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

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Message-ID: <a6bb0334-9aba-9fd8-6a9a-9d4a931b6da2@linux.ibm.com>
Date:   Tue, 2 May 2023 15:04:27 +0200
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     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>,
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        Arnaldo Carvalho de Melo <acme@...nel.org>,
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        "Theodore Ts'o" <tytso@....edu>, Peter Xu <peterx@...hat.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings



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.