Re: [PATCH v3 1/5] mm: page_alloc: remain memblock_next_valid_pfn() when CONFIG_HAVE_ARCH_PFN_VALID is enable

[Wei Yang <richard.weiyang@gmail.com>]

On Mon, Apr 02, 2018 at 05:17:35PM +0800, Jia He wrote:
>
>
>On 4/2/2018 4:12 PM, Wei Yang Wrote:
>> On Wed, Mar 28, 2018 at 05:49:23PM +0800, Jia He wrote:
>> > 
>> > On 3/28/2018 5:18 PM, Wei Yang Wrote:
>> > > Oops, I should reply this thread. Forget about the reply on another thread.
>> > > 
>> > > On Sun, Mar 25, 2018 at 08:02:15PM -0700, Jia He 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.
>> > > > 
>> > > Why this has a bug? Do you have some link about it?
>> > > 
>> > > If the audience could know the potential risk, it would be helpful to review
>> > > the code and decide whether to take it back.
>> > Hi Wei
>> > Paul firstly submit a commit b92df1de5 to improve the loop in
>> > memmap_init_zone.
>> > And Daniel tried to fix a bug_on panic issue on X86 in commit 864b75f9d6b
>> > because
>> > there is evidence that this bug_on was caused by b92df1de5 [1].
>> > 
>> > But things didn't get better, 864b75f9d6b caused booting hang issue on
>> > arm{64} [2]
>> > So maintainer decided to reverted both b92df1de5 and 864b75f9d6b
>> > 
>> > [1] https://patchwork.kernel.org/patch/10251145/
>> > [2] https://lkml.org/lkml/2018/3/14/469
>> I took some time to look into the discussion, while the root cause seems not
>> clear now?
>> 
>Frankly speaking, to me the root cause of that bug_on is not completedly
>clear :-) Daniel ever gave me some hints as followed, but currently I have
>no x86 platform to understand the details.
>
>"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."
>

This means a system with memblock is safe to use this function?

As I know, mem_section is based on memblock, so in which case
memblock_next_valid_pfn() skips a valid pfn? A little confused.

>-- 
>Cheers,
>Jia

-- 
Wei Yang
Help you, Help me