linux-kernel - Re: [Xen-devel] [PATCH v4 8/9] xen/gntdev.c: Convert to use vm_map

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Message-ID: <20190730142233.GR1250@mail-itl>
Date:   Tue, 30 Jul 2019 16:22:33 +0200
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Souptick Joarder <jrdr.linux@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Juergen Gross <jgross@...e.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        robin.murphy@....com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>
Subject: Re: [Xen-devel] [PATCH v4 8/9] xen/gntdev.c: Convert to use
 vm_map_pages()

On Tue, Jul 30, 2019 at 10:05:42AM -0400, Boris Ostrovsky wrote:
> On 7/30/19 2:03 AM, Souptick Joarder wrote:
> > On Mon, Jul 29, 2019 at 7:06 PM Marek Marczykowski-Górecki
> > <marmarek@...isiblethingslab.com> wrote:
> >> On Mon, Jul 29, 2019 at 02:02:54PM +0530, Souptick Joarder wrote:
> >>> On Mon, Jul 29, 2019 at 1:35 PM Souptick Joarder <jrdr.linux@...il.com> wrote:
> >>>> On Sun, Jul 28, 2019 at 11:36 PM Marek Marczykowski-Górecki
> >>>> <marmarek@...isiblethingslab.com> wrote:
> >>>>> On Fri, Feb 15, 2019 at 08:18:31AM +0530, Souptick Joarder wrote:
> >>>>>> Convert to use vm_map_pages() to map range of kernel
> >>>>>> memory to user vma.
> >>>>>>
> >>>>>> map->count is passed to vm_map_pages() and internal API
> >>>>>> verify map->count against count ( count = vma_pages(vma))
> >>>>>> for page array boundary overrun condition.
> >>>>> This commit breaks gntdev driver. If vma->vm_pgoff > 0, vm_map_pages
> >>>>> will:
> >>>>>  - use map->pages starting at vma->vm_pgoff instead of 0
> >>>> The actual code ignores vma->vm_pgoff > 0 scenario and mapped
> >>>> the entire map->pages[i]. Why the entire map->pages[i] needs to be mapped
> >>>> if vma->vm_pgoff > 0 (in original code) ?
> >> vma->vm_pgoff is used as index passed to gntdev_find_map_index. It's
> >> basically (ab)using this parameter for "which grant reference to map".
> >>
> >>>> are you referring to set vma->vm_pgoff = 0 irrespective of value passed
> >>>> from user space ? If yes, using vm_map_pages_zero() is an alternate
> >>>> option.
> >> Yes, that should work.
> > I prefer to use vm_map_pages_zero() to resolve both the issues. Alternatively
> > the patch can be reverted as you suggested. Let me know you opinion and wait
> > for feedback from others.
> >
> > Boris, would you like to give any feedback ?
> 
> vm_map_pages_zero() looks good to me. Marek, does it work for you?

Yes, replacing vm_map_pages() with vm_map_pages_zero() fixes the
problem for me.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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