linux-kernel - Re: [BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM)

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Date:	Wed, 24 Feb 2016 11:02:22 +0000
From:	Will Deacon <will.deacon@....com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-s390@...r.kernel.org,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>
Subject: Re: [BUG] random kernel crashes after THP rework on s390 (maybe also
 on PowerPC and ARM)

On Wed, Feb 24, 2016 at 11:51:47AM +0100, Christian Borntraeger wrote:
> On 02/24/2016 11:41 AM, Will Deacon wrote:
> > On Wed, Feb 24, 2016 at 11:16:34AM +0100, Christian Borntraeger wrote:
> >> Without that fix we would clearly have stale tlb entries, no?
> > 
> > Yes, but AFAIU the sequence on arm64 is:
> > 
> > 1.  trans huge mapping (block mapping in arm64 speak)
> > 2.  faulting entry (pmd_mknotpresent)
> > 3.  tlb invalidation
> > 4.  table entry mapping the same pages as (1).
> > 
> > so if the microarchitecture we're on can tolerate a mixture of block
> > mappings and page mappings mapping the same VA to the same PA, then the
> > lack of TLB maintenance would go unnoticed. There are certainly systems
> > where that could cause an issue, but I believe the one I've been testing
> > on would be ok.
> 
> So in essence you say it does not matter that you flush the wrong range in 
> flush_pmd_tlb_range as long as it will be flushed later on when the pages
> really go away. Yes, then it really might be ok for arm64.

Indeed, although that's a property of the microarchitecture I'm using
rather than an architectural guarantee so the code should certainly be
fixed!

Will