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Re: O_DIRECT file access and cache aliasing...

To: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Subject: Re: O_DIRECT file access and cache aliasing...
From: David Daney <ddaney@avtrex.com>
Date: Tue, 28 Aug 2007 21:07:11 -0700
Cc: linux-mips@linux-mips.org, jschmidt@avtrex.com, sfrancis@avtrex.com
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Atsushi Nemoto wrote:
On Tue, 28 Aug 2007 18:04:28 -0700, David Daney <ddaney@avtrex.com> wrote:
When we write files that were opened with O_DIRECT set, we observe that there are many 16 byte chunks of data in the files that contain all zeros instead of the correct data.

My understanding is that the cache is virtually indexed. So I think what is happening is that when data is written to memory by a user application that does an O_DIRECT write, the IDE driver is given a list of pages to transfer to the disk. The driver then does a dma_cache_wback() on the KSEG0 address of the pages before initiating the DMA operation. Since the KSEG0 address and the USEG address of the physical memory are different, the data is never flushed to memory resulting in incorrect data being written to disk.

I think get_user_pages() should flush user data for O_DIRECT.

The get_user_pages() uses flush_anon_page() to do it, and MIPS
flush_anon_page() was added on Mar 2007.  Does your kernel have this
fix?
I doubt it.  We are currently using 2.6.15.

We will look at either moving to a recent kernel or back-porting the patch you mention.

Thanks for the help,
David Daney

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