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Date: Tue, 14 Oct 2014 13:08:15 +0100
From: "Daniel P. Berrange" <berrange@redhat.com>
Message-ID: <20141014120815.GA29671@redhat.com>
References: <543D10DE.2080109@huawei.com>
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Subject: Re: [Qemu-devel] [Bug?]When close VM the hugepage not freed
Reply-To: "Daniel P. Berrange" <berrange@redhat.com>
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To: Linhaifeng <haifeng.lin@huawei.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, lilijun <jerry.lilijun@huawei.com>

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.


Regards,
Daniel
-- 
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