Re: open(2) says O_DIRECT works on 512 byte boundries?

KAMEZAWA Hiroyuki
2009-02-03T03:42:30Z

On Tue, 3 Feb 2009 11:55:40 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org> wrote:

> On Tue, 3 Feb 2009 03:31:47 +0100
> Andrea Arcangeli <aarcange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
>
> > On Tue, Feb 03, 2009 at 10:29:20AM +0900, KAMEZAWA Hiroyuki wrote:
> > > On Mon, 2 Feb 2009 23:08:56 +0100
> > > Andrea Arcangeli <aarcange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> > >
> > > > Hi Greg!
> > > >
> > > > > Thanks for the pointers, I'll go read the thread and follow up there.
> > > >
> > > > If you also run into this final fix is attached below. Porting to
> > > > mainline is a bit hard because of gup-fast... Perhaps we can use mmu
> > > > notifiers to fix gup-fast... need to think more about it then I'll
> > > > post something.
> > > >
> > > > Please help testing the below on pre-gup-fast kernels, thanks!
> > > >
> > > I commented in FJ-Redhat Path but not forwared from unknown reason ;)
> > > I comment again.
> > >
> > > 1. Why TestSetLockPage() is necessary ?
> > > It seems not necesary.
> >
> > To avoid the VM to remove or add the page from/to swapcache and change
> > page_count/mapcount from under us. This most certainly wasn't the
> > reason of the slowdown (the slowdown were the false positives
> > generated by pagevec pinning) and removing it was more intrusive than
> > I wanted.
>
> My point is.
> - If TestSetLockPage() failes, force_cow=1.
> - If count/mapcount check fails, force_cow=1.
>
> So, lock_page() here seems meaningless. If you consider lock_page() is important,
> just use lock_page() seems better.
>
I changed my thinking. I understood *lock* is necessary here.
But I'm not sure this is enough.

> > > 3. Why "follow_page() successfully finds a page" case only ?
> > > not necessary to insert SetPageGUP() in following path ?
> > >
> > > - handle_mm_fault()
> > > => do_anonymos/swap/wp_page()
> > > or some.
> >
> > No need to change that either, all we need to know are the pages whose
> > count vs mapcount has a discrepancy that could have been caused by
> > get_user_pages. So only follow_page has to set it. More precisely
> > FOLL_GET|FOLL_WRITE is the only path we care about there.
> >
>
> Assume 3 threads in a process.
please ignore this, I'm shame ;(

-Kame