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

[Tiezhu Yang]

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.

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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.

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++;