Message-ID: <55599B95.9030800@plexistor.com>
Date: Mon, 18 May 2015 10:58:13 +0300
From: Boaz Harrosh <boaz@plexistor.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0
MIME-Version: 1.0
To: Rik van Riel <riel@redhat.com>, Daniel Phillips <daniel@phunq.net>,
        linux-kernel@vger.kernel.org
CC: linux-fsdevel@vger.kernel.org, tux3@tux3.org,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Subject: Re: [FYI] tux3: Core changes
References: <8f886f13-6550-4322-95be-93244ae61045@phunq.net> <55545C2F.8040207@phunq.net> <55549C2F.6000103@redhat.com> <555896EB.7040002@plexistor.com> <55594C8A.7000803@redhat.com>
In-Reply-To: <55594C8A.7000803@redhat.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2052
Lines: 56

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/