Message-ID: <4fd5f74f-3739-f469-fd8a-ad0ea22ec966@redhat.com>
Date:   Tue, 2 May 2023 17:45:40 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
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        Matthew Wilcox <willy@...radead.org>,
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        Oleg Nesterov <oleg@...hat.com>,
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        Dave Chinner <david@...morbit.com>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

On 02.05.23 17:36, Jason Gunthorpe wrote:
> On Tue, May 02, 2023 at 11:32:57AM -0400, Peter Xu wrote:
>>> How does s390 avoid mmu notifiers without having lots of problems?? It
>>> is not really optional to hook the invalidations if you need to build
>>> a shadow page table..
>>
>> Totally no idea on s390 details, but.. per my read above, if the firmware
>> needs to make sure the page is always available (so no way to fault it in
>> on demand), which means a longterm pinning seems appropriate here.
>>
>> Then if pinned a must, there's no need for mmu notifiers (as the page will
>> simply not be invalidated anyway)?
> 
> And what if someone deliberately changes the mapping?  memory hotplug
> in the VM, or whatever?

Besides s390 not supporting memory hotplug in VMs (yet): if the guest 
wants a different guest physical address, I guess that's the problem of 
the guest, and it can update it:

KVM_S390_ZPCIOP_REG_AEN is triggered from QEMU via 
s390_pci_kvm_aif_enable(), triggered by the guest via a special instruction.

If the hypervisor changes the mapping, it's just the same thing as 
mixing e.g. MADV_DONTNEED with longterm pinning in vfio: don't do it. 
And if you do it, you get to keep the mess you created for your VM.

Linux will make sure to not change the mapping: for example, page 
migration of a pinned page will fail.

But maybe I am missing something important here.

-- 
Thanks,

David / dhildenb

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