linux-kernel - Re: [PATCH 1/6] fs: Add flag to file_system

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Date:   Fri, 12 Feb 2021 15:01:54 +0000
From:   Luis Henriques <lhenriques@...e.de>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Amir Goldstein <amir73il@...il.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Ian Lance Taylor <iant@...gle.com>,
        Luis Lozano <llozano@...omium.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate
 content is generated

Greg KH <gregkh@...uxfoundation.org> writes:

> On Fri, Feb 12, 2021 at 12:41:48PM +0000, Luis Henriques wrote:
>> Greg KH <gregkh@...uxfoundation.org> writes:
...
>> >> >> Our option now are:
>> >> >> - Restore the cross-fs restriction into generic_copy_file_range()
>> >> >
>> >> > Yes.
>> >> >
>> >> 
>> >> Restoring this restriction will actually change the current cephfs CFR
>> >> behaviour.  Since that commit we have allowed doing remote copies between
>> >> different filesystems within the same ceph cluster.  See commit
>> >> 6fd4e6348352 ("ceph: allow object copies across different filesystems in
>> >> the same cluster").
>> >> 
>> >> Although I'm not aware of any current users for this scenario, the
>> >> performance impact can actually be huge as it's the difference between
>> >> asking the OSDs for copying a file and doing a full read+write on the
>> >> client side.
>> >
>> > Regression in performance is ok if it fixes a regression for things that
>> > used to work just fine in the past :)
>> >
>> > First rule, make it work.
>> 
>> 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);
>>  }
>>  
>>  /*
>
> That would make much more sense to me.

Great.  I can send a proper patch with changelog, if this is the really
what we want.  But I would rather hear from others first.  I guess that at
least the NFS devs have something to say here.

Cheers,
-- 
Luis