Re: [FYI] tux3: Core changes

[Daniel Phillips <daniel@phunq.net>]

On 05/15/2015 01:09 AM, Mel Gorman wrote:
> On Thu, May 14, 2015 at 11:06:22PM -0400, Rik van Riel wrote:
>> On 05/14/2015 08:06 PM, Daniel Phillips wrote:
>>>> The issue is that things like ptrace, AIO, infiniband
>>>> RDMA, and other direct memory access subsystems can take
>>>> a reference to page A, which Tux3 clones into a new page B
>>>> when the process writes it.
>>>>
>>>> However, while the process now points at page B, ptrace,
>>>> AIO, infiniband, etc will still be pointing at page A.
>>>>
>>>> This causes the process and the other subsystem to each
>>>> look at a different page, instead of at shared state,
>>>> causing ptrace to do nothing, AIO and RDMA data to be
>>>> invisible (or corrupted), etc...
>>>
>>> Is this a bit like page migration?
>>
>> Yes. Page migration will fail if there is an "extra"
>> reference to the page that is not accounted for by
>> the migration code.
> 
> When I said it's not like page migration, I was referring to the fact
> that a COW on a pinned page for RDMA is a different problem to page
> migration. The COW of a pinned page can lead to lost writes or
> corruption depending on the ordering of events.

I see the lost writes case, but not the corruption case, Do you
mean corruption by changing a page already in writeout? If so,
don't all filesystems have that problem?

If RDMA to a mmapped file races with write(2) to the same file,
maybe it is reasonable and expected to lose some data.

> Page migration fails
> when there are unexpected problems to avoid this class of issue which is
> fine for page migration but may be a critical failure in a filesystem
> depending on exactly why the copy is required.

Regards,

Daniel