linux-kernel - Re: [PATCH v2 00/19] mm: Support huge pfnmaps

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Message-ID: <CACw3F53ojc+m9Xq_2go3Fdn8aVumxwmBvPgiUJgmrQP3ExdT-g@mail.gmail.com>
Date: Wed, 4 Sep 2024 09:58:54 -0700
From: Jiaqi Yan <jiaqiyan@...gle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Gavin Shan <gshan@...hat.com>, Catalin Marinas <catalin.marinas@....com>, x86@...nel.org, 
	Ingo Molnar <mingo@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Alistair Popple <apopple@...dia.com>, kvm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, Sean Christopherson <seanjc@...gle.com>, 
	Oscar Salvador <osalvador@...e.de>, Borislav Petkov <bp@...en8.de>, Zi Yan <ziy@...dia.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, David Hildenbrand <david@...hat.com>, 
	Yan Zhao <yan.y.zhao@...el.com>, Will Deacon <will@...nel.org>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, Alex Williamson <alex.williamson@...hat.com>, 
	ankita@...dia.com
Subject: Re: [PATCH v2 00/19] mm: Support huge pfnmaps

On Wed, Sep 4, 2024 at 9:43 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Wed, Sep 04, 2024 at 09:38:22AM -0700, Jiaqi Yan wrote:
> > On Wed, Sep 4, 2024 at 8:52 AM Jason Gunthorpe <jgg@...dia.com> wrote:
> > >
> > > On Thu, Aug 29, 2024 at 12:21:39PM -0700, Jiaqi Yan wrote:
> > >
> > > > I think we still want to attempt to SIGBUS userspace, regardless of
> > > > doing unmap_mapping_range or not.
> > >
> > > IMHO we need to eliminate this path if we actually want to keep things
> > > mapped.
> > >
> > > There is no way to generate the SIGBUS without poking a 4k hole in the
> > > 1G page, as only that 4k should get SIGBUS, every other byte of the 1G
> > > is clean.
> >
> > Ah, sorry I wasn't clear. The SIGBUS will be only for poisoned PFN;
> > clean PFNs under the same PUD/PMD for sure don't need any SIGBUS,
> > which is the whole purpose of not unmapping.
>
> You can't get a SIGBUS if the things are still mapped. This is why the
> SIGBUS flow requires poking a non-present hole around the poisoned
> memory.
>
> So keeping things mapped at 1G also means giving up on SIGBUS.

SIGBUS during page fault is definitely impossible when memory is still
mapped, but the platform still MCE or SEA in case of poison
consumption, right? So I wanted to propose new code to SIGBUS (either
BUS_MCEERR_AR or BUS_OBJERR) as long as the platform notifies the
kernel in the synchronous poison consumption context, e.g. MCE on X86
and SEA on ARM64.

>
> Jason