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From:   Pasha Tatashin <Pavel.Tatashin@microsoft.com>
To:     Andrew Morton <akpm@linux-foundation.org>
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Subject: Re: [RESEND PATCH v10 0/6] optimize memblock_next_valid_pfn and
 early_pfn_valid on arm and arm64
Thread-Topic: [RESEND PATCH v10 0/6] optimize memblock_next_valid_pfn and
 early_pfn_valid on arm and arm64
Thread-Index: AQHUNZO1/cRNVxmBf0uQR82FDfwseg==
Date:   Thu, 16 Aug 2018 19:02:42 +0000
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On 18-08-15 15:34:56, Andrew Morton wrote:
> On Fri,  6 Jul 2018 17:01:09 +0800 Jia He <hejianet@gmail.com> wrote:
>=20
> > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
> > where possible") optimized the loop in memmap_init_zone(). But it cause=
s
> > possible panic bug. So Daniel Vacek reverted it later.
> >=20
> > 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.
> >=20
> > 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."
> >=20
> > 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/arm=
64.
> >=20
> > 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
>=20
> This patchset is basically unreviewed at this stage.  Could people
> please find some time to check it carefully?

Working on it.

Pavel=