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Date: Wed, 31 Jan 2024 15:20:03 +0100
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Subject: Re: [PATCH v1 0/9] mm/memory: optimize unmap/zap with PTE-mapped THP
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To: Michal Hocko <mhocko@suse.com>, Ryan Roberts <ryan.roberts@arm.com>
Cc: Yin Fengwei <fengwei.yin@intel.com>, linux-kernel@vger.kernel.org,
 Linus Torvalds <torvalds@linux-foundation.org>, linux-mm@kvack.org,
 Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
 Nick Piggin <npiggin@gmail.com>, Peter Zijlstra <peterz@infradead.org>,
 Michael Ellerman <mpe@ellerman.id.au>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Heiko Carstens <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>,
 Alexander Gordeev <agordeev@linux.ibm.com>,
 Christian Borntraeger <borntraeger@linux.ibm.com>,
 Sven Schnelle <svens@linux.ibm.com>, Arnd Bergmann <arnd@arndb.de>,
 linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
 linux-s390@vger.kernel.org, "Huang, Ying" <ying.huang@intel.com>
References: <20240129143221.263763-1-david@redhat.com>
 <4ef64fd1-f605-4ddf-82e6-74b5e2c43892@intel.com>
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 <1fd26a83-8e6f-4b96-9d27-dd46de9488cc@arm.com> <ZbpUVXa-Aujp6gWO@tiehlicka>
From: David Hildenbrand <david@redhat.com>
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On 31.01.24 15:08, Michal Hocko wrote:
> On Wed 31-01-24 10:26:13, Ryan Roberts wrote:
>> IIRC there is an option to zero memory when it is freed back to the buddy? So
>> that could be a place where time is proportional to size rather than
>> proportional to folio count? But I think that option is intended for debug only?
>> So perhaps not a problem in practice?
> 
> init_on_free is considered a security/hardening feature more than a
> debugging one. It will surely add an overhead and I guess this is
> something people who use it know about. The batch size limit is a latency
> reduction feature for !PREEMPT kernels but by no means it should be
> considered low latency guarantee feature. A lot of has changed since
> the limit was introduced and the current latency numbers will surely be
> different than back then. As long as soft lockups do not trigger again
> this should be acceptable IMHO.

It could now be zeroing out ~512 MiB. That shouldn't take double-digit 
seconds unless we are running in a very problematic environment 
(over-committed VM). But then, we might have different problems already.

I'll do some sanity checks with an extremely large processes (as much as 
I can fit on my machines), with a !CONFIG_PREEMPT kernel and 
init_on_free, to see if anything pops up.

Thanks Michal!

-- 
Cheers,

David / dhildenb