Re: [Qemu-devel] [Bug?]When close VM the hugepage not freed

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Oct 14, 2014 at 07:28:39PM +0300, Michael S. Tsirkin wrote:
> On Tue, Oct 14, 2014 at 01:08:15PM +0100, Daniel P. Berrange wrote:
> > On Tue, Oct 14, 2014 at 08:02:38PM +0800, Linhaifeng wrote:
> > > Hi,all
> > > 
> > > I was trying to use hugepage with VM and found that the hugepage not freed when close VM.
> > > 
> > > 
> > > 1.Before start VM the /proc/meminfo is:
> > > AnonHugePages:    124928 kB
> > > HugePages_Total:    4096
> > > HugePages_Free:     3072
> > > HugePages_Rsvd:        0
> > > HugePages_Surp:        0
> > > Hugepagesize:       2048 kB
> > > 
> > > 2.Start VM the /proc/meminfo is:
> > > AnonHugePages:    139264 kB
> > > HugePages_Total:    4096
> > > HugePages_Free:     2048
> > > HugePages_Rsvd:        0
> > > HugePages_Surp:        0
> > > Hugepagesize:       2048 kB
> > > 
> > > 3.Close VM the /proc/meminfo is:
> > > AnonHugePages:    124928 kB
> > > HugePages_Total:    4096
> > > HugePages_Free:     2048
> > > HugePages_Rsvd:        0
> > > HugePages_Surp:        0
> > > Hugepagesize:       2048 kB
> > > 
> > > We can see there are 1024 hugepage leak!
> > > 
> > > I try to found which function used to free hugepage but i'm not sure
> > > where the qemu_ram_free is the function to free hugepage.
> > > I found that the qemu_ram_free function not call unlink and we know
> > > unlink is used to free hugepage(see example of hugepage-mmap.c in
> > > kernel source).
> > 
> > We can't rely on 'qemu_ram_free' ever executing because we must
> > ensure hugepages are freed upon QEMU crash.
> > 
> > It seems we should rely on UNIX filesytstem semantics and simply
> > unlink the memory segment the moment we create it & open the FD.
> > That way the kernel will automatically free it when the FD is
> > closed when QEMU process exits.
> > 
> We being libvirt?

No, QEMU is responsible for creating the mmap file, so QEMU should
take care of unlinking it after opening it.

Regards,
Daniel
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