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

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Date:	Thu, 11 Feb 2016 21:09:42 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Gerald Schaefer <gerald.schaefer@...ibm.com>
Cc:	"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>,
	Will Deacon <will.deacon@....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 Thu, Feb 11, 2016 at 07:22:23PM +0100, Gerald Schaefer wrote:
> Hi,
> 
> Sebastian Ott reported random kernel crashes beginning with v4.5-rc1 and
> he also bisected this to commit 61f5d698 "mm: re-enable THP". Further
> review of the THP rework patches, which cannot be bisected, revealed
> commit fecffad "s390, thp: remove infrastructure for handling splitting PMDs"
> (and also similar commits for other archs).
> 
> This commit removes the THP splitting bit and also the architecture
> implementation of pmdp_splitting_flush(), which took care of the IPI for
> fast_gup serialization. The commit message says
> 
>     pmdp_splitting_flush() is not needed too: on splitting PMD we will do
>     pmdp_clear_flush() + set_pte_at().  pmdp_clear_flush() will do IPI as
>     needed for fast_gup
> 
> The assumption that a TLB flush will also produce an IPI is wrong on s390,
> and maybe also on other architectures, and I thought that this was actually
> the main reason for having an arch-specific pmdp_splitting_flush().
> 
> At least PowerPC and ARM also had an individual implementation of
> pmdp_splitting_flush() that used kick_all_cpus_sync() instead of a TLB
> flush to send the IPI, and those were also removed. Putting the arch
> maintainers and mailing lists on cc to verify.
> 
> On s390 this will break the IPI serialization against fast_gup, which
> would certainly explain the random kernel crashes, please revert or fix
> the pmdp_splitting_flush() removal.

Sorry for that.

I believe, the problem was already addressed for PowerPC:

http://lkml.kernel.org/g/454980831-16631-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com

I think kick_all_cpus_sync() in arch-specific pmdp_invalidate() would do
the trick, right?

If yes, I'll prepare patch tomorrow (some sleep required).

-- 
 Kirill A. Shutemov