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

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Oct 21, 2026 at 10:36:00PM +0200, 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.
> 
> 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>

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

-- 
  Kiryl Shutsemau / Kirill A. Shutemov