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xen-devel
[Xen-devel] Re: Question about x86/mm/gup.c's use of disabled interrupts
Nick Piggin wrote:
Also, assuming that disabling the interrupt is enough to get the
guarantees we need here, there's a Xen problem because we don't use IPIs
for cross-cpu tlb flushes (well, it happens within Xen). I'll have to
think a bit about how to deal with that, but I'm thinking that we could
add a per-cpu "tlb flushes blocked" flag, and maintain some kind of
per-cpu deferred tlb flush count so we can get around to doing the flush
eventually.
But I want to make sure I understand the exact algorithm here.
FWIW, powerpc actually can flush tlbs without IPIs, and it also has
a gup_fast. powerpc RCU frees its page _tables_ so we can walk them,
and then I use speculative page references in order to be able to
take a reference on the page without having it pinned.
Ah, interesting. So disabling interrupts prevents the RCU free from
happening, and non-atomic pte fetching is a non-issue. So it doesn't
address the PAE side of the problem.
Turning gup_get_pte into a pvop would be a bit nasty because on !PAE
it is just a single load, and even on PAE it is pretty cheap.
Well, it wouldn't be too bad; for !PAE it would turn into something we
could inline, so there'd be little to no cost. For PAE it would be out
of line, but a direct function call, which would be nicely cached and
very predictable once we've gone through the the loop once (and for Xen
I think I'd just make it a cmpxchg8b-based implementation, assuming that
the tlb flush hypercall would offset the cost of making gup_fast a bit
slower).
But it would be better if we can address it at a higher level.
J
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