linux-kernel - Re: [PATCH] copy_file_range.2: Kernel v5.12 updates

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Message-ID: <ffd92bb4-8f72-cbec-045f-a2ad7869ab3b@gmail.com>
Date:   Sat, 27 Feb 2021 13:20:31 +0100
From:   "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To:     Amir Goldstein <amir73il@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Luis Henriques <lhenriques@...e.de>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Jeff Layton <jlayton@...nel.org>,
        Steve French <sfrench@...ba.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dave Chinner <dchinner@...hat.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Ian Lance Taylor <iant@...gle.com>,
        Luis Lozano <llozano@...omium.org>,
        Andreas Dilger <adilger@...ger.ca>,
        Olga Kornievskaia <aglo@...ch.edu>,
        Christoph Hellwig <hch@...radead.org>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        samba-technical <samba-technical@...ts.samba.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH] copy_file_range.2: Kernel v5.12 updates

Hi Amir,

On 2/27/21 6:41 AM, Amir Goldstein wrote:
> On Sat, Feb 27, 2021 at 12:19 AM Alejandro Colomar (man-pages)
>> On 2/24/21 5:10 PM, Amir Goldstein wrote:
>>> On Wed, Feb 24, 2021 at 4:22 PM Luis Henriques <lhenriques@...e.de> wrote:
>>>>    .TP
>>>> +.B EOPNOTSUPP
>>
>> I'll add the kernel version here:
>>
>> .BR EOPNOTSUPP " (since Linux 5.12)"
> 
> Error could be returned prior to 5.3 and would be probably returned
> by future stable kernels 5.3..5.12 too

OK, I think I'll state <5.3 and >=5.12 for the moment, and if Greg adds 
that to stable 5.3..5.11 kernels, please update me.

>>>>    .B EXDEV
>>>>    The files referred to by
>>>>    .IR fd_in " and " fd_out
>>>> -are not on the same mounted filesystem (pre Linux 5.3).
>>>> +are not on the same mounted filesystem (pre Linux 5.3 and post Linux 5.12).
>>
>> I'm not sure that 'mounted' adds any value here.  Would you remove the
>> word here?
> 
> See rename(2). 'mounted' in this context is explained there.
> HOWEVER, it does not fit here.
> copy_file_range() IS allowed between two mounts of the same filesystem instance.

Also allowed for <5.3 ?

> 
> To make things more complicated, it appears that cross mount clone is not
> allowed via FICLONE/FICLONERANGE ioctl, so ioctl_ficlonerange(2) man page
> also uses the 'mounted filesystem' terminology for EXDEV
> 
> As things stand now, because of the fallback to clone logic,
> copy_file_range() provides a way for users to clone across different mounts
> of the same filesystem instance, which they cannot do with the FICLONE ioctl.
> 
> Fun :)
> 
> BTW, I don't know if preventing cross mount clone was done intentionally,
> but as I wrote in a comment in the code once:
> 
>          /*
>           * FICLONE/FICLONERANGE ioctls enforce that src and dest files are on
>           * the same mount. Practically, they only need to be on the same file
>           * system.
>           */

:)

> 
>>
>> It reads as if two separate devices with the same filesystem type would
>> still give this error.
>>
>> Per the LWN.net article Amir shared, this is permitted ("When called
>> from user space, copy_file_range() will only try to copy a file across
>> filesystems if the two are of the same type").
>>
>> This behavior was slightly different before 5.3 AFAICR (was it?) ("until
>> then, copy_file_range() refused to copy between files that were not
>> located on the same filesystem.").  If that's the case, I'd specify the
>> difference, or more probably split the error into two, one before 5.3,
>> and one since 5.12.
>>
> 
> True.
> 
>>>
>>> I think you need to drop the (Linux range) altogether.
>>
>> I'll keep the range.  Users of 5.3..5.11 might be surprised if the
>> filesystems are different and they don't get an error, I think.
>>
>> I reworded it to follow other pages conventions:
>>
>> .BR EXDEV " (before Linux 5.3; or since Linux 5.12)"
>>
>> which renders as:
>>
>>          EXDEV (before Linux 5.3; or since Linux 5.12)
>>                 The files referred to by fd_in and fd_out are not on
>>                 the same mounted filesystem.
>>
> 
> drop 'mounted'

Yes

> 
>>
>>> What's missing here is the NFS cross server copy use case.
>>> Maybe:
>>>
>>> ...are not on the same mounted filesystem and the source and target filesystems
>>> do not support cross-filesystem copy.
>>
>> Yes.
>>
>> Again, this wasn't true before 5.3, right?
>>
> 
> Right.
> Actually, v5.3 provides the vfs capabilities for filesystems to support
> cross fs copy. I am not sure if NFS already implements cross fs copy in
> v5.3 and not sure about cifs. Need to get input from nfs/cis developers
> or dig in the release notes for server-side copy.

Okay
> Thanks to LWN :)

:)

Thanks,

Alex


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/