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Date: Sat, 18 Jan 2014 20:10:31 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH 0/5] splice: locking changes and code refactoring
Message-ID: <20140118201031.GI10323@ZenIV.linux.org.uk>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jens Axboe <axboe@kernel.dk>, Steve French <sfrench@samba.org>, Sage Weil <sage@inktank.com>, Mark Fasheh <mfasheh@suse.com>, xfs@oss.sgi.com, Christoph Hellwig <hch@infradead.org>, Joel Becker <jlbec@evilplan.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>

On Sat, Jan 18, 2014 at 11:59:56AM -0800, Linus Torvalds wrote:
> On Fri, Jan 17, 2014 at 11:46 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > On Fri, Jan 17, 2014 at 11:22:04PM -0800, Linus Torvalds wrote:
> >> On Fri, Jan 17, 2014 at 10:40 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >> >
> >> > Objections, comments?
> >>
> >> I certainly object to the "map, then unmap" approach. No VM games.
> >
> > Um...
> >
> > If we are going to copy that data (and all users of generic_file_splice_write()
> > do that memcpy() to page cache), we have to kmap the source ;-/
> 
> Yeah, the kmap/kunmap we have to do. But that's a no-op on 64-bit, and
> has to be done one page at a time (well, I guess you could do a
> couple).
> 
> But you can't do that *around* the default_file_splice_write(), so I
> thought you meant some kind of "map into user space". And I absolutely
> *detest* that kind of approach.

Ouch...  No, I hadn't meant that kind of insanity, but I'd missed the
problem with scarcity of mappings completely...

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