linux-kernel - Re: [PATCH v2 09/17] mm: Add unsafe_follow

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Message-ID: <20201011083642.06ea8062@coco.lan>
Date:   Sun, 11 Oct 2020 08:36:42 +0200
From:   Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Tomasz Figa <tfiga@...omium.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Kees Cook <keescook@...omium.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jérôme Glisse <jglisse@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>, Hans Verkuil <hverkuil@...all.nl>,
        "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Subject: Re: [PATCH v2 09/17] mm: Add unsafe_follow_pfn

Em Sun, 11 Oct 2020 08:27:41 +0200
Mauro Carvalho Chehab <mchehab+huawei@...nel.org> escreveu:

> Em Sat, 10 Oct 2020 23:50:27 +0200
> Daniel Vetter <daniel.vetter@...ll.ch> escreveu:
> 
> > On Sat, Oct 10, 2020 at 11:36 PM Laurent Pinchart
> > <laurent.pinchart@...asonboard.com> wrote:
> > >
> 
> > > > We probably still have a few legacy drivers using videobuf (non-2),
> > > > but IMHO those should be safe to put behind some disabled-by-default
> > > > Kconfig symbol or even completely drop, as the legacy framework has
> > > > been deprecated for many years already.  
> > >
> > > There's 8 drivers left, and they support a very large number of devices.
> > > I expect unhappy users distros stop shipping them. On the other hand,
> > > videobuf has been deprecated for a loooooooong time, so there has been
> > > plenty of time to convert the remaining drivers to videobuf2. If nobody
> > > can do it, then we'll have to drop support for these devices given the
> > > security issues.  
> > 
> > Again, the issue here is _only_ with follow_pfn. For videobuf1 this
> > means videbuf-dma-contig.c userptr support is broken. Unlike videobuf2
> > it means it's broken for all usage (not just zero-copy userptr),
> > because videbuf-dma-contig.c lacks the pin_user_pages path.
> 
> Well, follow_pfn() is used only by videbuf-dma-contig.c. If this is 
> the only part of VB1 that will have userptr broken, then there's
> just one driver that might be affected: davinci.
> 
> Yet, taking a deeper look:
> 
> 	$ git grep include drivers/media/platform/davinci/|grep -i videobuf
> 	drivers/media/platform/davinci/vpif_capture.h:#include <media/videobuf2-dma-contig.h>
> 	drivers/media/platform/davinci/vpif_display.h:#include <media/videobuf2-dma-contig.h>
> 
> It sounds to me that it was already converted to VB2, but some VB1
> symbols were not converted at its Kconfig.
> 
> It sounds to me that there are other drivers with some VB1 left overs
> at Kconfig, as those are the only ones using VB1 those days:
> 
> 	$ for i in $(git grep media/videobuf drivers |grep -v videobuf2 |grep -v v4l2-core|cut -d: -f1); do dirname $i; done|sort|uniq
> 	drivers/media/pci/bt8xx
> 	drivers/media/pci/cx18
> 	drivers/media/platform
> 	drivers/media/usb/tm6000
> 	drivers/media/usb/zr364xx
> 	drivers/staging/media/atomisp/pci

This is incomplete. There are two drivers that include videobuf
indirectly:

	include/media/davinci/vpfe_capture.h
	include/media/drv-intf/saa7146_vv.h

I double-checked that DaVinci still uses VB1. There are 
actually two clients for videbuf-dma-contig.c: davinci and fsl-viu.

Those two will be affected, if we don't add pin_user_pages_fast()
support to VB1 or convert them to VB2.

> 
> > But that
> > would be easy to add if this poses a  problem I think - we just need
> > to carry over the pin_user_pages_fast logic from videbuf2, no driver
> > changes required. But of course I don't think we should do that before
> > someone reports the regression, since videobuf1 userptr is doubly
> > deprecated :-)
> 
> I think otherwise. Keeping a broken component at the Kernel is 
> a bad idea. 
> 
> Yet, from my quick search above, it sounds to me that it is time for 
> us to retire the VB1 DMA contig support as a hole, as there's no client 
> for it anymore.
> 
> I'll work on some patches cleaning up the VB1 left overs at
> Kconfig and removing videbuf-dma-contig.c for good, if there's
> no hidden dependency on it.
> 
> 
> Thanks,
> Mauro



Thanks,
Mauro