linux-kernel - Re: [RFC] theoretical race between memory hotplug and pfn iterator

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Date:	Mon, 21 Dec 2015 15:00:08 +0800
From:	Zhu Guihua <zhugh.fnst@...fujitsu.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>, <linux-mm@...ck.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Gu Zheng <guz.fnst@...fujitsu.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Toshi Kani <toshi.kani@....com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Vlastimil Babka <vbabka@...e.cz>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC] theoretical race between memory hotplug and pfn iterator


On 12/21/2015 11:15 AM, Joonsoo Kim wrote:
> Hello, memory-hotplug folks.
>
> I found theoretical problems between memory hotplug and pfn iterator.
> For example, pfn iterator works something like below.
>
> for (pfn = zone_start_pfn; pfn < zone_end_pfn; pfn++) {
>          if (!pfn_valid(pfn))
>                  continue;
>
>          page = pfn_to_page(pfn);
>          /* Do whatever we want */
> }
>
> Sequence of hotplug is something like below.
>
> 1) add memmap (after then, pfn_valid will return valid)
> 2) memmap_init_zone()
>
> So, if pfn iterator runs between 1) and 2), it could access
> uninitialized page information.
>
> This problem could be solved by re-ordering initialization steps.
>
> Hot-remove also has a problem. If memory is hot-removed after
> pfn_valid() succeed in pfn iterator, access to page would cause NULL
> deference because hot-remove frees corresponding memmap. There is no
> guard against free in any pfn iterators.
>
> This problem can be solved by inserting get_online_mems() in all pfn
> iterators but this looks error-prone for future usage. Another idea is
> that delaying free corresponding memmap until synchronization point such
> as system suspend. It will guarantee that there is no running pfn
> iterator. Do any have a better idea?
>
> Btw, I tried to memory-hotremove with QEMU 2.5.5 but it didn't work. I
> followed sequences in doc/memory-hotplug. Do you have any comment on this?

I tried memory hot remove with qemu 2.5.5 and RHEL 7, it works well.
Maybe you can provide more details, such as guest version, err log.

Thanks,
Zhu

>
> Thanks.
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