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

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Date:   Wed, 4 Dec 2019 14:06:28 +0100
From:   Jean-Louis Biasini <jl.biasini@...oste.net>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Matthew Wilcox <willy@...radead.org>
Cc:     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

Please UNSUBSCRIBE ME from this list of tell how to!!!

Le 03/12/2019 à 03:07, Tiezhu Yang a écrit :
> 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.
>
> 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++;
>