Re: [PATCH 1/4] mm: page_alloc: reduce unnecessary binary search in memblock_next_valid_pfn()

[Jia He]

On 3/21/2018 6:14 PM, Daniel Vacek Wrote:
> On Wed, Mar 21, 2018 at 9:09 AM, 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 there is
> > still some room for improvement. E.g. if pfn and pfn+1 are in the same
> > memblock region, we can simply pfn++ instead of doing the binary search
> > in memblock_next_valid_pfn.
> There is a revert-mm-page_alloc-skip-over-regions-of-invalid-pfns-where-possible.patch
> in -mm reverting b92df1de5d289c0b as it is fundamentally wrong by
> design causing system panics on some machines with rare but still
> valid mappings. Basically it skips valid pfns which are outside of
> usable memory ranges (outside of memblock memory regions).
Thanks for the infomation.
quote from you patch description:
>But given some specific memory mapping on x86_64 (or more generally 
theoretically anywhere but on arm with CONFIG_HAVE_ARCH_PFN_VALID) > the 
implementation also skips valid pfns which is plain wrong and causes > 
'kernel BUG at mm/page_alloc.c:1389!'

Do you think memblock_next_valid_pfn can remain to be not reverted on 
arm64 with CONFIG_HAVE_ARCH_PFN_VALID? Arm64 can benifit from this 
optimization.

Cheers,
Jia