Messages in this thread | | | Date | Mon, 15 Aug 2022 12:51:02 +0200 | Subject | Re: [PATCH v1] drm/ttm: Refcount allocated tail pages | From | Christian König <> |
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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.
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