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

Ma, Yu
2024-07-04T14:45:37Z

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 ...