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

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Date:	Fri, 16 Jan 2009 14:51:19 +0900
From:	MinChan Kim <minchan.kim@...il.com>
To:	Matthew Wilcox <matthew@....cx>
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 Thu, Jan 15, 2009 at 10:33:38PM -0700, Matthew Wilcox wrote:
> 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.

I see. :)

Thanks for quick reponse and good explaination.
Hmm,.. one more question. 

I can't find flush_dcache_page call in mpage_readpage which is 
generic read function. In case of ext fs, it use mpage_readpage 
with readpage.

who and where call flush_dcache_page in mpage_readpage call path?


> 
> -- 
> 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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