Re: [PATCH 1/1] btrfs: Simplify parsing max_inline mount option

[David Sterba <dsterba@suse.cz>]

On Thu, Aug 15, 2019 at 02:04:53AM +0000, Vladimir Panteleev wrote:
> - Avoid an allocation;
> - Properly handle non-numerical argument and garbage after numerical
>   argument.
> 
> Signed-off-by: Vladimir Panteleev <git@thecybershadow.net>
> ---
>  fs/btrfs/super.c | 24 +++++++++---------------
>  1 file changed, 9 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index f56617dfb3cf..6fe8ef6667f3 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -426,7 +426,7 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
>  			unsigned long new_flags)
>  {
>  	substring_t args[MAX_OPT_ARGS];
> -	char *p, *num;
> +	char *p, *retptr;
>  	u64 cache_gen;
>  	int intarg;
>  	int ret = 0;
> @@ -630,22 +630,16 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
>  			info->thread_pool_size = intarg;
>  			break;
>  		case Opt_max_inline:
> -			num = match_strdup(&args[0]);
> -			if (num) {
> -				info->max_inline = memparse(num, NULL);
> -				kfree(num);
> -
> -				if (info->max_inline) {
> -					info->max_inline = min_t(u64,
> -						info->max_inline,
> -						info->sectorsize);
> -				}
> -				btrfs_info(info, "max_inline at %llu",
> -					   info->max_inline);
> -			} else {
> -				ret = -ENOMEM;
> +			info->max_inline = memparse(args[0].from, &retptr);

I don't think this is a good idea, match_strdup takes an opaque type
substring_t that is used by the parser. So accessing ::from directly in
memparse does not respect the API boundary. I've checked some other
usages in the tree and the match_strdup/memparse/kstrtoull is quite
common.