linux-kernel - Re: O_DIRECT question

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Date:	Fri, 12 Jan 2007 20:03:15 +0300
From:	Viktor <vvp01@...ox.ru>
To:	Linus Torvalds <torvalds@...l.org>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	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
Subject: Re: O_DIRECT question

Linus Torvalds wrote:
>>>>O_DIRECT is still crazily racy versus pagecache operations.
>>>
>>>Yes. O_DIRECT is really fundamentally broken. There's just no way to fix 
>>>it sanely.
>>
>>How about aliasing O_DIRECT to POSIX_FADV_NOREUSE (sortof) ?
> 
> 
> That is what I think some users could do. If the main issue with O_DIRECT 
> is the page cache allocations, if we instead had better (read: "any") 
> support for POSIX_FADV_NOREUSE, one class of reasons O_DIRECT usage would 
> just go away.
> 
> See also the patch that Roy Huang posted about another approach to the 
> same problem: just limiting page cache usage explicitly.
> 
> That's not the _only_ issue with O_DIRECT, though. It's one big one, but 
> people like to think that the memory copy makes a difference when you do 
> IO too (I think it's likely pretty debatable in real life, but I'm totally 
> certain you can benchmark it, probably even pretty easily especially if 
> you have fairly studly IO capabilities and a CPU that isn't quite as 
> studly).
> 
> So POSIX_FADV_NOREUSE kind of support is one _part_ of the O_DIRECT 
> picture, and depending on your problems (in this case, the embedded world) 
> it may even be the *biggest* part. But it's not the whole picture.

>From 2.6.19 sources it looks like POSIX_FADV_NOREUSE is no-op there

> 		Linus

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