Re: [Bug #14141] order 2 page allocation failures in iwlagn

[Frans Pop <elendil-EIBgga6/0yRmR6Xm/wNWPw@public.gmane.org>]

Sorry for the delayed reply.

On Monday 19 October 2009, Chris Mason wrote:
> On Mon, Oct 19, 2009 at 03:01:52PM +0100, Mel Gorman wrote:
> > > During the 2nd phase I see the first SKB allocation errors with a
> > > music skip between reading commits 95.000 and 110.000.
> > > About commit 115.000 there is a very long pause during which the
> > > counter does not increase, music stops and the desktop freezes
> > > completely. The first 30 seconds of that freeze there is only very
> > > low disk activity (which seems strange);
> >
> > I'm just going to have to depend on Jens here. Jens, the
> > congestion_wait() is on BLK_RW_ASYNC after the commit. Reclaim usually
> > writes pages asynchronously but lumpy reclaim actually waits of pages
> > to write out synchronously so it's not always async.
>
> Waiting doesn't make it synchronous from the elevator point of view ;)
> If you're using WB_SYNC_NONE, it's a async write.  WB_SYNC_ALL makes it
> a sync write.  I only see WB_SYNC_NONE in vmscan.c, so we should be
> using the async congestion wait.  (the exception is xfs which always
> does async writes).
>
> But I'm honestly not 100% sure.  Looking back through the emails, the
> test case is doing IO on top of a whole lot of things on top of
> dm-crypt?  I just tried to figure out if dm-crypt is turning the async
> IO into sync IOs, but didn't quite make sense of it.
>
> Could you also please include which filesystems were being abused during
> the test and how?  Reading through the emails, I think you've got:
>
> gitk being run 3 times on some FS (NFS?)

gitk is run on an ext3 logical volume in a volume group that's on a LUKS 
encrypted partition of the local hard disk.

So it's: SATA harddisk -> dm-crypt (dmsetup) -> LVM (lvm2) -> ext3

> streaming reads on NFS

Correct. My music share is a remote (nfs4) read-only mounted ext3 
partition.

> swap on dm-crypt

Correct. Swap is another logical volume in the same volume group as 
mentioned above.

So kcrypt gets to (de)encrypt both the gitk data *and* any swapping caused 
by that [1].

> If other filesystems are being used, please correct me.  Also please
> include if they are on crypto or straight block device.

All my file systems are ext3. Nothing newfangled or exotic ;-)
There are some bind mounts involved, but I expect that's transparent.

Cheers,
FJP

[1] I've plans to move some of my data outside the encrypted volume, but 
currently everything except /boot is in the encrypted VG.