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Date: Thu, 6 Sep 2018 11:47:07 +0100
From: Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v11 0/3] remain and optimize memblock_next_valid_pfn on
 arm and arm64
Message-ID: <20180906104707.GL3592@arm.com>
References: <1534907237-2982-1-git-send-email-jia.he@hxt-semitech.com>
 <20180905145755.cc89819d446f311e4b8e8f95@linux-foundation.org>
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: Jia He <hejianet@gmail.com>, Russell King <linux@armlinux.org.uk>, Catalin Marinas <catalin.marinas@arm.com>, Mark Rutland <mark.rutland@arm.com>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Michal Hocko <mhocko@suse.com>, Wei Yang <richard.weiyang@gmail.com>, Kees Cook <keescook@chromium.org>, Laura Abbott <labbott@redhat.com>, Vladimir Murzin <vladimir.murzin@arm.com>, Philip Derrin <philip@cog.systems>, AKASHI Takahiro <takahiro.akashi@linaro.org>, James Morse <james.morse@arm.com>, Steve Capper <steve.capper@arm.com>, Gioh Kim <gi-oh.kim@profitbricks.com>, Vlastimil Babka <vbabka@suse.cz>, Mel Gorman <mgorman@suse.de>, Johannes Weiner <hannes@cmpxchg.org>, Kemi Wang <kemi.wang@intel.com>, Petr Tesarik <ptesarik@suse.com>, YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Nikolay Borisov <nborisov@suse.com>, Daniel Jordan <daniel.m.jordan@oracle.com>, Daniel Vacek <neelx@redhat.com>, Eugeniu Rosca <erosca@de.adit-jv.com>, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Jia He <jia.he@hxt-semitech.com>

On Wed, Sep 05, 2018 at 02:57:55PM -0700, Andrew Morton wrote:
> On Wed, 22 Aug 2018 11:07:14 +0800 Jia He <hejianet@gmail.com> wrote:
> 
> > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
> > where possible") optimized the loop in memmap_init_zone(). But it causes
> > possible panic bug. So Daniel Vacek reverted it later.
> > 
> > But as suggested by Daniel Vacek, it is fine to using memblock to skip
> > gaps and finding next valid frame with CONFIG_HAVE_ARCH_PFN_VALID.
> > 
> > More from what Daniel said:
> > "On arm and arm64, memblock is used by default. But generic version of
> > pfn_valid() is based on mem sections and memblock_next_valid_pfn() does
> > not always return the next valid one but skips more resulting in some
> > valid frames to be skipped (as if they were invalid). And that's why
> > kernel was eventually crashing on some !arm machines."
> > 
> > About the performance consideration:
> > As said by James in b92df1de5,
> > "I have tested this patch on a virtual model of a Samurai CPU with a
> > sparse memory map.  The kernel boot time drops from 109 to 62 seconds."
> > Thus it would be better if we remain memblock_next_valid_pfn on arm/arm64.
> > 
> > Besides we can remain memblock_next_valid_pfn, there is still some room
> > for improvement. After this set, I can see the time overhead of memmap_init
> > is reduced from 27956us to 13537us in my armv8a server(QDF2400 with 96G
> > memory, pagesize 64k). I believe arm server will benefit more if memory is
> > larger than TBs
> 
> Thanks.  I switched to v11.  It would be nice to see some confirmation
> from ARM people please?

I'll take a look...

Will