linux-kernel - Re: [PATCH] Remove needless flush_dcache

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Date:	Thu, 15 Jan 2009 22:33:38 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	MinChan Kim <minchan.kim@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, npiggin@...e.de,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] Remove needless flush_dcache_page call

On Fri, Jan 16, 2009 at 02:28:04PM +0900, MinChan Kim wrote:
> Now, Anyone don't maintain cramfs.
> I don't know who is maintain romfs. so I send this patch to linux-mm, 
> lkml, linux-dev. 
> 
> I am not sure my thought is right. 
> 
> When readpage is called, page with argument in readpage is just new 
> allocated because kernel can't find that page in page cache. 
> 
> At this time, any user process can't map the page to their address space. 
> so, I think D-cache aliasing probelm never occur. 
> 
> It make sense ?

Sorry, no.  You have to call fluch_dcache_page() in two situations --
when the kernel is going to read some data that userspace wrote, *and*
when userspace is going to read some data that the kernel wrote.  From a
quick look at the patch, this seems to be the second case.  The kernel
wrote data to a pagecache page, and userspace should be able to read it.

To understand why this is necessary, consider a processor which is
virtually indexed and has a writeback cache.  The kernel writes to a
page, then a user process reads from the same page through a different
address.  The cache doesn't find the data the kernel wrote because it
has a different virtual index, so userspace reads stale data.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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