netdev - Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <203a8ed7-47fa-0830-c691-71d00517fecb@redhat.com>
Date:   Tue, 2 May 2023 16:57:08 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
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        Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

On 02.05.23 15:35, Matthew Rosato wrote:
> On 5/2/23 9:04 AM, Christian Borntraeger wrote:
>>
>>
>> Am 02.05.23 um 14:54 schrieb Lorenzo Stoakes:
>>> On Tue, May 02, 2023 at 02:46:28PM +0200, Christian Borntraeger wrote:
>>>> Am 02.05.23 um 01:11 schrieb Lorenzo Stoakes:
>>>>> Writing to file-backed dirty-tracked mappings via GUP is inherently broken
>>>>> as we cannot rule out folios being cleaned and then a GUP user writing to
>>>>> them again and possibly marking them dirty unexpectedly.
>>>>>
>>>>> This is especially egregious for long-term mappings (as indicated by the
>>>>> use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as
>>>>> we have already done in the slow path.
>>>>
>>>> Hmm, does this interfer with KVM on s390 and PCI interpretion of interrupt delivery?
>>>> It would no longer work with file backed memory, correct?
>>>>
>>>> See
>>>> arch/s390/kvm/pci.c
>>>>
>>>> kvm_s390_pci_aif_enable
>>>> which does have
>>>> FOLL_WRITE | FOLL_LONGTERM
>>>> to
>>>>
>>>
>>> Does this memory map a dirty-tracked file? It's kind of hard to dig into where
>>> the address originates from without going through a ton of code. In worst case
>>> if the fast code doesn't find a whitelist it'll fall back to slow path which
>>> explicitly checks for dirty-tracked filesystem.
>>
>> It does pin from whatever QEMU uses as backing for the guest.
>>>
>>> We can reintroduce a flag to permit exceptions if this is really broken, are you
>>> able to test? I don't have an s390 sat around :)
>>
>> Matt (Rosato on cc) probably can. In the end, it would mean having
>>    <memoryBacking>
>>      <source type="file"/>
>>    </memoryBacking>
>>
>> In libvirt I guess.
> 
> I am running with this series applied using a QEMU guest with memory-backend-file (using the above libvirt snippet) for a few different PCI device types and AEN forwarding (e.g. what is setup in kvm_s390_pci_aif_enable) is still working.
> 

That's ... unexpected. :)

Either this series doesn't work as expected or you end up using a 
filesystem that is still compatible. But I guess most applicable 
filesystems (ext4, btrfs, xfs) all have a page_mkwrite callback and 
should, therefore, disallow long-term pinning with this series.

-- 
Thanks,

David / dhildenb