linux-kernel - Re: [PATCH v4 4/9] drm/rockchip/rockchip_drm_gem.c: Convert to use vm_insert

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Message-ID: <20181218123318.GN26090@n2100.armlinux.org.uk>
Date:   Tue, 18 Dec 2018 12:33:19 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     Michal Hocko <mhocko@...e.com>, Heiko Stuebner <heiko@...ech.de>,
        linux-rockchip@...ts.infradead.org, airlied@...ux.ie,
        hjc@...k-chips.com, Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        dri-devel@...ts.freedesktop.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 4/9] drm/rockchip/rockchip_drm_gem.c: Convert to use
 vm_insert_range

On Tue, Dec 18, 2018 at 05:36:04PM +0530, Souptick Joarder wrote:
> On Tue, Dec 18, 2018 at 3:27 PM Russell King - ARM Linux
> <linux@...linux.org.uk> wrote:
> > This looks like a change in behaviour.
> >
> > If user_count is zero, and offset is zero, then we pass into
> > vm_insert_range() a page_count of zero, and vm_insert_range() does
> > nothing and returns zero.
> >
> > However, as we can see from the above code, the original behaviour
> > was to return -ENXIO in that case.
> 
> I think these checks are not necessary. I am not sure if we get into mmap
> handlers of driver with user_count = 0.

I'm not sure either, I'm just pointing out the change of behaviour.

> > The other thing that I'm wondering is that if (eg) count is 8 (the
> > object is 8 pages), offset is 2, and the user requests mapping 6
> > pages (user_count = 6), then we call vm_insert_range() with a
> > pages of rk_obj->pages + 2, and a pages_count of 6 - 2 = 4. So we
> > end up inserting four pages.
> 
> Considering the scenario, user_count will remain 8 (user_count =
> vma_pages(vma) ). ? No ?
> Then we call vm_insert_range() with a pages of rk_obj->pages + 2, and
> a pages_count
> of 8 - 2 = 6. So we end up inserting 6 pages.
> 
> Please correct me if I am wrong.

vma_pages(vma) is the number of pages that the user requested, it is
the difference between vma->vm_end and vma->vm_start in pages.  As I
said above, "the user requests mapping 6 pages", so vma_pages() will
be 6, and so user_count will also be 6.  You are passing
user_count - offset into vm_insert_range(), which will be 6 - 2 = 4
in my example.  This is two pages short of what the user requested.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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