linux-kernel - Re: [PATCH] fs: introduce is_dot

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Date:   Mon, 2 Dec 2019 18:39:54 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Tyler Hicks <tyhicks@...onical.com>,
        linux-fsdevel@...r.kernel.org, ecryptfs@...r.kernel.org,
        linux-fscrypt@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: introduce is_dot_dotdot helper for cleanup

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?

--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++;
>