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
Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping
Date: Thu, 28 Jun 2018 17:59:25 -0700 [thread overview]
Message-ID: <b7b8ed15-183b-9afe-8e72-d2751672e24a@linux.alibaba.com> (raw)
In-Reply-To: <2ecdb667-f4de-673d-6a5f-ee50df505d0c@linux.alibaba.com>
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.
>
next prev parent reply other threads:[~2018-06-29 1:00 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-18 23:34 [RFC v2 0/2] mm: zap pages with read mmap_sem in munmap for large mapping Yang Shi
2018-06-18 23:34 ` [RFC v2 PATCH 1/2] uprobes: make vma_has_uprobes non-static Yang Shi
2018-06-18 23:34 ` [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping Yang Shi
2018-06-19 10:02 ` Peter Zijlstra
2018-06-19 21:13 ` Yang Shi
2018-06-20 7:17 ` Michal Hocko
2018-06-20 16:23 ` Yang Shi
2018-06-19 22:17 ` Nadav Amit
[not found] ` <158a4e4c-d290-77c4-a595-71332ede392b@linux.alibaba.com>
2018-06-20 0:31 ` Nadav Amit
2018-06-20 7:18 ` Michal Hocko
2018-06-20 17:12 ` Nadav Amit
2018-06-20 18:42 ` Yang Shi
2018-06-23 1:01 ` Yang Shi
2018-06-25 9:14 ` Michal Hocko
2018-06-26 0:06 ` Yang Shi
2018-06-26 7:43 ` Peter Zijlstra
2018-06-27 1:03 ` Yang Shi
2018-06-27 7:24 ` Michal Hocko
2018-06-27 17:23 ` Yang Shi
2018-06-28 11:51 ` Michal Hocko
2018-06-28 19:10 ` Yang Shi
2018-06-29 0:59 ` Yang Shi [this message]
2018-06-29 11:39 ` Michal Hocko
2018-06-29 16:50 ` Yang Shi
2018-06-29 11:34 ` Michal Hocko
2018-06-29 16:45 ` Yang Shi
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