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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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