm_normal_page_pud() resolves to a page, as copy_present_ptes() does for
> a PTE, with a new folio_dup_file_rmap_pud() helper. copy_huge_pud() now
> needs both VMAs, so pass only them and take the mms from them.
>
> Fixes: aed877c2b425 ("device/dax: properly refcount device dax pages when mapping")
> Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Konstantin Khorenko <khorenko@xxxxxxxxxxxxx>
> ---
>  include/linux/huge_mm.h |  5 ++---
>  include/linux/rmap.h    | 22 ++++++++++++++++++++++
>  mm/huge_memory.c        | 27 +++++++++++++++++++--------
>  mm/memory.c             |  4 +---
>  4 files changed, 44 insertions(+), 14 deletions(-)
>

I assume this supersedes David's version[1], which uses
folio_add_file_rmap_pud().

[1] https://lore.kernel.org/all/20261031-fixes_core-v3-11-d6437171ecc3@xxxxxxxxxx/

LGTM.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>


-- 
Best Regards,
Yan, Zi