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

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Message-ID: <20240904170041.GR3915968@nvidia.com>
Date: Wed, 4 Sep 2024 14:00:41 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Jiaqi Yan <jiaqiyan@...gle.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 04, 2024 at 09:58:54AM -0700, Jiaqi Yan wrote:
> 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.

So you want a SIGBUS that is delivered asynchronously instead of via
the page fault handler? Something like that is sort of what I ment by
"eliminate this path", though I didn't think keeping an async SIGBUS
was an option?

Jason