lkml.org 
[lkml]   [2015]   [Dec]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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.
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>
>
> .
>





\
 
 \ /
  Last update: 2015-12-21 08:21    [W:0.058 / U:1.492 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site