From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christopher Lameter <cl@linux.com>
Subject: Re: [PATCH v5 02/11] powerpc/mm: Adds counting method to monitor
 lockless pgtable walks
Date: Tue, 8 Oct 2019 18:27:29 +0000 (UTC)
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To: Leonardo Bras <leonardo@linux.ibm.com>
Cc: Song Liu <songliubraving@fb.com>, Michal Hocko <mhocko@suse.com>, "Peter Zijlstra (Intel)" <peterz@infradead.org>, Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>, "Dmitry V. Levin" <ldv@altlinux.org>, Keith Busch <keith.busch@intel.com>, linux-mm@kvack.org, Paul Mackerras <paulus@samba.org>, Christian Brauner <christian.brauner@ubuntu.com>, Ira Weiny <ira.weiny@intel.com>, Ingo Molnar <mingo@kernel.org>, Elena Reshetova <elena.reshetova@intel.com>, linux-arch@vger.kernel.org, Santosh Sivaraj <santosh@fossix.org>, Davidlohr Bueso <dave@stgolabs.net>, "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>, Jann Horn <jannh@google.com>, Mike Rapoport <rppt@linux.ibm.com>, Jason Gunthorpe <jgg@ziepe.ca>, Allison Randal <allison@lohutok.net>, Jesper Dangaard Brouer <brouer@redhat.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Alexey Dobriyan <adobriyan@gmail.com>, Andrea
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On Tue, 8 Oct 2019, Leonardo Bras wrote:

> So you say that the performance impact of using my approach is the same
> as using locks? (supposing that lock never waits)
>
> So, there are 'lockless pagetable walks' only for the sake of better
> performance?

I thought that was the major motivation here.

> I thought they existed to enable doing pagetable walks in states where
> locking was not safe.

That sounds profoundly dangerous. Usually locking makes things safe to
access. Accesses without locking require a lot of care.