Re: Regression in Linux 5.5.0-rc[1-5]: btrfs send/receive out of memory

[Craig Andrews <candrews@integralblue.com>]

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On 2020-01-13 08:37, David Sterba wrote:
> On Mon, Jan 13, 2020 at 07:41:13AM -0500, Craig Andrews wrote:
>> If I perform a btrfs send receive like so:
>> 
>> sh -c btrfs send -p /mnt/everything/.snapshots/root.20191230
>> /mnt/everything/.snapshots/root.20191231 | btrfs receive
>> /mnt/backup/.snapshots/
>> 
>> On Linux 5.4.0, the process completes successfully.
>> 
>> Starting with Linux 5.5.0-rc1 up to the current 5.5 rc, 5.5.0-rc5, the
>> result is the OOM killer being invoked which (among other process
>> carnage) kills the btrfs processes stopping the backup.
> 
> As this is on the -rc1, it's possible that changes done in the MM
> subsystem could change the logic of OOM and that send now is able to
> trigger the OOM.
> 
> There are 2 btrfs patches in send.c but they reduce amount of work,
> namely for heavily reflinked extents so the effects should be opposite.
> 
> To find out where's the cause, is it possible that you build a kernel
> from the 5.4-based branch and run the send again? It's the same set of
> btrfs patches that gets merged to 5.5.
> 
> From repository: 
> git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux.git
> branch: for-5.5-rc4
> 
> If this can't be done we'll find another way to debug it.

I finally was able to test with this kernel.

With the kernel compiled from the branch "for-5.5-rc4" I do not get the 
out of memory error. However, I do this error:
btrfs send -p /mnt/everything/.snapshots/root.20200119 
/mnt/everything/.snapshots/root.20200120 | mbuffer -v 1 | btrfs receive 
/mnt/backup/.snapshots/
ERROR: failed to clone extents to var/amavis/db/__db.001: Invalid 
argument

I rebooted then immediately ran the same command with the 5.4 kernel and 
it completed successfully.

So I guess it's nice that I didn't get an out of memory error...

Thanks,
~Craig

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