Re: [RFC v4 PATCH] mm: shmem: make stat.st_blksize return huge page size if THP is on

[Kirill A. Shutemov]

On Tue, Apr 24, 2018 at 12:00:50PM +0800, Yang Shi wrote:
> Since tmpfs THP was supported in 4.8, hugetlbfs is not the only
> filesystem with huge page support anymore. tmpfs can use huge page via
> THP when mounting by "huge=" mount option.
> 
> When applications use huge page on hugetlbfs, it just need check the
> filesystem magic number, but it is not enough for tmpfs. Make
> stat.st_blksize return huge page size if it is mounted by appropriate
> "huge=" option to give applications a hint to optimize the behavior with
> THP.
> 
> Some applications may not do wisely with THP. For example, QEMU may mmap
> file on non huge page aligned hint address with MAP_FIXED, which results
> in no pages are PMD mapped even though THP is used. Some applications
> may mmap file with non huge page aligned offset. Both behaviors make THP
> pointless.
> 
> statfs.f_bsize still returns 4KB for tmpfs since THP could be split, and it
> also may fallback to 4KB page silently if there is not enough huge page.
> Furthermore, different f_bsize makes max_blocks and free_blocks
> calculation harder but without too much benefit. Returning huge page
> size via stat.st_blksize sounds good enough.
> 
> Since PUD size huge page for THP has not been supported, now it just
> returns HPAGE_PMD_SIZE.
> 
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> ---
> v3 --> v4:
> * Rework the commit log per the education from Michal and Kirill
> * Fix build error if CONFIG_TRANSPARENT_HUGEPAGE is disabled
> v2 --> v3:
> * Use shmem_sb_info.huge instead of global variable per Michal's comment
> v2 --> v1:
> * Adopted the suggestion from hch to return huge page size via st_blksize
>   instead of creating a new flag.
> 
>  mm/shmem.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index b859192..19b8055 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -988,6 +988,7 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
>  {
>  	struct inode *inode = path->dentry->d_inode;
>  	struct shmem_inode_info *info = SHMEM_I(inode);
> +	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
>  
>  	if (info->alloced - info->swapped != inode->i_mapping->nrpages) {
>  		spin_lock_irq(&info->lock);
> @@ -995,6 +996,11 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
>  		spin_unlock_irq(&info->lock);
>  	}
>  	generic_fillattr(inode, stat);
> +#ifdef CONFIG_TRANSPARENT_HUGE_PAGECACHE
> +	if (sbinfo->huge > 0)

No ifdeffery, please.

And we probably want to check if shmem_huge is 'force'.

Something like this?

	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
		 (shmem_huge == SHMEM_HUGE_FORCE || sbinfo->huge))

> +		stat->blksize = HPAGE_PMD_SIZE;
> +#endif
> +	
>  	return 0;
>  }
>  
> -- 
> 1.8.3.1
> 

-- 
 Kirill A. Shutemov