Re: [PATCH v11 0/3] remain and optimize memblock_next_valid_pfn on arm and arm64

[Will Deacon]

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@xxxxxxxxx> 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