RE: [RFC PATCH] xen/privcmd: Convert get_user_pages*() to pin_user_pages*()

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Sent: 17 June 2020 18:57
> To: Souptick Joarder <jrdr.linux@xxxxxxxxx>; jgross@xxxxxxxx; 
> sstabellini@xxxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; John 
> Hubbard <jhubbard@xxxxxxxxxx>;
> paul@xxxxxxx
> Subject: Re: [RFC PATCH] xen/privcmd: Convert get_user_pages*() to 
> pin_user_pages*()
> 
> On 6/16/20 11:14 PM, Souptick Joarder wrote:
> > In 2019, we introduced pin_user_pages*() and now we are converting
> > get_user_pages*() to the new API as appropriate. [1] & [2] could
> > be referred for more information.
> >
> > [1] Documentation/core-api/pin_user_pages.rst
> >
> > [2] "Explicit pinning of user-space pages":
> >         https://lwn.net/Articles/807108/
> >
> > Signed-off-by: Souptick Joarder <jrdr.linux@xxxxxxxxx>
> > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > ---
> > Hi,
> >
> > I have compile tested this patch but unable to run-time test,
> > so any testing help is much appriciated.
> >
> > Also have a question, why the existing code is not marking the
> > pages dirty (since it did FOLL_WRITE) ?
> 
> 
> Indeed, seems to me it should. Paul?
> 

Yes, it looks like that was an oversight. The hypercall may well result in data 
being copied back into the buffers so the whole pages array should be 
considered dirty.

  Paul

> 
> >
> >  drivers/xen/privcmd.c | 7 ++-----
> >  1 file changed, 2 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> > index a250d11..543739e 100644
> > --- a/drivers/xen/privcmd.c
> > +++ b/drivers/xen/privcmd.c
> > @@ -594,7 +594,7 @@ static int lock_pages(
> >             if (requested > nr_pages)
> >                     return -ENOSPC;
> >
> > -           pinned = get_user_pages_fast(
> > +           pinned = pin_user_pages_fast(
> >                     (unsigned long) kbufs[i].uptr,
> >                     requested, FOLL_WRITE, pages);
> >             if (pinned < 0)
> > @@ -614,10 +614,7 @@ static void unlock_pages(struct page *pages[], 
> > unsigned int nr_pages)
> >     if (!pages)
> >             return;
> >
> > -   for (i = 0; i < nr_pages; i++) {
> > -           if (pages[i])
> > -                   put_page(pages[i]);
> > -   }
> > +   unpin_user_pages(pages, nr_pages);
> 
> 
> Why are you no longer checking for valid pages?
> 
> 
> -boris
> 
> 
>