linux-kernel - Re: [GIT PULL] PMEM driver for v4.1

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Message-ID: <20150413172225.GC14243@lst.de>
Date:	Mon, 13 Apr 2015 19:22:25 +0200
From:	Christoph Hellwig <hch@....de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Boaz Harrosh <boaz@...xistor.com>, Christoph Hellwig <hch@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>
Subject: Re: [GIT PULL] PMEM driver for v4.1

On Mon, Apr 13, 2015 at 02:35:35PM +0200, Ingo Molnar wrote:
> How does splice work with DAX files?

By falling back to default_file_splice_read/default_file_splice_write
which doesn't use the iter ops, but instead requires a copy in the
splice code.  But given that the actual underlying reads and writes
bypass the pagecache it's not any less effiecient than the normal
pagecache based splice.

> AFAICS vmsplice() won't work, as 
> it uses get_user_pages(), which needs struct page backing.

Exactly.

> Also, how 
> will f_op->sendpage() work? That too needs page backing.

default_file_splice_read allocates it's own kernel pages, which are
then passed to ->sendpage.
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