Re: Potential problem with the page_mapping macro and flush_dcache_page()

["David S. Miller" <davem@davemloft.net>]

From: James Bottomley <James.Bottomley@SteelEye.com>
Date: Sun, 19 Mar 2006 14:53:08 -0600

> Actually, this is my specific worry.  On parisc, the coherence index
> associated with the DMA is that of the kernel mapping, not the user
> mappings, so DMA only flushes the kernel cache for the line, not the
> user one.  If nothing happens to purge the user line, DMA to anon pages
> won't work on parisc either.

You have physically indexed caches, thus cache coherency transactions
should purge all cache lines with that same physical TAG.

If not, you have much larger problems.

> I do think we need to begin considering it.  The user space device
> managers, like hal, are starting to use SG_IO to interrogate devices,
> making it much more prevalent than before.  It's got to be only a matter
> of time before this issue bites something like hal.
> 
> Is the principle that get_user_pages() should return coherent pages OK?
> If so, I think the fix is to have get_user_pages do a specific user page
> flush for anonymous pages.

Direct I/O is another problematic case.