Re: [PATCH v3] tmpfs: don't enable large folios if not supported

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/10/11 14:59, Kefeng Wang wrote:
> The tmpfs could support large folio, but there is some configurable
> options(mount options and runtime deny/force) to enable/disable large
> folio allocation, so there is a performance issue when perform write
> without large folio, the issue is similar to commit 4e527d5841e2
> ("iomap: fault in smaller chunks for non-large folio mappings").
> 
> Don't call mapping_set_large_folios() in __shmem_get_inode() when
> large folio is disabled to fix it.
> 
> Fixes: 9aac777aaf94 ("filemap: Convert generic_perform_write() to support large folios")
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> 
> v3:
> - don't enable large folio suppport in __shmem_get_inode() if disabled,
>    suggested by Matthew.
> 
> v2:
> - Don't use IOCB flags
> 
>   mm/shmem.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 0a2f78c2b919..2b859ac4ddc5 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2850,7 +2850,10 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
>   	cache_no_acl(inode);
>   	if (sbinfo->noswap)
>   		mapping_set_unevictable(inode->i_mapping);
> -	mapping_set_large_folios(inode->i_mapping);
> +
> +	if ((sbinfo->huge && shmem_huge != SHMEM_HUGE_DENY) ||
> +	    shmem_huge == SHMEM_HUGE_FORCE)
> +		mapping_set_large_folios(inode->i_mapping);

IMHO, I'm still a little concerned about the 'shmem_huge' validation. 
Since the 'shmem_huge' can be set at runtime, that means file mapping 
with 'huge=always' option might miss the opportunity to allocate large 
folios if the 'shmem_huge' is changed from 'deny' from 'always' at runtime.

So I'd like to drop the 'shmem_huge' validation and add some comments to 
indicate 'deny' and 'force' options are only for testing purpose and 
performence issue should not be a problem in the real production 
environments.

That's just my 2 cents:)