Re: [PATCH v5] fs: introduce is_dot_or_dotdot helper for cleanup

[Gao Xiang]

On Wed, Dec 11, 2019 at 02:38:34PM +0800, Tiezhu Yang wrote:
> On 12/11/2019 12:47 PM, Al Viro wrote:
> > On Wed, Dec 11, 2019 at 11:59:40AM +0800, Tiezhu Yang wrote:
> > 
> > > static inline bool is_dot_or_dotdot(const unsigned char *name, size_t len)
> > > {
> > >          if (len == 1 && name[0] == '.')
> > >                  return true;
> > > 
> > >          if (len == 2 && name[0] == '.' && name[1] == '.')
> > >                  return true;
> > > 
> > >          return false;
> > > }
> > > 
> > > Hi Matthew,
> > > 
> > > How do you think? I think the performance influence is very small
> > > due to is_dot_or_dotdot() is a such short static inline function.
> > It's a very short inline function called on a very hot codepath.
> > Often.
> > 
> > I mean it - it's done literally for every pathname component of
> > every pathname passed to a syscall.
> 
> OK. I understand. Let us do not use the helper function in fs/namei.c,
> just use the following implementation for other callers:
> 
> static inline bool is_dot_or_dotdot(const unsigned char *name, size_t len)
> {
>         if (len >= 1 && unlikely(name[0] == '.')) {


And I suggest drop "unlikely" here since files start with prefix
'.' (plus specical ".", "..") are not as uncommon as you expected...


Thanks,
Gao Xiang


>                 if (len < 2 || (len == 2 && name[1] == '.'))
>                         return true;
>         }
> 
>         return false;
> }
> 
> Special thanks for Matthew, Darrick, Al and Eric.
> If you have any more suggestion, please let me know.
> 
> Thanks,
> 
> Tiezhu Yang
>