Am 15.08.22 um 12:18 schrieb Dmitry Osipenko:
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 :)
Well exactly that's the problem. It does not work, you are just lucky :)
I will provide a patch to set the reference count to zero even for
non-compound pages. Maybe that will yield more backtrace to abusers of
this interface.
Regards,
Christian.
I'll try to dig out the older discussions, thank you for the quick reply!
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