On Wed, Jan 05, 2022, David Matlack wrote: > On Thu, Dec 23, 2021 at 10:23:12PM +0000, Sean Christopherson wrote: > > + /* > > + * Invalidate the root to prevent it from being reused by a vCPU, and > > + * mark it defunct so that kvm_tdp_mmu_zap_invalidated_roots() doesn't > > + * try to put a reference it didn't acquire. > > + */ > > + root->role.invalid = true; > > + root->tdp_mmu_defunct_root = true; > > Ah ok so tdp_mmu_defunct_root indicates the root became invalid due to > losing all its references while it was valid. This is in contrast to > kvm_tdp_mmu_invalidate_all_roots() which marks roots invalid while they > still have valid references. > > But I wonder if tdp_mmu_defunct_root is necessary? It's only used to > skip a put in zap_invalidated_roots. Could we instead unconditionally > grab a reference in invalidate_all_roots and then unconditionally drop > it? Hmm, it's probably not necessary. I added tdp_mmu_defunct_root before realizing that that kvm_tdp_mmu_invalidate_all_roots() was wrong about this: * Roots which have a zero refcount should be skipped as * they're already being torn down. IIRC, I added the second flag because I was trying to honor that (incorrect) logic, and never reconsidered the need for a second flag once I got everything working. The only downside would be keeping the memory for defunct roots around a wee bit longer, and that's not a big deal. I'll yank it out for the next version, assuming I didn't forget some detail...
