Re: arm64 cache maintenance on read only address loops forever

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On 2/26/2014 6:03 AM, Catalin Marinas wrote:
On Tue, Feb 25, 2014 at 08:59:46PM -0800, Laura Abbott wrote:
On arm64, set_pte_at currently write protects user ptes that are not
dirty. The expected behavior is that the fault handler will fix this
up on a write to the address. do_page_fault will not mark the fault
as a write though if ESR has the CM (cache maintenance) bit set.
This has the unfortunate side effect that if cache maintenance is
performed on a user address that has not yet been marked as dirty,
handle_mm_fault may return without actually adjusting the pte or
returning an error. This means that the fault will be infinitely
retried.

Calling cache maintenance on an address that hasn't actually been
written to isn't all that useful but looping forever seems like a
poor result. It seems like the check in do_page_fault is too
restrictive and we need to be able to fault in pages via cache
maintenance.

Which kernel are you using? We had a fix in this area, commit
db6f41063cbdb58b14846e600e6bc3f4e4c2e888 (arm64: mm: don't treat user
cache maintenance faults as writes).


I'm using a 3.10 based kernel with stable fixes pulled in. db6f41063cbdb58b14846e600e6bc3f4e4c2e888 is present in the tree and reverting does not make a different.

Thanks,
Laura


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