linux-kernel - Re: O_DIRECT question

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Date:	Thu, 11 Jan 2007 23:57:40 -0800 (PST)
From:	dean gaudet <dean@...tic.org>
To:	Linus Torvalds <torvalds@...l.org>
cc:	Viktor <vvp01@...ox.ru>, Aubrey <aubreylee@...il.com>,
	Hua Zhong <hzhong@...il.com>, Hugh Dickins <hugh@...itas.com>,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	kenneth.w.chen@...el.com, akpm@...l.org, mjt@....msk.ru
Subject: Re: O_DIRECT question

On Thu, 11 Jan 2007, Linus Torvalds wrote:

> On Thu, 11 Jan 2007, Viktor wrote:
> > 
> > OK, madvise() used with mmap'ed file allows to have reads from a file
> > with zero-copy between kernel/user buffers and don't pollute cache
> > memory unnecessarily. But how about writes? How is to do zero-copy
> > writes to a file and don't pollute cache memory without using O_DIRECT?
> > Do I miss the appropriate interface?
> 
> mmap()+msync() can do that too.
> 
> Also, regular user-space page-aligned data could easily just be moved into 
> the page cache. We actually have a lot of the infrastructure for it. See 
> the "splice()" system call.

it seems to me that if splice and fadvise and related things are 
sufficient for userland to take care of things "properly" then O_DIRECT 
could be changed into splice/fadvise calls either by a library or in the 
kernel directly...

looking at the splice(2) api it seems like it'll be difficult to implement 
O_DIRECT pread/pwrite from userland using splice... so there'd need to be 
some help there.

i'm probably missing something.

-dean
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