[PATCH] makedumpfile: change the wrong code to calculate bufsize_cyclic for elf dump

[Atsushi Kumagai]

>On 04/17/14 at 12:52pm, Baoquan He wrote:
>> On 04/17/14 at 04:01am, Atsushi Kumagai wrote:
>> > Hello Baoquan,
>> >
>> > >Hi Atsushi,
>> > >
>> > >I have got the test machine where bug reported and did a test. The
>> > >changed code can make elf dump successful.
>> >
>> > Great, thanks for your help!
>> > However, I still have questions.
>> >
>> > First, what is the difference between yours and mine?
>> >
>> > http://lists.infradead.org/pipermail/kexec/2014-April/011535.html
>>
>> Yeah, you are right, it's the same on changing the code bug. I mush
>> haven't read your patch carefully.
>                                                          must<--
>>
>> >
>> > My patch includes renaming some values, but the purpose looks
>> > the same as yours.
>> > Further, you described as below,
>> >
>> > >On 04/14/14 at 04:02pm, Baoquan He wrote:
>> > but I still don't think this bug causes OOM.
>> > Even if needed_size is calculated as so much size wrongly, bufsize_cyclic
>> > will not exceed 40% of free memory by the check below:
>> >
>> >     info->bufsize_cyclic = (free_size <= needed_size) ? free_size : needed_size;
>> >
>> > So it looks that bitmap1(40%) and bitmap2(40%) will fit in 80% of free
>> > memory in any case.
>> >
>> > I may misunderstand something since your patch has an effect on this
>> > issue in practice, could you correct me?
>>
>> It definitely will cause OOM. On my test machine, it has 100G memory. So
>> per old code, its needed_size is 3200K*2 == 6.4M, if currently free
>> memory is only 15M left, the free_size will be 15M*0.4 which is 6M. So
>> info->bufsize_cyclic is assigned to be 6M. and only 3M is left for other
>> use, e.g page cache, dynamic allocation. OOM will happen.
>>
>
>BTW, in our case, there's about 30M free memory when we started saving
>dump. It should be caused by my coarse estimation above.

Thanks for your description, I understand that situation and
the nature of the problem.

That is, the assumption that 20% of free memory is enough for
makedumpfile can be broken if free memory is too small.
If your machine has 200GB memory, OOM will happen even after fix
the too allocation bug.

I don't think this is a problem, it's natural that a lack of memory
causes OOM. However, there is a thing we can do for improvement. 

What I think is:

  1. Use a constant value as safe limit to calculate bufsize_cyclic
     instead of 80% of free memory. This value must be enough for
     makedumpfile's work except bitmap.

  2. If free memory is smaller than the value, makedumpfile gives up
     to work early.

This change may reduce the possibility of lack of memory, but the
required memory size will be changing every version, so maintaining
it sounds tough to me.

Any comments are welcome.


Thanks
Atsushi Kumagai