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Date:   Fri, 6 Jul 2018 15:41:35 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Jia He <hejianet@gmail.com>
Cc:     Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
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        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>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Gioh Kim <gi-oh.kim@profitbricks.com>,
        Vlastimil Babka <vbabka@suse.cz>, Mel Gorman <mgorman@suse.de>,
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        Kemi Wang <kemi.wang@intel.com>,
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        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
Subject: Re: [RESEND PATCH v10 0/6] optimize memblock_next_valid_pfn and
 early_pfn_valid on arm and arm64
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On Fri,  6 Jul 2018 17:01:09 +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

It's a shame that we're at v10, still with very little evidence of
review activity.

Oh well, it's a nice speedup.  I'll toss it in and see what happens,
but I'm not very familiar with memblock so we should try to find
reviewers, please.