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

Andres Lagar-Cavilla
2014-09-17T17:41:06Z

On Wed, Sep 17, 2014 at 10:21 AM, Gleb Natapov <gleb@kernel.org> wrote:
> On Wed, Sep 17, 2014 at 10:13:45AM -0700, Andres Lagar-Cavilla wrote:
>> On Wed, Sep 17, 2014 at 10:08 AM, Gleb Natapov <gleb@kernel.org> wrote:
>> > On Wed, Sep 17, 2014 at 10:00:32AM -0700, Andres Lagar-Cavilla wrote:
>> >> On Wed, Sep 17, 2014 at 4:42 AM, Gleb Natapov <gleb@kernel.org> wrote:
>> >> > On Wed, Sep 17, 2014 at 01:27:14PM +0200, Radim Krčmář wrote:
>> >> >> 2014-09-17 13:26+0300, Gleb Natapov:
>> >> >> > For async_pf_execute() you do not need to even retry. Next guest's page fault
>> >> >> > will retry it for you.
>> >> >>
>> >> >> Wouldn't that be a waste of vmentries?
>> >> > This is how it will work with or without this second gup. Page is not
>> >> > mapped into a shadow page table on this path, it happens on a next fault.
>> >>
>> >> The point is that the gup in the async pf completion from the work
>> >> queue will not relinquish the mmap semaphore. And it most definitely
>> >> should, given that we are likely looking at swap/filemap.
>> >>
>> > I get this point and the patch looks good in general, but my point is
>> > that when _retry() is called from async_pf_execute() second gup is not
>> > needed. In the original code gup is called to do IO and nothing else.
>> > In your patch this is accomplished by the first gup already, so you
>> > can skip second gup if pagep == nullptr.
>>
>> I see. However, if this function were to be used elsewhere in the
>> future, then the "if pagep == NULL don't retry" semantics may not
>> match the new caller's intention. Would you prefer an explicit flag?
>>
> We can add explicit flag whenever such caller will be added, if ever.

Ok. Patch forthcoming.

Paolo, I'm not sure the split will buy anything, because the first
patch will be a one-liner (no point in the new kvm_gup_something
function if the impl won't match the intended semantics of the
function). But if you push back, I'll cut a v3.

Thanks all,
Andres
>
> --
> Gleb.

--
Andres Lagar-Cavilla | Google Kernel Team | andreslc@google.com