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SubjectQuestion about x86/mm/gup.c's use of disabled interrupts
Hi Nick,

The comment in arch/x86/mm/gup.c:gup_get_pte() says:

[...] What
* we do have is the guarantee that a pte will only either go from not
* present to present, or present to not present or both -- it will not
* switch to a completely different present page without a TLB flush in
* between; something that we are blocking by holding interrupts off.


Disabling the interrupt will prevent the tlb flush IPI from coming in
and flushing this cpu's tlb, but I don't see how it will prevent some
other cpu from actually updating the pte in the pagetable, which is what
we're concerned about here. Is this the only reason to disable
interrupts? Would we need to do it for the !PAE cases?

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.

(I couldn't find an instance of a pte update going from present->present
anyway; the only caller of set_pte_present is set_pte_vaddr which only
operates on kernel mappings, so perhaps this is moot. Oh, look,
native_set_pte_present thinks its only called on user mappings... In
fact set_pte_present seems to have completely lost its rationale for
existing.)

Thanks,
J


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