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

[Lin Feng <linfeng@cn.fujitsu.com>]

On 11/30/2012 01:57 PM, 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?
hi Andrew,

Yes, better to find an approach to solve them all.
 
But I'm worried about that if we just confine get_user_pages() to use 
none-movable pages, it will drain the none-movable pages soon. Because
there are many places using get_user_pages() such as some drivers. 

IMHO in most cases get_user_pages() callers should release the pages soon, 
so pages allocated from movable zone should be OK. But I'm not sure if
we get such rule upon get_user_pages(). 
And in other cases we specify get_user_pages() to allocate pages from
none-movable zone. 

So could we add a zone-alloc flags when we call get_user_pages()?

Thanks,
linfeng

>