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

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Message-ID: <ZFEJz4s/95t1I4+7@nvidia.com>
Date:   Tue, 2 May 2023 10:02:07 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
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        Leon Romanovsky <leon@...nel.org>,
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        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
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        Matthew Rosato <mjrosato@...ux.ibm.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

On Tue, May 02, 2023 at 01:54:41PM +0100, Lorenzo Stoakes wrote:
> 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.

This looks like the same stuff David was talking about, a qemu guest
with VFIO backed by a filesystem file..

Broadly though, arch kvm code should not call pin_user_pages().

Either it is KVM focused and it should use the shadow table and it's
existing mmu_notifier synchronization scheme

Or it is VFIO focused so it should use mdev/etc and have an unmap call
back.

I'm not really sure what this is for though..

Jason