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

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Date:   Fri, 19 Jun 2020 08:42:39 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Juergen Gross <jgross@...e.com>, sstabellini@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        John Hubbard <jhubbard@...dia.com>, paul@....org
Subject: Re: [RFC PATCH] xen/privcmd: Convert get_user_pages*() to pin_user_pages*()

On Wed, Jun 17, 2020 at 11:29 PM Boris Ostrovsky
<boris.ostrovsky@...cle.com> wrote:
>
> 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@...il.com>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > ---
> > 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?
>
>
> >
> >  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?

My understanding is, in case of lock_pages() end up returning partial
mapped pages,
we should pass no. of partial mapped pages to unlock_pages(), not nr_pages.
This will avoid checking extra check to validate the pages[i].

and if lock_pages() returns 0 in success, anyway we have all the pages[i] valid.
I will try to correct it in v2.

But I agree, there is no harm to check for pages[i] and I believe,
unpin_user_pages()
is the right place to do so.

John any thought ?