Re: GUP guarantees wrt to userspace mappings

[Andrea Arcangeli]

On Mon, May 02, 2016 at 06:00:13PM +0300, Kirill A. Shutemov wrote:
> Switching to non-fast GUP would help :-P

If we had a race in khugepaged or ksmd against gup_fast O_DIRECT we'd
get flood of bugreports of data corruption with KVM run with
cache=direct.

Just wanted to reassure there's no race, explained how the
serialization to force a fallback to non-fast GUP works in previous
email.

This issue we're fixing for the COW is totally unrelated to KVM too,
because it uses MADV_DONTFORK, but the other races with O_DIRECT
against khugepaged/kksmd would still happen if we didn't already have
proper serialization against get_user_pages_fast.

> Alternatively, we have mmu_notifiers to track changes in userspace
> mappings.

This is always the absolute best solution, then no gup pins are used
at all and all VM functionality is activated regardless of the
secondary MMU, just most IOMMUs can't generate a synchronous page
fault, when they fault the I/O is undefined. It'd be like if when you
get a page fault in the CPU, when you return from the fault you go to
then next instruction and during the fault you've no way to even
emulate the faulting instruction.