linux-kernel - Re: cp --reflink and target file open flags

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Message-ID: <CADT32eJHi--oChq5TJLjv5GsVYAOGdnpY_=nxTBE10dKasr8Hg@mail.gmail.com>
Date:	Thu, 17 Oct 2013 13:27:52 -0500
From:	Shirish Pargaonkar <shirishpargaonkar@...il.com>
To:	David Disseldorp <ddiss@...e.de>
Cc:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	samba-technical <samba-technical@...ts.samba.org>,
	"linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: cp --reflink and target file open flags

I think it would not matter which ioctl is used if the target file is
not opened with flag O_RDWR.

Perhaps cifs.ko can make the decision to change O_WRONLY
to O_RDWR based on the flag and location of the source and
destination files.


On Thu, Oct 17, 2013 at 5:45 AM, David Disseldorp <ddiss@...e.de> wrote:
> On Wed, 16 Oct 2013 18:36:19 -0500
> Steve French <smfrench@...il.com> wrote:
>
>> cp --reflink opens the target file for O_WRONLY before invoking the
>> (BTRFS) ioctl for clone file, but for copy offload over the network
>> the SMB2 specification requires that the target file be open O_RDWR.
>>
>> I may be able to upgrade the target file handle on the fly by
>> reopening it in cifs.ko, and of course I can write an SMB2/SMB3
>> specific copy command, but it would be preferable to allow use of cp
>> --reflink since so many people are familiar with it.
>>
>> There is quite a bit of flexibility in server side copy offload  -
>> more than cp an offer, especially when using SMB3 or later dialects
>> (e.g. in number of chunks sent at one time, chunk size, attributes
>> copied, and even whether to use T10 style offload), but still it would
>> be nice to support "cp --reflink" over the network.  Any ideas on
>> this?
>>
>> After looking at copy.c in coreutils for cp - I couldn't think of any
>> trivial way to force cp to open the target RW.
>>
>> Ideas?
>
> You should be able to avoid this by using FSCTL_SRV_COPYCHUNK_WRITE
> instead of FSCTL_SRV_COPYCHUNK on the wire. The former doesn't require
> read access on the target, while the latter does. See [MS-SMB2] 2.2.31
> and smbtorture's copy_chunk_bad_access test.
>
> Samba only supported FSCTL_SRV_COPYCHUNK until now, as that's what
> Windows Server 2k12 uses for copies initiated by Explorer. I've just
> sent out the trivial patches adding FSCTL_SRV_COPYCHUNK_WRITE support.
>
> Cheers, David
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