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Subject: Re: [PATCH 1/4] mm: page_alloc: reduce unnecessary binary search in
 memblock_next_valid_pfn()
References: <1521619796-3846-1-git-send-email-hejianet@gmail.com>
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From: Jia He <hejianet@gmail.com>
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Date: Wed, 21 Mar 2018 20:28:18 +0800
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To: Daniel Vacek <neelx@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Michal Hocko <mhocko@suse.com>, Catalin Marinas <catalin.marinas@arm.com>, Mel Gorman <mgorman@suse.de>, Will Deacon <will.deacon@arm.com>, Mark Rutland <mark.rutland@arm.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Pavel Tatashin <pasha.tatashin@oracle.com>, Daniel Jordan <daniel.m.jordan@oracle.com>, AKASHI Takahiro <takahiro.akashi@linaro.org>, Gioh Kim <gi-oh.kim@profitbricks.com>, Steven Sistare <steven.sistare@oracle.com>, Eugeniu Rosca <erosca@de.adit-jv.com>, Vlastimil Babka <vbabka@suse.cz>, open list <linux-kernel@vger.kernel.org>, linux-mm@kvack.org, James Morse <james.morse@arm.com>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Steve Capper <steve.capper@arm.com>, x86@kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Kate Stewart <kstewart@linuxfoundation.org>, Philippe Ombredanne <pombredanne@nexb.com>, 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>, Jia He <jia.he@hxt-semitech.com>



On 3/21/2018 6:14 PM, Daniel Vacek Wrote:
> On Wed, Mar 21, 2018 at 9:09 AM, 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 there is
>> still some room for improvement. E.g. if pfn and pfn+1 are in the same
>> memblock region, we can simply pfn++ instead of doing the binary search
>> in memblock_next_valid_pfn.
> There is a revert-mm-page_alloc-skip-over-regions-of-invalid-pfns-where-possible.patch
> in -mm reverting b92df1de5d289c0b as it is fundamentally wrong by
> design causing system panics on some machines with rare but still
> valid mappings. Basically it skips valid pfns which are outside of
> usable memory ranges (outside of memblock memory regions).
Thanks for the infomation.
quote from you patch description:
 >But given some specific memory mapping on x86_64 (or more generally 
theoretically anywhere but on arm with CONFIG_HAVE_ARCH_PFN_VALID) > the 
implementation also skips valid pfns which is plain wrong and causes > 
'kernel BUG at mm/page_alloc.c:1389!'

Do you think memblock_next_valid_pfn can remain to be not reverted on 
arm64 with CONFIG_HAVE_ARCH_PFN_VALID? Arm64 can benifit from this 
optimization.

Cheers,
Jia