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

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Date:	Tue, 19 Mar 2013 11:04:10 +0000
From:	David Howells <dhowells@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.cz>, 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

Miklos Szeredi <miklos@...redi.hu> wrote:

> > 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.

Yes, but it could block the parent directory for a long time.  I suspect it's
fine if you can RCU walk through the parent, but if you have to grab a lock on
it...

David
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