Re: [Qemu-devel] [PATCH 14/23] userfaultfd: wake pending userfaults

[Andrea Arcangeli]

On Thu, Oct 22, 2015 at 05:15:09PM +0200, Peter Zijlstra wrote:
> Indefinitely is such a long time, we should try and finish
> computation before the computer dies etc. :-)

Indefinitely as read_seqcount_retry, eventually it makes progress.

Even returning 0 from the page fault can trigger it again
indefinitely, so VM_FAULT_RETRY isn't fundamentally different from
returning 0 and retrying the page fault again later. So it's not clear
why VM_FAULT_RETRY can only try once more.

FAULT_FLAG_TRIED as a message to the VM so it starts to do heavy
locking and block more aggressively is actually useful as such, but it
shouldn't be a replacement of FAULT_FLAG_ALLOW_RETRY. What I meant
with removing FAULT_FLAG_TRIED is really about converting it to an
hint, but not controlling if the page fault can keep retrying
in-kernel.