Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem

Paolo Bonzini
2014-09-17T07:43:46Z

Il 16/09/2014 20:42, Andres Lagar-Cavilla ha scritto:
> On Tue, Sep 16, 2014 at 11:29 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>> I think a first patch should introduce kvm_get_user_page_retry ("Retry a
>>> fault after a gup with FOLL_NOWAIT.") and the second would add
>>> FOLL_TRIED ("This properly relinquishes mmap semaphore if the
>>> filemap/swap has to wait on page lock (and retries the gup to completion
>>> after that").
>>>
>>> That's not what FOLL_TRIED does. The relinquishing of mmap semaphore is
>>> done by this patch minus the FOLL_TRIED bits. FOLL_TRIED will let the
>>> fault handler (e.g. filemap) know that we've been there and waited on
>>> the IO already, so in the common case we won't need to redo the IO.
>>
>> Yes, that's not what FOLL_TRIED does. But it's the difference between
>> get_user_pages and kvm_get_user_page_retry, right?
>
> Unfortunately get_user_pages does not expose the param (int
> *nonblocking) that __gup will use to set FAULT_FLAG_ALLOW_RETRY. So
> that's one difference. The second difference is that kvm_gup_retry
> will call two times if necessary (the second without _RETRY but with
> _TRIED).

Yeah, that's how it is in your patch. I can see that.

What I'm saying is that your patch is two changes in one:

1) do not use gup_fast in hva_to_pfn_slow, instead use gup as in
async_pf_execute. This change can already introduce a function called
kvm_get_user_page_retry, and can already use it in async_pf_execute and
hva_to_pfn_slow

2) introduce the two-phase RETRY + TRIED mechanism in
kvm_get_user_page_retry, so that the mmap semaphore is relinquished
properly if the filemap or swap has to wait on the page lock.

I would prefer to split it in two patches. Is it clearer now?

Paolo