Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for
 large mapping
From:   Yang Shi <yang.shi@linux.alibaba.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Nadav Amit <nadav.amit@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        ldufour@linux.vnet.ibm.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, acme@kernel.org,
        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        linux-kernel@vger.kernel.org
References: <1529364856-49589-3-git-send-email-yang.shi@linux.alibaba.com>
 <3DDF2672-FCC4-4387-9624-92F33C309CAE@gmail.com>
 <158a4e4c-d290-77c4-a595-71332ede392b@linux.alibaba.com>
 <BFD6A249-B1D7-43D5-8D7C-9FAED4A168A1@gmail.com>
 <20180620071817.GJ13685@dhcp22.suse.cz>
 <263935d9-d07c-ab3e-9e42-89f73f57be1e@linux.alibaba.com>
 <20180626074344.GZ2458@hirez.programming.kicks-ass.net>
 <e54e298d-ef86-19a7-6f6b-07776f9a43e2@linux.alibaba.com>
 <20180627072432.GC32348@dhcp22.suse.cz>
 <a52f0585-ebec-d098-2775-f55bde3519a4@linux.alibaba.com>
 <20180628115101.GE32348@dhcp22.suse.cz>
 <2ecdb667-f4de-673d-6a5f-ee50df505d0c@linux.alibaba.com>
Message-ID: <b7b8ed15-183b-9afe-8e72-d2751672e24a@linux.alibaba.com>
Date:   Thu, 28 Jun 2018 17:59:25 -0700
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0)
 Gecko/20100101 Thunderbird/52.7.0
MIME-Version: 1.0
In-Reply-To: <2ecdb667-f4de-673d-6a5f-ee50df505d0c@linux.alibaba.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 6/28/18 12:10 PM, Yang Shi wrote:
>
>
> On 6/28/18 4:51 AM, Michal Hocko wrote:
>> On Wed 27-06-18 10:23:39, Yang Shi wrote:
>>>
>>> On 6/27/18 12:24 AM, Michal Hocko wrote:
>>>> On Tue 26-06-18 18:03:34, Yang Shi wrote:
>>>>> On 6/26/18 12:43 AM, Peter Zijlstra wrote:
>>>>>> On Mon, Jun 25, 2018 at 05:06:23PM -0700, Yang Shi wrote:
>>>>>>> By looking this deeper, we may not be able to cover all the 
>>>>>>> unmapping range
>>>>>>> for VM_DEAD, for example, if the start addr is in the middle of 
>>>>>>> a vma. We
>>>>>>> can't set VM_DEAD to that vma since that would trigger SIGSEGV 
>>>>>>> for still
>>>>>>> mapped area.
>>>>>>>
>>>>>>> splitting can't be done with read mmap_sem held, so maybe just 
>>>>>>> set VM_DEAD
>>>>>>> to non-overlapped vmas. Access to overlapped vmas (first and 
>>>>>>> last) will
>>>>>>> still have undefined behavior.
>>>>>> Acquire mmap_sem for writing, split, mark VM_DEAD, drop mmap_sem. 
>>>>>> Acquire
>>>>>> mmap_sem for reading, madv_free drop mmap_sem. Acquire mmap_sem for
>>>>>> writing, free everything left, drop mmap_sem.
>>>>>>
>>>>>> ?
>>>>>>
>>>>>> Sure, you acquire the lock 3 times, but both write instances 
>>>>>> should be
>>>>>> 'short', and I suppose you can do a demote between 1 and 2 if you 
>>>>>> care.
>>>>> Thanks, Peter. Yes, by looking the code and trying two different 
>>>>> approaches,
>>>>> it looks this approach is the most straight-forward one.
>>>> Yes, you just have to be careful about the max vma count limit.
>>> Yes, we should just need copy what do_munmap does as below:
>>>
>>> if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count)
>>>              return -ENOMEM;
>>>
>>> If the mas map count limit has been reached, it will return failure 
>>> before
>>> zapping mappings.
>> Yeah, but as soon as you drop the lock and retake it, somebody might
>> have changed the adddress space and we might get inconsistency.
>>
>> So I am wondering whether we really need upgrade_read (to promote read
>> to write lock) and do the
>>     down_write
>>     split & set up VM_DEAD
>>     downgrade_write
>>     unmap
>>     upgrade_read
>>     zap ptes
>>     up_write

Promoting to write lock may be a trouble. There might be other users in 
the critical section with read lock, we have to wait them to finish.

>
> I'm supposed address space changing just can be done by mmap, mremap, 
> mprotect. If so, we may utilize the new VM_DEAD flag. If the VM_DEAD 
> flag is set for the vma, just return failure since it is being unmapped.
>
> Does it sounds reasonable?

It looks we just need care about MAP_FIXED (mmap) and MREMAP_FIXED 
(mremap), right?

How about letting them return -EBUSY or -EAGAIN to notify the 
application? This changes the behavior a little bit, MAP_FIXED and 
mremap may fail if they fail the race with munmap (if the mapping is 
larger than 1GB). I'm not sure if any multi-threaded application uses 
MAP_FIXED and MREMAP_FIXED very heavily which may run into the race 
condition. I guess it should be rare to meet all the conditions to 
trigger the race.

The programmer should be very cautious about MAP_FIXED.MREMAP_FIXED 
since they may corrupt its own address space as the man page noted.


Thanks,
Yang

>
> Thanks,
> Yang
>
>>
>> looks terrible, no question about that, but we won't drop the mmap sem
>> at any time.
>