On 18-07-06 17:01:11, Jia He wrote:

From: Jia He <redacted>

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.
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.

Suggested-by: Daniel Vacek <redacted>
Signed-off-by: Jia He <redacted>

The version of this patch in linux-next has few fixes, I reviewed that one
looks good to me.

Reviewed-by: Pavel Tatashin <redacted>