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Tue, 30 Jul 2024 13:00:51 -0700 (PDT) Date: Tue, 30 Jul 2024 13:00:49 -0700 In-Reply-To: <2da6b57e-d5c2-4016-b89b-d51700eeb845@redhat.com> Mime-Version: 1.0 References: <20240726235234.228822-1-seanjc@google.com> <20240726235234.228822-46-seanjc@google.com> <2da6b57e-d5c2-4016-b89b-d51700eeb845@redhat.com> Message-ID: Subject: Re: [PATCH v12 45/84] KVM: guest_memfd: Provide "struct page" as output from kvm_gmem_get_pfn() From: Sean Christopherson To: Paolo Bonzini Cc: Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, David Matlack , David Stevens Content-Type: text/plain; charset="us-ascii" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240730_130053_505258_8E35CD69 X-CRM114-Status: GOOD ( 20.07 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jul 30, 2024, Paolo Bonzini wrote: > On 7/27/24 01:51, Sean Christopherson wrote: > > Provide the "struct page" associated with a guest_memfd pfn as an output > > from __kvm_gmem_get_pfn() so that KVM guest page fault handlers can > ^^^^^^^^^^^^^^^^^^^^ > > Just "kvm_gmem_get_pfn()". > > > directly put the page instead of having to rely on > > kvm_pfn_to_refcounted_page(). > > This will conflict with my series, where I'm introducing > folio_file_pfn() and using it here: > > - page = folio_file_page(folio, index); > > + *page = folio_file_page(folio, index); > > - *pfn = page_to_pfn(page); > > + *pfn = page_to_pfn(*page); > > if (max_order) > > *max_order = 0; > > That said, I think it's better to turn kvm_gmem_get_pfn() into > kvm_gmem_get_page() here, and pull the page_to_pfn() or page_to_phys() > to the caller as applicable. This highlights that the caller always > gets a refcounted page with guest_memfd. I have mixed feelings on this. On one hand, it's silly/confusing to return a pfn+page pair and thus imply that guest_memfd can return a pfn without a page. On the other hand, if guest_memfd does ever serve pfns without a struct page, it could be quite painful to unwind all of the arch arch code we'll accrue that assumes guest_memfd only ever returns a refcounted page (as evidenced by this series). The probability of guest_memfd not having struct page for mapped pfns is likely very low, but at the same time, providing a pfn+page pair doesn't cost us much. And if it turns out that not having struct page is nonsensical, deferring the kvm_gmem_get_pfn() => kvm_gmem_get_page() conversion could be annoying, but highly unlikely to be painful since it should be 100% mechanical. Whereas reverting back to kvm_gmem_get_pfn() if we make the wrong decision now could mean doing surgery on a pile of arch code.