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

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Message-ID: <20160223184658.GA27281@arm.com>
Date:	Tue, 23 Feb 2016 18:47:14 +0000
From:	Will Deacon <will.deacon@....com>
To:	Gerald Schaefer <gerald.schaefer@...ibm.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Christian Borntraeger <borntraeger@...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>, steve.capper@....com
Subject: Re: [BUG] random kernel crashes after THP rework on s390 (maybe also
 on PowerPC and ARM)

[adding Steve, since he worked on THP for 32-bit ARM]

On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote:
> On Tue, 23 Feb 2016 13:32:21 +0300
> "Kirill A. Shutemov" <kirill@...temov.name> wrote:
> > The theory is that the splitting bit effetely masked bogus pmd_present():
> > we had pmd_trans_splitting() in all code path and that prevented mm from
> > touching the pmd. Once pmd_trans_splitting() has gone, mm proceed with the
> > pmd where it shouldn't and here's a boom.
> 
> Well, I don't think pmd_present() == true is bogus for a trans_huge pmd under
> splitting, after all there is a page behind the the pmd. Also, if it was
> bogus, and it would need to be false, why should it be marked !pmd_present()
> only at the pmdp_invalidate() step before the pmd_populate()? It clearly
> is pmd_present() before that, on all architectures, and if there was any
> problem/race with that, setting it to !pmd_present() at this stage would
> only (marginally) reduce the race window.
> 
> BTW, PowerPC and Sparc seem to do the same thing in pmdp_invalidate(),
> i.e. they do not set pmd_present() == false, only mark it so that it would
> not generate a new TLB entry, just like on s390. After all, the function
> is called pmdp_invalidate(), and I think the comment in mm/huge_memory.c
> before that call is just a little ambiguous in its wording. When it says
> "mark the pmd notpresent" it probably means "mark it so that it will not
> generate a new TLB entry", which is also what the comment is really about:
> prevent huge and small entries in the TLB for the same page at the same
> time.
> 
> FWIW, and since the ARM arch-list is already on cc, I think there is
> an issue with pmdp_invalidate() on ARM, since it also seems to clear
> the trans_huge (and formerly trans_splitting) bit, which actually makes
> the pmd !pmd_present(), but it violates the other requirement from the
> comment:
> "the pmd_trans_huge and pmd_trans_splitting must remain set at all times
> on the pmd until the split is complete for this pmd"

I've only been testing this for arm64 (where I'm yet to see a problem),
but we use the generic pmdp_invalidate implementation from
mm/pgtable-generic.c there. On arm64, pmd_trans_huge will return true
after pmd_mknotpresent. On arm, it does look to be buggy, since it nukes
the entire entry... Steve?

Will