linux-kernel - Re: [PATCH v1 0/9] mm/memory: optimize unmap/zap with PTE-mapped THP

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Message-ID: <ZbpUVXa-Aujp6gWO@tiehlicka>
Date: Wed, 31 Jan 2024 15:08:21 +0100
From: Michal Hocko <mhocko@...e.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: David Hildenbrand <david@...hat.com>,
	Yin Fengwei <fengwei.yin@...el.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
	Nick Piggin <npiggin@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, "Huang, Ying" <ying.huang@...el.com>
Subject: Re: [PATCH v1 0/9] mm/memory: optimize unmap/zap with PTE-mapped THP

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.

-- 
Michal Hocko
SUSE Labs