On 11/14/2014 08:08 AM, Michal Hocko wrote:
Hi,
back in 2012 [1] there was a discussion about a forking load which
accumulates anon_vmas. There was a trivial test case which triggers this
and can potentially deplete the memory by local user.
We have a report for an older enterprise distribution where nsd is
suffering from this issue most probably (I haven't debugged it throughly
but accumulating anon_vma structs over time sounds like a good enough
fit) and has to be restarted after some time to release the accumulated
anon_vma objects.
There was a patch which tried to work around the issue [2] but I do not
see any follow ups nor any indication that the issue would be addressed
in other way.
The test program from [1] was running for around 39 mins on my laptop
and here is the result:
$ date +%s; grep anon_vma /proc/slabinfo
1415960225
anon_vma 11664 11900 160 25 1 : tunables 0 0 0 :
slabdata 476 476 0
$ ./a # The reproducer
$ date +%s; grep anon_vma /proc/slabinfo
1415962592
anon_vma 34875 34875 160 25 1 : tunables 0 0 0 :
slabdata 1395 1395 0
$ killall a
$ date +%s; grep anon_vma /proc/slabinfo
1415962607
anon_vma 11277 12175 160 25 1 : tunables 0 0 0 :
slabdata 487 487 0
So we have accumulated 23211 objects over that time period before the
offender was killed which released all of them.
The proposed workaround is kind of ugly but do people have a better idea
than reference counting? If not should we merge it?
I believe we should just merge that patch.
I have not seen any better ideas come by.
The comment should probably be fixed to reflect the
chain length of 5 though :)
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