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SubjectRe: [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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