linux-kernel - Re: [PATCH v3 kvm/queue 05/16] KVM: Maintain ofs_tree for fast memslot lookup by file offset

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Message-ID: <YdSHViDXGkjz5t/Q@google.com>
Date:   Tue, 4 Jan 2022 17:43:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, john.ji@...el.com, susie.li@...el.com,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com
Subject: Re: [PATCH v3 kvm/queue 05/16] KVM: Maintain ofs_tree for fast
 memslot lookup by file offset

On Fri, Dec 31, 2021, Chao Peng wrote:
> On Tue, Dec 28, 2021 at 09:48:08PM +0000, Sean Christopherson wrote:
> >KVM handles
> > reverse engineering the memslot to get the offset and whatever else it needs.
> > notify_fallocate() and other callbacks are unchanged, though they probably can
> > drop the inode.
> > 
> > E.g. likely with bad math and handwaving on the overlap detection:
> > 
> > int kvm_private_fd_fallocate_range(void *owner, pgoff_t start, pgoff_t end)
> > {
> > 	struct kvm_memory_slot *slot = owner;
> > 	struct kvm_gfn_range gfn_range = {
> > 		.slot	   = slot,
> > 		.start	   = (start - slot->private_offset) >> PAGE_SHIFT,
> > 		.end	   = (end - slot->private_offset) >> PAGE_SHIFT,
> > 		.may_block = true,
> > 	};
> > 
> > 	if (!has_overlap(slot, start, end))
> > 		return 0;
> > 
> > 	gfn_range.end = min(gfn_range.end, slot->base_gfn + slot->npages);
> > 
> > 	kvm_unmap_gfn_range(slot->kvm, &gfn_range);
> > 	return 0;
> > }
> 
> I understand this KVM side handling, but again one fd can have multiple
> memslots. How shmem decides to notify which memslot from a list of
> memslots when it invokes the notify_fallocate()? Or just notify all
> the possible memslots then let KVM to check? 

Heh, yeah, those are the two choices.  :-)

Either the backing store needs to support registering callbacks for specific,
arbitrary ranges, or it needs to invoke all registered callbacks.  Invoking all
callbacks has my vote; it's much simpler to implement and is unlikely to incur
meaningful overhead.  _Something_ has to find the overlapping ranges, that cost
doesn't magically go away if it's pushed into the backing store.

Note, invoking all notifiers is also aligned with the mmu_notifier behavior.