Re: Page Allocation Failures/OOM with dm-crypt on software RAID10 (Intel Rapid Storage)

[Michal Hocko]

On Tue 12-07-16 16:56:32, Matthias Dahl wrote:
> Hello Michal...
> 
> On 2016-07-12 16:07, Michal Hocko wrote:
> 
> > /proc/slabinfo could at least point on who is eating that memory.
> 
> Thanks. I have made another test (and thus again put the RAID10 out of
> sync for the 100th time, sigh) and made regular snapshots of slabinfo
> which I have attached to this mail.
> 
> > Direct IO doesn't get throttled like buffered IO.
> 
> Is buffered i/o not used in both cases if I don't explicitly request
> direct i/o?
> 
>     dd if=/dev/zero /dev/md126p5 bs=512K
> and dd if=/dev/zero /dev/mapper/test-device bs=512K

OK, I misunderstood your question though. You were mentioning the direct
IO earlier so I thought you were referring to it here as well.
 
> Given that the test-device is dm-crypt on md125p5. Aren't both using
> buffered i/o?

Yes they are.

> > the number of pages under writeback was more or less same throughout
> > the time but there are some local fluctuations when some pages do get
> > completed.
> 
> The pages under writeback are those directly destined for the disk, so
> after dm-crypt had done its encryption?

Those are submitted for the IO. dm-crypt will allocate a "shadow" page
for each of them to perform the encryption and only then submit the IO
to the storage underneath see
http://lkml.kernel.org/r/alpine.LRH.2.02.1607121907160.24806@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

> > If not you can enable allocator trace point for a particular object
> > size (or range of sizes) and see who is requesting them.
> 
> If that support is baked into the Fedora provided kernel that is. If
> you could give me a few hints or pointers, how to properly do a allocator
> trace point and get some decent data out of it, that would be nice.

You need to have a kernel with CONFIG_TRACEPOINTS and then enable them
via debugfs. You are interested in kmalloc tracepoint and specify a size
as a filter to only see those that are really interesting. I haven't
checked your slabinfo yet - hope to get to it later today.
-- 
Michal Hocko
SUSE Labs