Re: [PATCH v2] mm/shmem.c: make shmem_mapping() inline

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 14 Nov 2020 13:51:34 +0800 Hui Su <sh_def@163.com> wrote:

> inline the shmem_mapping(), and use shmem_mapping()
> instead of 'inode->i_mapping->a_ops == &shmem_aops'
> in shmem_evict_inode().
> 
> ...
>
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1152,7 +1152,7 @@ static void shmem_evict_inode(struct inode *inode)
>  	struct shmem_inode_info *info = SHMEM_I(inode);
>  	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
>  
> -	if (inode->i_mapping->a_ops == &shmem_aops) {
> +	if (shmem_mapping(inode->i_mapping)) {
>  		shmem_unacct_size(info->flags, inode->i_size);
>  		inode->i_size = 0;
>  		shmem_truncate_range(inode, 0, (loff_t)-1);
> @@ -2352,7 +2352,7 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
>  	return inode;
>  }
>  
> -bool shmem_mapping(struct address_space *mapping)
> +inline bool shmem_mapping(struct address_space *mapping)
>  {
>  	return mapping->a_ops == &shmem_aops;
>  }

huh.  I'd have expected the inlining in shmem_evict_inode() to not work
because the compiler hasn't seen the definition yet.  But gcc has
evidently become smarter about that.

But really, shmem_mapping() isn't worth an out-of-line call from any
callsite - it would be best to make it inlined everywhere.

- make shmem_aops global
- declare shmem_aops in shmem_fs.h
- export shmem_aops to modules for drivers/dma-buf/udmabuf.c
- include linux/fs.h in shmem_fs.h for address_space_operations (we already
  include fs.h via swap.h, but we shouldn't depend on that)
- make shmem_mapping() a static inline in shmem_fs.h.