linux-kernel - Re: [FYI] tux3: Core changes

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Date:	Mon, 18 May 2015 10:58:13 +0300
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Rik van Riel <riel@...hat.com>, Daniel Phillips <daniel@...nq.net>,
	linux-kernel@...r.kernel.org
CC:	linux-fsdevel@...r.kernel.org, tux3@...3.org,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: [FYI] tux3: Core changes

On 05/18/2015 05:20 AM, Rik van Riel wrote:
> On 05/17/2015 09:26 AM, Boaz Harrosh wrote:
>> On 05/14/2015 03:59 PM, Rik van Riel wrote:
>>> On 05/14/2015 04:26 AM, Daniel Phillips wrote:
>>>> Hi Rik,
>> <>
>>>
>>> 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.
>>>
>>
>> All these problems can also happen with truncate+new-extending-write
>>
>> It is the responsibility of the application to take file/range locks
>> to prevent these page-pinned problems.
> 
> It is unreasonable to expect a process that is being ptraced
> (potentially without its knowledge) to take special measures
> to protect the ptraced memory from disappearing.

If the memory disappears that's a bug. No the memory is just there
it is just not reflecting the latest content of the fs-file.

> 
> It is impossible for the debugger to take those special measures
> for anonymous memory, or unlinked inodes.
> 

Why? one line of added code after the open and before the mmap do an flock

> I don't think your requirement is workable or reasonable.
> 

Therefor it is unreasonable to write/modify a ptraced process
file.

Again what I'm saying is COWing a page on write, has the same effect
as truncate+write. They are both allowed and both might give you the same
"stale" effect. So the presidence is there. We are not introducing a new
anomaly, just introducing a new instance of it. I guess the question
is what applications/procedures are going to break. Need lots of testing
and real life installations to answer that, I guess.

Thanks
Boaz

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