Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

[Christian König]

Am 15.08.22 um 12:47 schrieb Dmitry Osipenko:
> On 8/15/22 13:18, Dmitry Osipenko wrote:
> > On 8/15/22 13:14, Christian König wrote:
> > > Am 15.08.22 um 12:11 schrieb Christian König:
> > > > Am 15.08.22 um 12:09 schrieb Dmitry Osipenko:
> > > > > On 8/15/22 13:05, Christian König wrote:
> > > > > > Am 15.08.22 um 11:54 schrieb Dmitry Osipenko:
> > > > > > > Higher order pages allocated using alloc_pages() aren't refcounted and
> > > > > > > they
> > > > > > > need to be refcounted, otherwise it's impossible to map them by
> > > > > > > KVM. This
> > > > > > > patch sets the refcount of the tail pages and fixes the KVM memory
> > > > > > > mapping
> > > > > > > faults.
> > > > > > >
> > > > > > > Without this change guest virgl driver can't map host buffers into
> > > > > > > guest
> > > > > > > and can't provide OpenGL 4.5 profile support to the guest. The host
> > > > > > > mappings are also needed for enabling the Venus driver using host GPU
> > > > > > > drivers that are utilizing TTM.
> > > > > > >
> > > > > > > Based on a patch proposed by Trigger Huang.
> > > > > > Well I can't count how often I have repeated this: This is an
> > > > > > absolutely
> > > > > > clear NAK!
> > > > > >
> > > > > > TTM pages are not reference counted in the first place and because of
> > > > > > this giving them to virgl is illegal.
> > > > > A? The first page is refcounted when allocated, the tail pages are not.
> > > > No they aren't. The first page is just by coincident initialized with
> > > > a refcount of 1. This refcount is completely ignored and not used at all.
> > > >
> > > > Incrementing the reference count and by this mapping the page into
> > > > some other address space is illegal and corrupts the internal state
> > > > tracking of TTM.
> > > See this comment in the source code as well:
> > >
> > >          /* Don't set the __GFP_COMP flag for higher order allocations.
> > >           * Mapping pages directly into an userspace process and calling
> > >           * put_page() on a TTM allocated page is illegal.
> > >           */
> > >
> > > I have absolutely no idea how somebody had the idea he could do this.
> > I saw this comment, but it doesn't make sense because it doesn't explain
> > why it's illegal. Hence it looks like a bogus comment since the
> > refcouting certainly works, at least to a some degree because I haven't
> > noticed any problems in practice, maybe by luck :)
> >
> > I'll try to dig out the older discussions, thank you for the quick reply!
> Are you sure it was really discussed in public previously? All I can
> find is yours two answers to a similar patches where you're saying that
> this it's a wrong solution without in-depth explanation and further
> discussions.

Yeah, that's my problem as well I can't find that of hand.

But yes it certainly was discussed in public.

> Maybe it was discussed privately? In this case I will be happy to get
> more info from you about the root of the problem so I could start to
> look at how to fix it properly. It's not apparent where the problem is
> to a TTM newbie like me.

Well this is completely unfixable. See the whole purpose of TTM is to 
allow tracing where what is mapped of a buffer object.

If you circumvent that and increase the page reference yourself than 
that whole functionality can't work correctly any more.

Regards,
Christian.