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

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Date:	Fri, 16 Jan 2009 15:08:30 +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 10:57:30PM -0700, Matthew Wilcox wrote:
> On Fri, Jan 16, 2009 at 02:51:19PM +0900, MinChan Kim wrote:
> > On Thu, Jan 15, 2009 at 10:33:38PM -0700, Matthew Wilcox wrote:
> > > 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?
> 
> Most I/O devices will do DMA to the page in question and thus the kernel
> hasn't written to it and the CPU won't have the data in cache.  For the
> few devices which can't do DMA, it's the responsibility of the device
> driver to call flush_dcache_page() (or some other flushing primitive).

Hmm.. Now I am confusing. 
If devicer driver or with DMA makes sure cache consistency,
Why filesystem code have to handle it ?

> See the gdth driver for an example:
> 
>             address = kmap_atomic(sg_page(sl), KM_BIO_SRC_IRQ) + sl->offset;
>             memcpy(address, buffer, cpnow);
>             flush_dcache_page(sg_page(sl));
>             kunmap_atomic(address, KM_BIO_SRC_IRQ);
> 
> -- 
> 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."

--
Kinds regards,
MinChan Kim

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