Re: [BUG] Invalid return address of mmap() followed by mbind() in multithreaded context

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

(2011/06/28 2:18), Kornilios Kourtis wrote:
> 
> Hi,
> 
> On Fri, Jun 17, 2011 at 06:21:49PM +0300, Vasileios Karakasis wrote:
>> Hi,
>>
>> I am implementing a multithreaded numa aware code where each thread
>> mmap()'s an anonymous private region and then mbind()'s it to its local
>> node. The threads are performing a series of such mmap() + mbind()
>> operations. My program crashed with SIGSEGV and I noticed that mmap()
>> returned an invalid address.
> 
> I've taken a closer look at this issue.
> 
> As Vasileios said, it can be reproduced by having two threads doing the
> following loop:
> | for {
> | 	addr = mmap(4096, MAP_ANONUMOUS)
> | 	if (addr == (void *)-1)
> | 		continue
> | 	mbind(addr, 4096, 0x1) // do mbind on first NUMA node
> | }
> After a couple of iterations, mbind() will return EFAULT, although the addr is
> valid.
> 
> Doing a bisect, pins it down to the following commit (Author added to To:):
> 	9d8cebd4bcd7c3878462fdfda34bbcdeb4df7ef4
> 	mm: fix mbind vma merge problem
> Which adds merging of vmas in the mbind() path.
> Reverting this commit, seems to fix the issue.
> 
> I 've added some printks to track down the issue, and EFAULT is returned on:
> mm/mempolicy.c: mbind_range()
> |   vma = find_vma_prev(mm. start, &prev);
> |   if (!vma |vma->vm_start > start)
> |       return EFAULT;
> Where: vma->start > start
> 
> I am not sure what exactly happens, but concurrent merges and splits
> of (already mapped) VMAs do not seem to work well together.

Hi

Thank you for digging this! I look it at soon as far as possible.

 - kosaki

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