Re: [PATCH v3 1/3] fs/file.c: remove sanity_check and add likely/unlikely in alloc_fd()

["Ma, Yu" <yu.ma@intel.com>]

On 7/4/2024 6:11 PM, Jan Kara wrote:
> On Wed 03-07-24 16:34:49, Christian Brauner wrote:
>> On Wed, Jul 03, 2024 at 10:33:09AM GMT, Yu Ma wrote:
>>> alloc_fd() has a sanity check inside to make sure the struct file mapping to the
>>> allocated fd is NULL. Remove this sanity check since it can be assured by
>>> exisitng zero initilization and NULL set when recycling fd. Meanwhile, add
>>> likely/unlikely and expand_file() call avoidance to reduce the work under
>>> file_lock.
>>>
>>> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
>>> Signed-off-by: Yu Ma <yu.ma@intel.com>
>>> ---
>>>   fs/file.c | 38 ++++++++++++++++----------------------
>>>   1 file changed, 16 insertions(+), 22 deletions(-)
>>>
>>> diff --git a/fs/file.c b/fs/file.c
>>> index a3b72aa64f11..5178b246e54b 100644
>>> --- a/fs/file.c
>>> +++ b/fs/file.c
>>> @@ -515,28 +515,29 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>>>   	if (fd < files->next_fd)
>>>   		fd = files->next_fd;
>>>   
>>> -	if (fd < fdt->max_fds)
>>> +	if (likely(fd < fdt->max_fds))
>>>   		fd = find_next_fd(fdt, fd);
>>>   
>>> +	error = -EMFILE;
>>> +	if (unlikely(fd >= fdt->max_fds)) {
>>> +		error = expand_files(files, fd);
>>> +		if (error < 0)
>>> +			goto out;
>>> +		/*
>>> +		 * If we needed to expand the fs array we
>>> +		 * might have blocked - try again.
>>> +		 */
>>> +		if (error)
>>> +			goto repeat;
>>> +	}
>> So this ends up removing the expand_files() above the fd >= end check
>> which means that you can end up expanding the files_struct even though
>> the request fd is past the provided end. That seems odd. What's the
>> reason for that reordering?
> Yeah, not only that but also:
>
>>>   	/*
>>>   	 * N.B. For clone tasks sharing a files structure, this test
>>>   	 * will limit the total number of files that can be opened.
>>>   	 */
>>> -	error = -EMFILE;
>>> -	if (fd >= end)
>>> -		goto out;
>>> -
>>> -	error = expand_files(files, fd);
>>> -	if (error < 0)
>>> +	if (unlikely(fd >= end))
>>>   		goto out;
> We could then exit here with error set to 0 instead of -EMFILE. So this
> needs a bit of work. But otherwise the patch looks good to me.
>
> 								Honza

Do you mean that we return 0 here is fd >=end, I'm afraid that might 
broke the original design of this function. And all the callers of it 
are using ret < 0 for error handling, if ret=0, that should mean the fd 
allocated is 0 ...