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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20160212154116.GA15142@node.shutemov.name>
Date:	Fri, 12 Feb 2016 17:41:16 +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 08:57:02PM +0100, Gerald Schaefer wrote:
> On Thu, 11 Feb 2016 21:09:42 +0200
> "Kirill A. Shutemov" <kirill@...temov.name> wrote:
> 
> > 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?
> 
> Hmm, not sure about that. After pmdp_invalidate(), a pmd_none() check in
> fast_gup will still return false, because the pmd is not empty (at least
> on s390). So I don't see spontaneously how it will help fast_gup to break
> out to the slow path in case of THP splitting.

What pmdp_flush_direct() does in pmdp_invalidate()? It's hard to unwrap for me :-/
Does it make the pmd !pmd_present()?

I'm also confused by pmd_none() is equal to !pmd_present() on s390. Hm?

-- 
 Kirill A. Shutemov