Messages in this thread | | | Date | Mon, 15 Aug 2022 12:42:22 +0200 | Subject | Re: [PATCH v1] drm/ttm: Refcount allocated tail pages | From | Christian König <> |
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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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