linux-kernel - Re: [PATCH 2/9] vfs: export do_splice

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Message-ID: <CAJfpegt8Zi3f5VxWc0kGRFcra3x_rze8j6dKCCTb4t+ViMCVYA@mail.gmail.com>
Date:	Tue, 19 Mar 2013 11:29:41 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Jan Kara <jack@...e.cz>, David Howells <dhowells@...hat.com>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	akpm@...ux-foundation.org, apw@...onical.com, nbd@...nwrt.org,
	neilb@...e.de, jordipujolp@...il.com, ezk@....cs.sunysb.edu,
	sedat.dilek@...glemail.com, hooanon05@...oo.co.jp, mszeredi@...e.cz
Subject: Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

On Tue, Mar 19, 2013 at 2:38 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Mon, Mar 18, 2013 at 11:01:03PM +0000, Al Viro wrote:
>
>> I'm looking at the existing callers and I really wonder if we ought to
>> push sb_start_write() from ->splice_write()/->aio_write()/etc. into the
>> callers.
>>
>> Something like file_start_write()/file_end_write(), with check for file
>> being regular one might be a good starting point.  As it is, copyup is
>> really fucked both in unionmount and overlayfs...
>
> BTW, I wonder what's the right locking for that sucker; overlayfs is probably
> too heavy - we are talking about copying a file from one fs to another, which
> can obviously take quite a while, so holding ->i_mutex on _parent_ all along
> is asking for very serious contention.

Copy up is a once-in-a-lifetime event for an object.  Optimizing it is
way down in the list of things to do.  I'd drop splice in a jiffy if
it's in the way.

Much more interesting question:  what happens if we crash during a
rename?  Whiteout implemented in the filesystem won't save us.  And
the results are interesting: old versions of files become visible and
similar fun.  Far from likely to happen, but ...

Add a rename-with-whiteout primitive on filesystems?  That one is not
going to be as simple as plain whiteout.  Or?

Thanks,
Miklos
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