Re: [BUG REPORT] [mm-hotplug, aio] aio ring_pages can't be offlined

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/11/30 14:57), Andrew Morton wrote:
> On Fri, 30 Nov 2012 11:42:05 +0800 Lin Feng <linfeng@cn.fujitsu.com> wrote:
>
>> hi Andrew,
>>
>> On 11/30/2012 07:39 AM, Andrew Morton wrote:
>>> Tricky.
>>>
>>> I expect the same problem would occur with pages which are under
>>> O_DIRECT I/O.  Obviously O_DIRECT pages won't be pinned for such long
>>> periods, but the durations could still be lengthy (seconds).
>> the offline retry timeout duration is 2 minutes, so to O_DIRECT pages
>> seem maybe not a problem for the moment.
>>>
>>> Worse is a futex page, which could easily remain pinned indefinitely.
>>>
>>> The best I can think of is to make changes in or around
>>> get_user_pages(), to steal the pages from userspace and replace them
>>> with non-movable ones before pinning them.  The performance cost of
>>> something like this would surely be unacceptable for direct-io, but
>>> maybe OK for the aio ring and futexes.
>> thanks for your advice.
>> I want to limit the impact as little as possible, as mentioned above,
>> direct-io seems not a problem, we needn't touch them. Maybe we can
>> just change the use of get_user_pages()(in or around) such as aio
>> ring pages. I will try to find a way to do this.
>
> What about futexes?
>

IIUC, futex's key is now a pair of (mm,address) or (inode, pgoff).
Then, get_user_page() in futex.c will release the page by put_page().
'struct page' is just touched by get_futex_key() to obtain page->mapping info.

Thanks,
-Kame