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

[Christian König]

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.

Regards,
Christian.

> > > Please immediately stop this completely broken approach. We have
> > > discussed this multiple times now.
> > Could you please give me a link to these discussions?
>
> Not of hand, please search the dri-devel list for similar patches. 
> This was brought up multiple times now.
>
> Regards,
> Christian.