Re: [PATCH RFC] pram: persistent over-kexec memory file system

From: Marco Stornelli
Date: Sun Jul 28 2013 - 07:09:56 EST

Il 28/07/2013 12:05, Vladimir Davydov ha scritto:
On 07/27/2013 09:37 PM, Marco Stornelli wrote:
Il 27/07/2013 19:35, Vladimir Davydov ha scritto:
On 07/27/2013 07:41 PM, Marco Stornelli wrote:
Il 26/07/2013 14:29, Vladimir Davydov ha scritto:

We want to propose a way to upgrade a kernel on a machine without
restarting all the user-space services. This is to be done with CRIU
project, but we need help from the kernel to preserve some data in
memory while doing kexec.

The key point of our implementation is leaving process memory in-place
during reboot. This should eliminate most io operations the services
would produce during initialization. To achieve this, we have
implemented a pseudo file system that preserves its content during
kexec. We propose saving CRIU dump files to this file system,
and then restoring the processes in the newly booted kernel.

AFAIU it's a bit different thing: PRAMFS as well as pstore, which has
already been merged, requires hardware support for over-reboot
persistency, so called non-volatile RAM, i.e. RAM which is not directly
accessible and so is not used by the kernel. On the contrary, what we'd
like to have is preserving usual RAM on kexec. It is possible, because
RAM is not reset during kexec. This would allow leaving applications
working set as well as filesystem caches in place, speeding the reboot
process as a whole and reducing the downtime significantly.


Actually not. You can use normal system RAM reserved at boot with mem
parameter without any kernel change. Until an hard reset happens, that
area will be "persistent".

Thank you, we'll look at PRAMFS closer, but right now, after trying it I
have a couple of concerns I'd appreciate if you could clarify:

1) As you advised, I tried to reserve a range of memory (passing
memmap=4G$4G at boot) and mounted PRAMFS using the following options:

# mount -t pramfs -o physaddr=0x100000000,init=4G,bs=4096 none /mnt/pramfs

And it turned out that PRAMFS is very slow as compared to ramfs:

# dd if=/dev/zero of=/mnt/pramfs if=/dev/zero of=/mnt/pramfs/dummy
bs=4096 count=$[100*1024]
102400+0 records in
102400+0 records out
419430400 bytes (419 MB) copied, 9.23498 s, 45.4 MB/s
# dd if=/dev/zero of=/mnt/pramfs if=/dev/zero of=/mnt/pramfs/dummy
bs=4096 count=$[100*1024] conv=notrunc
102400+0 records in
102400+0 records out
419430400 bytes (419 MB) copied, 3.04692 s, 138 MB/s

We need it to be as fast as usual RAM, because otherwise the benefit of
it over hdd disappears. So before diving into the code, I'd like to ask
you if it's intrinsic to PRAMFS, or can it be fixed? Or, perhaps, I used
wrong mount/boot/config options (btw, I enabled only CONFIG_PRAMFS)?

In x86 you should have the write protection enabled. Turn it off or mount it with noprotect option.

2) To enable saving application dump files in memory using PRAMFS, one
should reserve half of RAM for it. That's too expensive. While with
ramfs, once SPLICE_F_MOVE flag is implemented, one could move anonymous
memory pages to ramfs page cache and after kexec move it back so that
almost no extra memory space costs would be required. Of course,
SPLICE_F_MOVE is to be yet implemented, but with PRAMFS significant
memory costs are inevitable... or am I wrong?


From this point of view you are right. Pramfs (or other solution like that) are out of page cache, so you can't do any memory transfer. It's like to have a disk but it's actually a separate piece of RAM. We could talk about it again when this kind of implementation will be done.

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