Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated

[Luis Henriques]

Amir Goldstein <amir73il@xxxxxxxxx> writes:

> On Fri, Feb 12, 2021 at 2:40 PM Luis Henriques <lhenriques@xxxxxxx> wrote:
...
>> Sure, I just wanted to point out that *maybe* there are other options than
>> simply reverting that commit :-)
>>
>> Something like the patch below (completely untested!) should revert to the
>> old behaviour in filesystems that don't implement the CFR syscall.
>>
>> Cheers,
>> --
>> Luis
>>
>> diff --git a/fs/read_write.c b/fs/read_write.c
>> index 75f764b43418..bf5dccc43cc9 100644
>> --- a/fs/read_write.c
>> +++ b/fs/read_write.c
>> @@ -1406,8 +1406,11 @@ static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
>>                                                        file_out, pos_out,
>>                                                        len, flags);
>>
>> -       return generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
>> -                                      flags);
>> +       if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
>> +               return -EXDEV;
>> +       else
>> +               generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
>> +                                       flags);
>>  }
>>
>
> Which kernel is this patch based on?

It was v5.11-rc7.

> At this point, I am with Dave and Darrick on not falling back to
> generic_copy_file_range() at all.
>
> We do not have proof of any workload that benefits from it and the
> above patch does not protect from a wierd use case of trying to copy a file
> from sysfs to sysfs.
>

Ok, cool.  I can post a new patch doing just that.  I guess that function
do_copy_file_range() can be dropped in that case.

> I am indecisive about what should be done with generic_copy_file_range()
> called as fallback from within filesystems.
>
> I think the wise choice is to not do the fallback in any case, but this is up
> to the specific filesystem maintainers to decide.

I see what you mean.  You're suggesting to have userspace handle all the
-EOPNOTSUPP and -EXDEV errors.  Would you rather have a patch that also
removes all the calls to generic_copy_file_range() function?  And that
function can also be deleted too, of course.

Cheers,
-- 
Luis