On Fri,  6 Jul 2018 17:01:09 +0800 Jia He [off-list ref] 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.