Re: [Xen-devel] [PATCH for-4.5 v2] libxc: don't leak buffer containing the uncompressed PV kernel

[Gedalya <gedalya@xxxxxxxxxxx>]

On 11/21/2014 06:03 AM, Ian Campbell wrote:
> > So here's what happens now.
> > 1. Starts up tiny
> > 2. reboot: leak
> > 3. reboot: freed (process larger, but the delta is all/mostly shared pages)
> > 4. reboot: leak
> > 5. reboot: freed
> > etc..
> WTF, how very strange!
:-)

> > --- reboot domu ---
> >
> > root@xen:~/xen-pkgs# ps aux | grep asterisk_deb80
> > root     22981  0.6  3.3 131652 20008 ?        SLsl 21:55   0:00 
> > /usr/lib/xen-4.4/bin/xl cr /etc/xen/auto/asterisk_deb80.cfg
> > root@xen:~/xen-pkgs# pmap -x 22981
> > 22981:   /usr/lib/xen-4.4/bin/xl cr /etc/xen/auto/asterisk_deb80.cfg
> > Address           Kbytes     RSS   Dirty Mode  Mapping
> > 0000000000400000     144     144       0 r-x-- xl
> > 0000000000623000       4       4       4 r---- xl
> > 0000000000624000       8       8       8 rw--- xl
> > 0000000000626000       4       4       4 rw---   [ anon ]
> > 00000000009a6000     288     288     288 rw---   [ anon ]
> > 00000000009ee000   35676   16772   16772 rw---   [ anon ]
> This is the (temporarily) leaked mapping, right?
Yea that's the one that popped in after the reboot..
About 16 MB.

> > Tried valgrind, it doesn't look like it was able to see what was going on
> Indeed. The values for total heap usage at exist and still reachable etc
> also don't seem to account for the ~3M of mapping on each iteration.
>
> I don't know how glibc's allocator works, but I suppose it isn't
> impossible that it is retaining some mappings of free regions and
> collecting them to free later somehow, which just happens to only
> trigger every other reboot (e.g. perhaps it is based on some threshold
> of free memory).
>
> ...investigates...
>
> So, http://man7.org/linux/man-pages/man3/malloc.3.html talks about
> special behaviour using mmap for allocations above MMAP_THRESHOLD (128K
> by default), which we will be hitting here I think. That explains the
> anon mapping.
>
> http://man7.org/linux/man-pages/man3/mallopt.3.html also talks about
> various dynamic thresholds for growing and shrinking the heap. My guess
> is that we are bouncing up and down over some threshold with every other
> reboot.
>
> Ian.
OK this is way over my head.. I don't have a full and precise 
understanding of all of the above, but let me try to comment nevertheless.
There are two issues here. The added mappings (as I understand, files, 
non-anon, shared) do happen only on reboot, but they're not a real 
memory leak issue because they are shared with other processes, so no 
matter ho many xl processes we have, it's only another 2.6 or so MB 
added to the total memory usage of the server, right?
On the other hand we have this anon area, 16 MB, that pops in on one 
reboot and gets freed on the next. That's the real issue, as I see it..!
And all that only starts happening after the last line of valgrind 
output. Valgrind only had output up to the first boot of the VM, none later.
For a fresh, non-rebooted domu, the xl process shows up as in top as 
~588 KB RES, 0 SHR, how can the latter be anyway?? I don't understand.


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