Re: [PATCH v9 2/6] mm: page_alloc: remain memblock_next_valid_pfn() on arm/arm64

[Pavel Tatashin]

On Thu, Jun 28, 2018 at 10:30 PM 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.
>
> 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.

Hi Jia,

Is this a bug? Should we make other arches that support memblock to
use memblock_is_map_memory() ? it is more expensive, but if the
default is broken, maybe it makes sense to change?

Thank you,
Pavel