Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount

[David Hildenbrand (Arm)]

On 4/17/26 05:25, Baolin Wang wrote:
> Currently, when shmem mounts are initialized, they only use 'sbinfo->huge' to
> determine whether the shmem mount supports large folios. However, for anonymous
> shmem, whether it supports large folios can be dynamically configured via sysfs
> interfaces, so setting or not setting mapping_set_large_folios() during initialization
> cannot accurately reflect whether anonymous shmem actually supports large folios,
> which has already caused some confusion[1].
> 
> As discussed with David[2], for anonymous shmem we can treat it as always potentially
> having large folios. Therefore, always support large folios for the internal shmem
> mount (e.g., anonymous shmem), and which large order allocations are allowed can be
> configured dynamically via the 'shmem_enabled' interfaces.
> 
> [1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@xxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/all/875dc63b-0cd2-49e5-8b0d-3fb062789813@xxxxxxxxxx/
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---
> Changes from v2:
>  - Always support large folios for internal shmem mount, per David.
> Changes from v1:
>  - Update the comments and commit message, per Lance.
> ---
>  mm/shmem.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 4ecefe02881d..769ef37b1ea9 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3088,8 +3088,17 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
>  	if (sbinfo->noswap)
>  		mapping_set_unevictable(inode->i_mapping);
>  
> -	/* Don't consider 'deny' for emergencies and 'force' for testing */
> -	if (sbinfo->huge)
> +	/*
> +	 * Always support large folios for the internal shmem mount (e.g.,
> +	 * anonymous shmem), and which large order allocations are allowed
> +	 * can be configured dynamically via the 'shmem_enabled' interfaces.
> +	 *
> +	 * For tmpfs, honour the 'huge=' mount option to determine whether
> +	 * large folios are supported.
> +	 *
> +	 * Note: don't consider 'deny' for emergencies and 'force' for testing.
> +	 */
> +	if (sbinfo->huge || (sb->s_flags & SB_KERNMOUNT))
>  		mapping_set_large_folios(inode->i_mapping);

Two questions from a non-fs person about the semantics here:

a) Can sbinfo->huge be triggered later, for example, through a remount
(staring at shmem_reconfigure())

b) Do we cover all cases with the SB_KERNMOUNT where sbinfo->huge cannot
be changed later?

-- 
Cheers,

David