Re: [PATCH v4 1/4] mm/rmap: duplicate a PMD mapping as one in folio_dup_file_rmap_pmd()

[Zi Yan <ziy@nvidia.com>]

On 21 Oct 2026, at 16:36, Konstantin Khorenko wrote:

> folio_dup_file_rmap_pmd() passes PGTABLE_LEVEL_PTE to
> __folio_dup_file_rmap() for HPAGE_PMD_NR pages, so it adds HPAGE_PMD_NR
> PTE mappings to the folio instead of one PMD mapping. Each unmap of the


> PMD removes one PMD mapping:
>
> - the folio's mapcount never returns to zero, and removing the folio
>   from the page cache reports "Bad page cache", or hits a VM_BUG_ON
>   with CONFIG_DEBUG_VM;
> - the first unmap already counts as the last one and drops the
>   PMD-mapped statistics while the other mm still maps the folio;
> - the second one drives the entire mapcount below zero and drops them
>   again.

This chunk of text probably can go away, since no one is affected
by the code.

>
> The helper has no caller yet, the next patch adds the first one. Pass
> PGTABLE_LEVEL_PMD, as folio_try_dup_anon_rmap_pmd() does.
>
> Fixes: d8ef5e311d7b ("mm/rmap: convert page_dup_file_rmap() to folio_dup_file_rmap_[pte|ptes|pmd]()")
> Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
> ---
>  include/linux/rmap.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>

The code change looks good to me. I also find this when I am working
on PG_folio. Thanks.

Reviewed-by: Zi Yan <ziy@nvidia.com>


Best Regards,
Yan, Zi