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From: Oliver Upton To: Yu Zhao Subject: Re: [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables RCU safe Message-ID: References: <20230526234435.662652-1-yuzhao@google.com> <20230526234435.662652-5-yuzhao@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Jason A. Donenfeld" , x86@kernel.org, Gavin Shan , kvm@vger.kernel.org, linux-doc@vger.kernel.org, Catalin Marinas , Dave Hansen , Peter Xu , linux-mm@kvack.org, Ben Gardon , Chao Peng , Will Deacon , Gaosheng Cui , Marc Zyngier , "H. Peter Anvin" , Jonathan Corbet , Alistair Popple , Jason Gunthorpe , Ingo Molnar , Zenghui Yu , linux-trace-kernel@vger.kernel.org, linux-mm@google.com, Thomas Huth , Suzuki K Poulose , Nicholas Piggin , Borislav Petkov , Steven Rostedt , kvmarm@lists.linux.dev, Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Fab iano Rosas , Michael Larabel , Sean Christopherson , linux-kernel@vger.kernel.org, James Morse , Masami Hiramatsu , Anup Patel , Paolo Bonzini , Andrew Morton , linuxppc-dev@lists.ozlabs.org, Mike Rapoport Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Hi Yu, On Sat, May 27, 2023 at 02:13:07PM -0600, Yu Zhao wrote: > On Sat, May 27, 2023 at 12:08 PM Oliver Upton wrote: > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > > index 3d61bd3e591d..bfbebdcb4ef0 100644 > > --- a/arch/arm64/kvm/hyp/pgtable.c > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > @@ -1019,7 +1019,7 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx, > > kvm_granule_size(ctx->level)); > > > > if (childp) > > - mm_ops->put_page(childp); > > + mm_ops->free_removed_table(childp, ctx->level); > > Thanks, Oliver. > > A couple of things I haven't had the chance to verify -- I'm hoping > you could help clarify: > 1. For unmapping, with free_removed_table(), wouldn't we have to look > into the table we know it's empty unnecessarily? As it is currently implemented, yes. But, there's potential to fast-path the implementation by checking page_count() before starting the walk. > 2. For remapping and unmapping, how does free_removed_table() put the > final refcnt on the table passed in? (Previously we had > put_page(childp) in stage2_map_walk_table_post(). So I'm assuming we'd > have to do something equivalent with free_removed_table().) Heh, that's a bug, and an embarrassing one at that! Sent out a fix for that, since it would appear we leak memory on table->block transitions. PTAL if you have a chance. https://lore.kernel.org/all/20230530193213.1663411-1-oliver.upton@linux.dev/ -- Thanks, Oliver