Re: [PATCH] memory-hotplug: Fix bad area access on dissolve_free_huge_pages()

[Dave Hansen]

On 09/20/2016 07:45 AM, Rui Teng wrote:
> On 9/17/16 12:25 AM, Dave Hansen wrote:
>>
>> That's an interesting data point, but it still doesn't quite explain
>> what is going on.
>>
>> It seems like there might be parts of gigantic pages that have
>> PageHuge() set on tail pages, while other parts don't.  If that's true,
>> we have another bug and your patch just papers over the issue.
>>
>> I think you really need to find the root cause before we apply this
>> patch.
>>
> The root cause is the test scripts(tools/testing/selftests/memory-
> hotplug/mem-on-off-test.sh) changes online/offline status on memory
> blocks other than page header. It will *randomly* select 10% memory
> blocks from /sys/devices/system/memory/memory*, and change their
> online/offline status.

Ahh, that does explain it!  Thanks for digging into that!

> That's why we need a PageHead() check now, and why this problem does
> not happened on systems with smaller huge page such as 16M.
> 
> As far as the PageHuge() set, I think PageHuge() will return true for
> all tail pages. Because it will get the compound_head for tail page,
> and then get its huge page flag.
>     page = compound_head(page);
> 
> And as far as the failure message, if one memory block is in use, it
> will return failure when offline it.

That's good, but aren't we still left with a situation where we've
offlined and dissolved the _middle_ of a gigantic huge page while the
head page is still in place and online?

That seems bad.