On Tue, Oct 25, 2022 at 4:02 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Mon, Oct 24, 2022 at 09:58:07PM +0200, Jann Horn wrote: > > > Unless I'm completely misunderstanding what's going on here, the whole > > "remove_table" thing only happens when you "remove a table", meaning > > you free an entire *pagetable*. Just zapping PTEs doesn't trigger that > > logic. > > Aah; yes true. OTOH even it that were not so, I think it would still be > broken because the current code relies on the TLB flush to have > completed, whereas the RCU scheme is effectively async and can be > considered pending until the callback runs. > > Hurmph... easiest fix is probably to dis-allow kvm_flush_tlb_multi() > for i386-pae builds. > > Something like so... nobody in his right mind should care about i386-pae > virt performance much. I think Xen and HyperV have similar codepaths. hyperv_flush_tlb_multi() looks like it uses remote flush hypercalls, xen_flush_tlb_multi() too. On top of that, I think that theoretically, Linux doesn't even ensure that you have a TLB flush in between tearing down one PTE and installing another PTE (see https://lore.kernel.org/all/CAG48ez1Oz4tT-N2Y=Zs6jumu=zOp7SQRZ=V2c+b5bT9P4retJA@xxxxxxxxxxxxxx/), but I haven't tested that, and if it is true, I'm also not entirely sure if it's correct (in the sense that it only creates incoherent-TLB states when userspace is doing something stupid like racing MADV_DONTNEED and page faults on the same region). I think the more clearly correct fix would be to get rid of the split loads and use CMPXCHG16B instead (probably destroying the performance of GUP-fast completely), but that's complicated because some of the architectures that use the split loads path don't have cmpxchg_double (or at least don't have it wired up).
