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

Gleb Natapov
2014-09-17T17:21:37Z

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.

--
Gleb.