Re: [PATCH v8 07/14] mm: shmem: allow THP support determination at folio allocation time

[David Hildenbrand (Arm)]

On 9/18/26 03:45, Luiz Capitulino wrote:
> In order to enable THP support in shmem today, besides the user
> configuration required, the CPU must support PMD-sized pages. This
> is the case because of the following has_transparent_hugepage()
> usage:
> 
> - shmem_parse_one() and shmem_parse_huge(): Check if THP is built-in and
>   if the CPU supports PMD-sized pages
> 
> - shmem_init(): Since the CONFIG_TRANSPARENT_HUGEPAGE guard is outside
>   the code block calling has_transparent_hugepage(), the
>   has_transparent_hugepage() call is exclusively checking if the CPU
>   supports PMD-sized pages
> 
> While it's necessary to check if CONFIG_TRANSPARENT_HUGEPAGE is enabled
> in all cases, shmem can determine THP size support at folio allocation
> time. Therefore, drop the has_transparent_hugepage() usage listed above
> while keeping the CONFIG_TRANSPARENT_HUGEPAGE checks.
> 
> Additionally, we need to check if PMD size order is supported in
> shmem_getattr(). Use pgtable_has_pmd_leaves() for that.
> 
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Luiz Capitulino <luizcap@xxxxxxxxxx>
> ---
>  mm/shmem.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 776dff8a848e..930657d05375 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -690,7 +690,7 @@ static int shmem_parse_huge(const char *str)
>  	else
>  		return -EINVAL;
>  
> -	if (!has_transparent_hugepage() &&
> +	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>  	    huge != SHMEM_HUGE_NEVER && huge != SHMEM_HUGE_DENY)
>  		return -EINVAL;
>  
> @@ -1524,6 +1524,8 @@ static int shmem_getattr(struct mnt_idmap *idmap,
>  	generic_fillattr(idmap, request_mask, inode, stat);
>  
>  	orders = shmem_huge_global_enabled(inode, 0, 0, false, NULL, 0);

I'm curious: why is that not handled inside shmem_huge_global_enabled() ? If PMD
order is impossible (well, okay, it is possible, but we simply cannot map these
things through PMDs), I would expect that we never list them as "enabled".

(while shmem could allocate PMD folios, it would have to map them always through
PTEs. I think this would be possible through some changes, but we can defer that
to some future work).

-- 
Cheers,

David