linux-kernel - Re: [f2fs-dev] [PATCH 1/1] f2fs: fix args passed to trace_f2fs_lookup

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Message-ID: <0c15c662-903c-3504-d502-72f0a3b70cbf@kernel.org>
Date:   Mon, 29 May 2023 18:18:42 +0800
From:   Chao Yu <chao@...nel.org>
To:     Wu Bo <bo.wu@...o.com>
Cc:     jaegeuk@...nel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, wubo.oduw@...il.com
Subject: Re: [f2fs-dev] [PATCH 1/1] f2fs: fix args passed to
 trace_f2fs_lookup_end

On 2023/5/29 12:13, Wu Bo wrote:
> On Sat, May 27, 2023 at 09:01:41AM +0800, Chao Yu wrote:
>> On 2023/5/27 1:21, Jaegeuk Kim wrote:
>>> On 05/24, Wu Bo wrote:
>>>> The NULL return of 'd_splice_alias' dosen't mean error.
>>>>
>>>> Signed-off-by: Wu Bo <bo.wu@...o.com>
>>>> ---
>>>>    fs/f2fs/namei.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c
>>>> index 77a71276ecb1..e5a3e39ce90c 100644
>>>> --- a/fs/f2fs/namei.c
>>>> +++ b/fs/f2fs/namei.c
>>>> @@ -577,7 +577,7 @@ static struct dentry *f2fs_lookup(struct inode *dir, struct dentry *dentry,
>>>>    #endif
>>>>    	new = c(inode, dentry);
>>>>    	err = PTR_ERR_OR_ZERO(new);
>>>> -	trace_f2fs_lookup_end(dir, dentry, ino, !new ? -ENOENT : err);
>>>> +	trace_f2fs_lookup_end(dir, new ? new : dentry, ino, err);
>>>
>>> Shouldn't give an error pointer to the dentry field.
>>>
>>> How about just giving the err?
>>>
>>> -       err = PTR_ERR_OR_ZERO(new);
>>> -       trace_f2fs_lookup_end(dir, dentry, ino, !new ? -ENOENT : err);
>>> +       trace_f2fs_lookup_end(dir, dentry, ino, PTR_ERR_OR_ZERO(new));
>>
>> static inline int __must_check PTR_ERR_OR_ZERO(__force const void *ptr)
>> {
>> 	if (IS_ERR(ptr))
>> 		return PTR_ERR(ptr);
>> 	else
>> 		return 0;
>> }
>>
>> For below two cases, PTR_ERR_OR_ZERO(new) will return zero:
>> a) f2fs_lookup found existed dentry
>> b) f2fs_lookup didn't find existed dentry (-ENOENT case)
>>
>> So in below commit, I passed -ENOENT to tracepoint for case b), so we can
>> distinguish result of f2fs_lookup in tracepoint, actually, -ENOENT is expected
>> value when we create a new file/directory.
>>
>> Commit 84597b1f9b05 ("f2fs: fix wrong value of tracepoint parameter")
> I can see this commit is try to distinguish the dentry not existed case.
> But a normal case which dentry is exactly found will also go through
> 'd_splice_alias', and its return is also NULL. This makes the tracepoint always
> print 'err:-2' like the following:
>        ls-11676   [004] .... 329281.943118: f2fs_lookup_end: dev = (254,39), pino = 4451, name:Alarms, ino:7093, err:-2
>        ls-11676   [004] .... 329281.943145: f2fs_lookup_end: dev = (254,39), pino = 4451, name:Notifications, ino:7094, err:-2
>        ls-11676   [004] .... 329281.943172: f2fs_lookup_end: dev = (254,39), pino = 4451, name:Pictures, ino:7095, err:-2
> Even these lookup are acctually successful, this is a bit strange.

Ah, I misunderstand return value's meaning of .lookup.

So, how about this? it only update err if d_splice_alias() returns a negative
value?

if (IS_ERR(new))
	err = PTR_ERR(new);
trace_f2fs_lookup_end(dir, new ? new : dentry, ino, err);

Thanks,

>>
>>>
>>>
>>>>    	return new;
>>>>    out_iput:
>>>>    	iput(inode);
>>>> -- 
>>>> 2.35.3
>>
>>
>> _______________________________________________
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