Re: [PATCH] tmpfs: Restore functionality of nr_inodes=0

[Hugh Dickins]

On Tue, 1 Sep 2020, Byron Stanoszek wrote:

> Commit e809d5f0b5c9 ("tmpfs: per-superblock i_ino support") made changes to
> shmem_reserve_inode() in mm/shmem.c, however the original test for
> (sbinfo->max_inodes) got dropped. This causes mounting tmpfs with option
> nr_inodes=0 to fail:
> 
>   # mount -ttmpfs -onr_inodes=0 none /ext0
>   mount: /ext0: mount(2) system call failed: Cannot allocate memory.
> 
> This patch restores the nr_inodes=0 functionality.
> 
> Fixes: e809d5f0b5c9 ("tmpfs: per-superblock i_ino support")
> Cc: Chris Down <chris@xxxxxxxxxxxxxx>
> Signed-off-by: Byron Stanoszek <gandalf@xxxxxxxxx>

Yikes, thank you Byron, very bad of me not to have spotted that:

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

I've taken a quick look to see how I missed it: yes, I'd compared
against my own tree, knew I had to come back here sometime to replace
the SB_KERNMOUNT test by a max_inodes test like I had, to restore the
performance of nr_inodes=0; but thought the SB_KERNMOUNT test was good
enough for now - without realizing the effect on the code below it. The
error does seem to be localized just to this block, yes.  Many thanks.

> ---
>  mm/shmem.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 271548ca20f3..8e2b35ba93ad 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -279,11 +279,13 @@ static int shmem_reserve_inode(struct super_block *sb, ino_t *inop)
>  
>  	if (!(sb->s_flags & SB_KERNMOUNT)) {
>  		spin_lock(&sbinfo->stat_lock);
> -		if (!sbinfo->free_inodes) {
> -			spin_unlock(&sbinfo->stat_lock);
> -			return -ENOSPC;
> +		if (sbinfo->max_inodes) {
> +			if (!sbinfo->free_inodes) {
> +				spin_unlock(&sbinfo->stat_lock);
> +				return -ENOSPC;
> +			}
> +			sbinfo->free_inodes--;
>  		}
> -		sbinfo->free_inodes--;
>  		if (inop) {
>  			ino = sbinfo->next_ino++;
>  			if (unlikely(is_zero_ino(ino)))
> -- 
> 2.28.0