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SubjectRe: Question about x86/mm/gup.c's use of disabled interrupts
Jeremy Fitzhardinge wrote:
>>> 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.
>> The thread that cleared the pte holds the pte lock and is now waiting
>> for the IPI. The thread that wants to update the pte will wait for
>> the pte lock, thus also waits on the IPI and gup_fast()'s
>> local_irq_enable(). I think.
> But hasn't it already done the pte update at that point?
> (I think this conversation really is moot because the kernel never
> does P->P pte updates any more; its always P->N->P.)

I thought you were concerned about cpu 0 doing a gup_fast(), cpu 1 doing
P->N, and cpu 2 doing N->P. In this case cpu 2 is waiting on the pte lock.

>>> Is this the only reason to disable interrupts?
>> Another comment says it also prevents pagetable teardown.
> We could take a reference to the mm to get the same effect, no?

Won't stop munmap().

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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