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. 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. -- Best regards, Dmitry
