linux-kernel - Re: [PATCH 2/2] xen/privcmd: Convert get_user_pages*() to pin_user

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Message-ID: <CAFqt6zZdq_OMZ3EBDGC+Bn4uPBEhDGOYF=jB4B16z7rY6hpZ7g@mail.gmail.com>
Date:   Fri, 26 Jun 2020 10:56:53 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, sstabellini@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Paul Durrant <xadimgnik@...il.com>
Subject: Re: [PATCH 2/2] xen/privcmd: Convert get_user_pages*() to pin_user_pages*()

On Thu, Jun 25, 2020 at 11:19 AM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 2020-06-24 20:02, 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. This is case 5 as per document [1].
> >
> > [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@...il.com>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> > Cc: Paul Durrant <xadimgnik@...il.com>
> > ---
> > Hi,
> >
> > I'm compile tested this, but unable to run-time test, so any testing
> > help is much appriciated.
> >
> >   drivers/xen/privcmd.c | 10 ++--------
> >   1 file changed, 2 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> > index 0da417c..eb05254 100644
> > --- a/drivers/xen/privcmd.c
> > +++ b/drivers/xen/privcmd.c
> > @@ -595,7 +595,7 @@ static int lock_pages(
> >               if (requested > nr_pages)
> >                       return -ENOSPC;
> >
> > -             page_count = get_user_pages_fast(
> > +             page_count = pin_user_pages_fast(
> >                       (unsigned long) kbufs[i].uptr,
> >                       requested, FOLL_WRITE, pages);
> >               if (page_count < 0) {
> > @@ -612,13 +612,7 @@ static int lock_pages(
> >
> >   static void unlock_pages(struct page *pages[], unsigned int nr_pages)
> >   {
> > -     unsigned int i;
> > -
> > -     for (i = 0; i < nr_pages; i++) {
> > -             if (!PageDirty(page))
> > -                     set_page_dirty_lock(page);
> > -             put_page(pages[i]);
> > -     }
> > +     unpin_user_pages_dirty_lock(pages, nr_pages, 1);
>
> "true", not "1", is the correct way to call that function.

Ok.

>
> Also, this approach changes the behavior slightly, but I think it's
> reasonable to just set_page_dirty_lock() on the whole range--hard to
> see much benefit in checking PageDirty first.

unpin_user_pages_dirty_lock() internally will do the same check after
patch [2/2]
So I thought to keep old and new code in sync. Shall we avoid this check ?


>
>
> >   }
> >
> >   static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
> >
>
> thanks,
> --
> John Hubbard
> NVIDIA