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

[Atsushi Kumagai]

>>I am wondering if we are doing anything which requires kernel to allocate
>>memory and that leads to OOM.
>
>max_usage_in_bytes seems to include page cache, so I used simpler way
>like the patch at the bottom of this mail instead.
>
>Here is the result:
>
>            parameter                  |      result
> dump_lv | buffer[KiB] |  mmap (=4MiB) |    VmHWM [KiB]
> --------+-------------+---------------+------------------
>    d31  |       1     |       on      |        5872
>   Ed31  |       1     |       on      |        5804
>    d31  |       1     |      off      |        1784
>   Ed31  |       1     |      off      |        1720
>    d31  |    1000     |       on      |        8884  (*1)
>   Ed31  |    1000     |       on      |        7812
>    d31  |    1000     |      off      |        4788  (*1)
>   Ed31  |    1000     |      off      |        3720
>
>  *1. 2nd bitmap is allocated twice by the bug found by Arthur Zou.
>
>According to this result and the estimation below produced from the
>design of makedumpfile,
>
>  base size + (buffer size * 2) + (for mmap size) = VmHWM
>
>the base size may be about 2 MB, so about 6MB (base + mmap) will
>be the deadline. If 20% of the available memory is less than 6MB,
>OOM will happen.
>As for Baoquan's case, remained 17.2MB sounds enough even if
>makedumpfile consumes 6MB as for the other works.
>So I don't know why OOM happened yet, but the current safety
>limit looks still reasonable to me at least.
>
>By the way, I noticed it's better to remove 4MB(for mmap)
>from the available memory before calculate the bitmap buffer
>size. I'll do this anyway.

Sorry, I did my test on a 1st kernel environment.
The dump file was on a disk and I used mmap() for it, so __do_fault()
occurred and the VmHWM (MAX(MM_FILEPAGES + MM_ANONPAGES)) increased
by 4MB.

Here is the result on a 2nd kernel environment:

             parameter                  |      result
  dump_lv | buffer[KiB] |  mmap (=4MiB) |    VmHWM [KiB]
  --------+-------------+---------------+------------------
     d31  |       1     |       on      |         776
    Ed31  |       1     |       on      |         712
     d31  |       1     |      off      |         704
    Ed31  |       1     |      off      |         708
     d31  |    1000     |       on      |        1776
    Ed31  |    1000     |       on      |        2716
     d31  |    1000     |      off      |        1660
    Ed31  |    1000     |      off      |        2556


According to this, I think there is no need to take care of the memory
usage of mmap() because the actual memory usage is small enough.


Thanks
Atsushi Kumagai