Re: A bug in read operation for /dev/zero and a proposed fix.

[Salman Qazi]

On Thu, Jun 4, 2009 at 1:50 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 4 Jun 2009 13:32:55 -0700 (PDT)
> Salman Qazi <sqazi@xxxxxxxxxx> wrote:
>
>> While running 20 parallel instances of dd as follows:
>>
>> #!/bin/bash
>>
>> for i in `seq 1 20`; do
>>          dd if=/dev/zero of=/export/hda3/dd_$i bs=1073741824 count=1 &
>> done
>> wait
>>
>> on a 16G machine, we noticed that rather than just killing the
>> processes, the entire kernel went down.  Stracing dd reveals that it first
>> does an mmap2, which makes 1GB worth of zero page mappings.  Then it
>> performs
>> a read on those pages from /dev/zero, and finally it performs a write.
>> The
>> machine died during the reads.  Looking at the code, it was noticed that
>> /dev/zero's read operation had been changed at some point from giving
>> zero page mappings to actually zeroing the page.  The zeroing of the
>> pages causes physical pages to be allocated to the process.
>
> erk, Nick broke dd(1):
>
>  commit 557ed1fa2620dc119adb86b34c614e152a629a80
>  Author: Nick Piggin <npiggin@xxxxxxx>
>  Date:   Tue Oct 16 01:24:40 2007 -0700
>
>      remove ZERO_PAGE
>
>
> This is the first report I've seen of problems arising from that
> change.
>
>>  But, when
>> the process exhausts all the memory that it can, the kernel cannot kill
>> it, as it is still in the kernel mode allocating more memory.
>> Consequently,
>> the kernel eventually crashes.
>>
>> To fix this, I propose that when a fatal signal is pending during
>> /dev/zero read operation, we simply return and let the user process die.
>> Here is a patch that does that.
>>
>> Signed-off-by: Salman Qazi <sqazi@xxxxxxxxxx>
>> ---
>> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
>> index 8f05c38..2ffa36e 100644
>> --- a/drivers/char/mem.c
>> +++ b/drivers/char/mem.c
>> @@ -696,6 +696,11 @@ static ssize_t read_zero(struct file * file, char __user * buf,
>>                       break;
>>               buf += chunk;
>>               count -= chunk;
>> +             /* The exit code here doesn't actually matter, as userland
>> +              * will never see it.
>> +              */
>> +             if (fatal_signal_pending(current))
>> +                     return -ENOMEM;
>>               cond_resched();
>>       }
>>       return written ? written : -EFAULT;
>
> OK.  I think.
>
> It's presumptuous to return -ENOMEM: we don't _know_ that this signal
> came from the oom-killer.  It would be better to return -EINTR here.

agreed.

>
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