linux-kernel - Re: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

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Date:   Fri, 21 Jul 2023 15:33:15 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Yuan Yao <yuan.yao@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory

On Fri, Jul 21, 2023, Isaku Yamahata wrote:
> On Fri, Jul 21, 2023 at 02:13:14PM +0800,
> Yuan Yao <yuan.yao@...ux.intel.com> wrote:
> > > +static int kvm_gmem_error_page(struct address_space *mapping, struct page *page)
> > > +{
> > > +	struct list_head *gmem_list = &mapping->private_list;
> > > +	struct kvm_memory_slot *slot;
> > > +	struct kvm_gmem *gmem;
> > > +	unsigned long index;
> > > +	pgoff_t start, end;
> > > +	gfn_t gfn;
> > > +
> > > +	filemap_invalidate_lock_shared(mapping);
> > > +
> > > +	start = page->index;
> > > +	end = start + thp_nr_pages(page);
> > > +
> > > +	list_for_each_entry(gmem, gmem_list, entry) {
> > > +		xa_for_each_range(&gmem->bindings, index, slot, start, end - 1) {
> > > +			for (gfn = start; gfn < end; gfn++) {
> > 
> > Why the start end range used as gfn here ?

Math is hard?  I almost always mess up these types of things, and then catch my
bugs via tests.  But I don't have tests for this particular flow...   Which
reminds me, we need tests for this :-)  Hopefully error injection provides most
of what we need?

> > the page->index is offset of inode's page cache mapping and
> > gmem address space, IIUC, gfn calculation should follow same
> > way as kvm_gmem_invalidate_begin().
> 
> Also instead of sending signal multiple times, we can utilize lsb argument.

As Vishal pointed out, this code shouldn't be sending signals in the first place.