Re: unit tests and get_user_pages_ptes_fast()

[Avi Kivity <avi@redhat.com>]

  On 10/05/2010 04:32 PM, Andrea Arcangeli wrote:
> On Tue, Oct 05, 2010 at 04:25:05PM +0200, Avi Kivity wrote:
> >    On 10/05/2010 04:15 PM, Andrea Arcangeli wrote:
> >  >  On Tue, Oct 05, 2010 at 06:22:17AM -0300, Marcelo Tosatti wrote:
> >  >  >   It'll not be so advantageous for ksm because there should be read-faults
> >  >  >   very rarely on that case.
> >  >
> >  >  It'll also make all clean swapcache dirty for no good.
> >  >
> >  >  >   Will post.
> >  >
> >  >  If we've to walk pagetables twice, why don't you do this:
> >  >
> >  >  writable=1
> >  >  get_user_pages_fast(write=write_fault)
> >  >  if (!write_fault)
> >  >      writable = __get_user_pages_fast(write=1)
> >  >
> >  >  That will solve the debugging knob and it'll solve ksm and it'll be
> >  >  optimal for read swapins on exclusive clean swapcache too.
> >
> >  But it means an extra vmexit in the following case:
> >
> >  - read fault
> >  - page is present and writeable in the Linux page table
> >
> >  which is very common.  For this you need get_user_pages_ptes_fast().
>
> With a read fault, the VM already sets the pte as writable if the VM
> permissions allows that and the page isn't shared (i.e. if it's an
> exclusive swap page). We've just to check if it did that or not. So
> when it's a read fault we've to run __get_user_pages_fast(write=1)
> before we can assume the page is mapped writable in the pte.
>
> So I don't see the problem... in terms of page faults is optimal. Only
> downside is having to walk the pagetables twice, the second time to
> verify if the first gup_fast has marked the host pte writable or not.

You're right, I misread your pseudocode.

-- 
error compiling committee.c: too many arguments to function