Re: O_DIRECT file access and cache aliasing...

[David Daney <ddaney@avtrex.com>]

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