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

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Message-ID: <ZbpTQ80ebCUnAXW0@tiehlicka>
Date: Wed, 31 Jan 2024 15:03:47 +0100
From: Michal Hocko <mhocko@...e.com>
To: David Hildenbrand <david@...hat.com>
Cc: 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>,
	Ryan Roberts <ryan.roberts@....com>,
	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 11:16:01, David Hildenbrand wrote:
[...]
> This 10000 pages limit was introduced in 53a59fc67f97 ("mm: limit mmu_gather
> batching to fix soft lockups on !CONFIG_PREEMPT") where we wanted to handle
> soft-lockups.

AFAIR at the time of this patch this was mostly just to put some cap on
the number of batches to collect and free at once. If there is a lot of
free memory and a large process exiting this could grow really high. Now
that those pages^Wfolios can represent larger memory chunks it could
mean more physical memory being freed but from which might make the
operation take longer but still far from soft lockup triggering.

Now latency might suck on !PREEMPT kernels with too many pages to
free in a single batch but I guess this is somehow expected for this
preemption model. The soft lockup has to be avoided because this can
panic the machine in some configurations.

-- 
Michal Hocko
SUSE Labs