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SubjectRe: still nfs problems [Was: Linux 2.6.37-rc8]
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On Wed, 2011-01-05 at 18:27 +0000, Russell King - ARM Linux wrote: 
> On Wed, Jan 05, 2011 at 01:12:25PM -0500, Trond Myklebust wrote:
> > On Wed, 2011-01-05 at 17:26 +0000, Russell King - ARM Linux wrote:
> > > On Wed, Jan 05, 2011 at 12:17:27PM -0500, Trond Myklebust wrote:
> > > > We should already be flushing the kernel direct mapping after writing by
> > > > means of the calls to flush_dcache_page() in xdr_partial_copy_from_skb()
> > > > and all the helpers in net/sunrpc/xdr.c.
> > > Hmm, we're getting into the realms of what flush_dcache_page() is supposed
> > > to do and what it's not supposed to do.
> > >
> > > Is this page an associated with a mapping (iow, page_mapping(page) is non-
> > > NULL)? If not, flush_dcache_page() won't do anything, and from my
> > > understanding, its flush_anon_page() which you want to be using there
> > > instead.
> >
> > Actually, none of these pages are ever mapped into userspace, nor are
> > they mapped into the page cache.
> >
> > They are allocated directly using alloc_page() by the thread that called
> > the readdir() syscall, so afaics there should be no incoherent mappings
> > other than the kernel direct mapping and the one created by
> > vm_map_ram().
> >
> > So, yes, you are right that we don't need the flush_dcache_page() here.
>
> I do still think you need _something_ there, otherwise data can remain
> in the direct map alias and not be visible via the vmap alias. I don't
> see that we have anything in place to handle this at present though.

Is that perhaps what flush_kernel_dcache_page() is supposed to do?

> jejb mentioned something about making kunmap_atomic() always flush the
> cache, even for lowmem pages, but I think that's going to be exceedingly
> painful, to the extent that I believe it will prevent our PIO-only MMC
> drivers working - or we need a scatterlist API that will let drivers
> iterate over the scatterlist without needing to continually kmap_atomic
> and kunmap_atomic each page.

--
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com



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