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

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Date:   Fri, 19 Jun 2020 10:03:42 +0100
From:   Paul Durrant <xadimgnik@...il.com>
To:     "'Boris Ostrovsky'" <boris.ostrovsky@...cle.com>,
        "'Souptick Joarder'" <jrdr.linux@...il.com>, <jgross@...e.com>,
        <sstabellini@...nel.org>
Cc:     <xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
        "'John Hubbard'" <jhubbard@...dia.com>
Subject: RE: [RFC PATCH] xen/privcmd: Convert get_user_pages*() to pin_user_pages*()

> -----Original Message-----
> From: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Sent: 17 June 2020 18:57
> To: Souptick Joarder <jrdr.linux@...il.com>; jgross@...e.com; sstabellini@...nel.org
> Cc: 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 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?
> 

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
> 
> 
>