Re: [RFC PATCH 1/2] mm/secretmem: try to restore large page mappings in direct map

[Mike Rapoport]

Hi Lance,

On Wed, Jun 03, 2026 at 06:46:23PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@xxxxxxxxx>
> 
> secretmem removes the pages backing secretmem mappings from the direct map.
> 
> Removing one base page from the direct map can split the covering large
> mapping down to PTE mappings. Repeated splits can leave more of the direct
> map mapped with PTEs, meaning more TLB entries for the same range and
> potentially more TLB pressure.
> 
> So let's try to restore large page mappings whenever secretmem restores a
> folio to the direct map.
> 
> Tested-by: Xueyuan Chen <xueyuan.chen21@xxxxxxxxx>
> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> ---
>  include/linux/set_memory.h |  6 ++++++
>  mm/secretmem.c             | 12 ++++++++++--
>  2 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
> index 3030d9245f5a..ad2fa414a22d 100644
> --- a/include/linux/set_memory.h
> +++ b/include/linux/set_memory.h
> @@ -58,6 +58,12 @@ static inline bool can_set_direct_map(void)
>  #endif
>  #endif /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
>  
> +#ifndef arch_try_collapse_direct_map
> +static inline void arch_try_collapse_direct_map(struct page *page)
> +{
> +}
> +#endif

Did you explore what would it mean to hook collapse_large_pages() into
set_direct_map_default_noflush()?

-- 
Sincerely yours,
Mike.