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Date:   Tue, 31 Oct 2023 07:10:08 -0700
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Subject: Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory
From:   Sean Christopherson <seanjc@google.com>
To:     Chao Gao <chao.gao@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
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        kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Xiaoyao Li <xiaoyao.li@intel.com>,
        Xu Yilun <yilun.xu@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        Fuad Tabba <tabba@google.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Anish Moorthy <amoorthy@google.com>,
        David Matlack <dmatlack@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Isaku Yamahata <isaku.yamahata@intel.com>,
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        Wang <wei.w.wang@intel.com>,
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        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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On Tue, Oct 31, 2023, Chao Gao wrote:
> >+int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
> >+{
> >+	loff_t size = args->size;
> >+	u64 flags = args->flags;
> >+	u64 valid_flags = 0;
> >+
> >+	if (flags & ~valid_flags)
> >+		return -EINVAL;
> >+
> >+	if (size < 0 || !PAGE_ALIGNED(size))
> >+		return -EINVAL;
> 
> is size == 0 a valid case?

Nope, this is a bug.

> >+	if (!xa_empty(&gmem->bindings) &&
> >+	    xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT)) {
> >+		filemap_invalidate_unlock(inode->i_mapping);
> >+		goto err;
> >+	}
> >+
> >+	/*
> >+	 * No synchronize_rcu() needed, any in-flight readers are guaranteed to
> >+	 * be see either a NULL file or this new file, no need for them to go
> >+	 * away.
> >+	 */
> >+	rcu_assign_pointer(slot->gmem.file, file);
> >+	slot->gmem.pgoff = start;
> >+
> >+	xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
> >+	filemap_invalidate_unlock(inode->i_mapping);
> >+
> >+	/*
> >+	 * Drop the reference to the file, even on success.  The file pins KVM,
> >+	 * not the other way 'round.  Active bindings are invalidated if the
> >+	 * file is closed before memslots are destroyed.
> >+	 */
> >+	fput(file);
> >+	return 0;
> >+
> >+err:
> >+	fput(file);
> >+	return -EINVAL;
> 
> The cleanup, i.e., filemap_invalidate_unlock() and fput(), is common. So, I think it
> may be slightly better to consolidate the common part e.g.,

I would prefer to keep this as-is.  Only goto needs the unlock, and I find it easier
to understand the success vs. error paths with explicit returns.  But it's not a
super strong preference.