[FYI] tux3: Core changes

[Rik van Riel]

On 05/14/2015 08:06 PM, Daniel Phillips wrote:
> Hi Rik,
> 
> Added Mel, Andrea and Peterz to CC as interested parties. There are
> probably others, please just jump in.
> 
> On 05/14/2015 05:59 AM, Rik van Riel wrote:
>> On 05/14/2015 04:26 AM, Daniel Phillips wrote:
>>> Hi Rik,
>>>
>>> Our linux-tux3 tree currently currently carries this 652 line diff
>>> against core, to make Tux3 work. This is mainly by Hirofumi, except
>>> the fs-writeback.c hook, which is by me. The main part you may be
>>> interested in is rmap.c, which addresses the issues raised at the
>>> 2013 Linux Storage Filesystem and MM Summit 2015 in San Francisco.[1]
>>>
>>>    LSFMM: Page forking
>>>    http://lwn.net/Articles/548091/
>>>
>>> This is just a FYI. An upcoming Tux3 report will be a tour of the page
>>> forking design and implementation. For now, this is just to give a
>>> general sense of what we have done. We heard there are concerns about
>>> how ptrace will work. I really am not familiar with the issue, could
>>> you please explain what you were thinking of there?
>>
>> 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.

Only pages that have no extra refcount can be migrated.

Similarly, your cow code needs to fail if there is an
extra reference count pinning the page. As long as
the page has a user that you cannot migrate, you cannot
move any of the other users over. They may rely on data
written by the hidden-to-you user, and the hidden-to-you
user may write to the page when you think it is a read
only stable snapshot.

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