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SubjectRe: still nfs problems [Was: Linux 2.6.37-rc8]
On Wed, 2011-01-05 at 13:30 -0800, Linus Torvalds wrote: 
> On Wed, Jan 5, 2011 at 1:16 PM, Trond Myklebust
> <> wrote:
> >
> > So what should be the preferred way to ensure data gets flushed when
> > you've written directly to a page, and then want to read through the
> > vm_map_ram() virtual range? Should we be adding new semantics to
> > flush_kernel_dcache_page()?
> The "preferred way" is actually simple: "don't do that". IOW, if some
> page is accessed through a virtual mapping you've set up, then
> _always_ access it through that virtual mapping.
> Now, when that is impossible (and yes, it sometimes is), then you
> should flush after doing all writes. And if you do the write through
> the regular kernel mapping, you should use flush_dcache_page(). And if
> you did it through the virtual mapping, you should use
> "flush_kernel_vmap_range()" or whatever.
> NOTE! I really didn't look those up very closely, and if the accesses
> can happen concurrently you are basically screwed, so you do need to
> do locking or something else to guarantee that there is some nice
> sequential order. And maybe I forgot something. Which is why I do
> suggest "don't do that" as a primary approach to the problem if at all
> possible.
> Oh, and you may need to flush before reading too (and many writes do
> end up being "read-modify-write" cycles) in case it's possible that
> you have stale data from a previous read that was then invalidated by
> a write to the aliasing address. Even if that write was flushed out,
> the stale read data may exist at the virtual address. I forget what
> all we required - in the end the only sane model is "virtual caches
> suck so bad that anybody who does them should be laughed at for being
> a retard".

Yes. The fix I sent out was a call to invalidate_kernel_vmap_range(),
which takes care of invalidating the cache prior to a virtual address

My question was specifically about the write through the regular kernel
mapping: according to Russell and my reading of the cachetlb.txt
documentation, flush_dcache_page() is only guaranteed to have an effect
on page cache pages.
flush_kernel_dcache_page() (not to be confused with flush_dcache_page)
would appear to be the closest fit according to my reading of the
documentation, however the ARM implementation appears to be a no-op...

Trond Myklebust
Linux NFS client maintainer


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