Re: [PATCH] fs: introduce is_dot_dotdot helper for cleanup

[Tiezhu Yang]

On 12/03/2019 10:39 AM, Darrick J. Wong wrote:
> On Tue, Dec 03, 2019 at 10:07:41AM +0800, Tiezhu Yang wrote:
> > On 12/03/2019 04:03 AM, Matthew Wilcox wrote:
> > > On Mon, Dec 02, 2019 at 06:10:13PM +0800, Tiezhu Yang wrote:
> > > > There exists many similar and duplicate codes to check "." and "..",
> > > > so introduce is_dot_dotdot helper to make the code more clean.
> > > The idea is good.  The implementation is, I'm afraid, badly chosen.
> > > Did you benchmark this change at all?  In general, you should prefer the
> > > core kernel implementation to that of some less-interesting filesystems.
> > > I measured the performance with the attached test program on my laptop
> > > (Core-i7 Kaby Lake):
> > >
> > > qstr . time_1 0.020531 time_2 0.005786
> > > qstr .. time_1 0.017892 time_2 0.008798
> > > qstr a time_1 0.017633 time_2 0.003634
> > > qstr matthew time_1 0.011820 time_2 0.003605
> > > qstr .a time_1 0.017909 time_2 0.008710
> > > qstr , time_1 0.017631 time_2 0.003619
> > >
> > > The results are quite stable:
> > >
> > > qstr . time_1 0.021137 time_2 0.005780
> > > qstr .. time_1 0.017964 time_2 0.008675
> > > qstr a time_1 0.017899 time_2 0.003654
> > > qstr matthew time_1 0.011821 time_2 0.003620
> > > qstr .a time_1 0.017889 time_2 0.008662
> > > qstr , time_1 0.017764 time_2 0.003613
> > >
> > > Feel free to suggest some different strings we could use for testing.
> > > These seemed like interesting strings to test with.  It's always possible
> > > I've messed up something with this benchmark that causes it to not
> > > accurately represent the performance of each algorithm, so please check
> > > that too.
> > [Sorry to resend this email because the mail list server
> > was denied due to it is not plain text.]
> >
> > Hi Matthew,
> >
> > Thanks for your reply and suggestion. I measured the
> > performance with the test program, the following
> > implementation is better for various of test cases:
> >
> > bool is_dot_dotdot(const struct qstr *str)
> > {
> >          if (unlikely(str->name[0] == '.')) {
> >                  if (str->len < 2 || (str->len == 2 && str->name[1] == '.'))
> >                          return true;
> >          }
> >
> >          return false;
> > }
> >
> > I will send a v2 patch used with this implementation.
> Can you make it a static inline since it's such a short function?

Thanks for your suggestion, I will make it static inline and
move it to include/linux/fs.h in the v2 patch.

Thanks,

Tiezhu Yang

> --D
>
> > Thanks,
> >
> > Tiezhu Yang
> >
> > > > +bool is_dot_dotdot(const struct qstr *str)
> > > > +{
> > > > +	if (str->len == 1 && str->name[0] == '.')
> > > > +		return true;
> > > > +
> > > > +	if (str->len == 2 && str->name[0] == '.' && str->name[1] == '.')
> > > > +		return true;
> > > > +
> > > > +	return false;
> > > > +}
> > > > +EXPORT_SYMBOL(is_dot_dotdot);
> > > > diff --git a/fs/namei.c b/fs/namei.c
> > > > index 2dda552..7730a3b 100644
> > > > --- a/fs/namei.c
> > > > +++ b/fs/namei.c
> > > > @@ -2458,10 +2458,8 @@ static int lookup_one_len_common(const char *name, struct dentry *base,
> > > >    	if (!len)
> > > >    		return -EACCES;
> > > > -	if (unlikely(name[0] == '.')) {
> > > > -		if (len < 2 || (len == 2 && name[1] == '.'))
> > > > -			return -EACCES;
> > > > -	}
> > > > +	if (unlikely(is_dot_dotdot(this)))
> > > > +		return -EACCES;
> > > >    	while (len--) {
> > > >    		unsigned int c = *(const unsigned char *)name++;