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! -- Best regards, Dmitry
