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Date: Thu, 8 Apr 2021 13:40:59 +0200 From: Paolo Bonzini <pbonzini@...hat.com> To: Jason Gunthorpe <jgg@...dia.com>, DRI Development <dri-devel@...ts.freedesktop.org>, LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org, linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org, linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org, 3pvd@...gle.com, Jann Horn <jannh@...gle.com>, Cornelia Huck <cohuck@...hat.com>, Peter Xu <peterx@...hat.com>, Alex Williamson <alex.williamson@...hat.com>, Daniel Vetter <daniel.vetter@...el.com> Subject: Re: [PATCH 3/3] mm: unexport follow_pfn On 08/04/21 12:05, Daniel Vetter wrote: > On Mon, Mar 29, 2021 at 10:31:01AM -0300, Jason Gunthorpe wrote: >> On Tue, Mar 16, 2021 at 04:33:03PM +0100, Daniel Vetter wrote: >>> Both kvm (in bd2fae8da794 ("KVM: do not assume PTE is writable after >>> follow_pfn")) and vfio (in 07956b6269d3 ("vfio/type1: Use >>> follow_pte()")) have lost their callsites of follow_pfn(). All the >>> other ones have been switched over to unsafe_follow_pfn because they >>> cannot be fixed without breaking userspace api. >>> >>> Argueably the vfio code is still racy, but that's kinda a bigger >> >> vfio and kvm > > Hm I thought kvm is non-racy due to the mmu notifier catch races? No, but the plan is indeed to have some struct for each page that uses follow_pfn and update it from the MMU notifiers. Paolo >> >>> picture. But since it does leak the pte beyond where it drops the pt >>> lock, without anything else like an mmu notifier guaranteeing >>> coherence, the problem is at least clearly visible in the vfio code. >>> So good enough with me. >>> >>> I've decided to keep the explanation that after dropping the pt lock >>> you must have an mmu notifier if you keep using the pte somehow by >>> adjusting it and moving it into the kerneldoc for the new follow_pte() >>> function. >>> >>> Cc: 3pvd@...gle.com >>> Cc: Jann Horn <jannh@...gle.com> >>> Cc: Paolo Bonzini <pbonzini@...hat.com> >>> Cc: Jason Gunthorpe <jgg@...dia.com> >>> Cc: Cornelia Huck <cohuck@...hat.com> >>> Cc: Peter Xu <peterx@...hat.com> >>> Cc: Alex Williamson <alex.williamson@...hat.com> >>> Cc: linux-mm@...ck.org >>> Cc: linux-arm-kernel@...ts.infradead.org >>> Cc: linux-samsung-soc@...r.kernel.org >>> Cc: linux-media@...r.kernel.org >>> Cc: kvm@...r.kernel.org >>> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com> >>> --- >>> include/linux/mm.h | 2 -- >>> mm/memory.c | 26 +++++--------------------- >>> mm/nommu.c | 13 +------------ >>> 3 files changed, 6 insertions(+), 35 deletions(-) >> >> Reviewed-by: Jason Gunthorpe <jgg@...dia.com> > > Thanks for your r-b tags, I'll add them. > -Daniel > >> >> Jason >
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