Re: iomap infrastructure and multipage writes V5

[Dave Chinner <david@fromorbit.com>]

On Mon, Jul 18, 2016 at 09:14:00PM +1000, Dave Chinner wrote:
> On Thu, Jun 30, 2016 at 07:22:39PM +0200, Christoph Hellwig wrote:
> > On Tue, Jun 28, 2016 at 10:26:49AM +1000, Dave Chinner wrote:
> > > Christoph, it look slike there's an ENOSPC+ENOMEM behavioural regression here.
> > > generic/224 on my 1p/1GB RAM VM using a 1k lock size filesystem has
> > > significantly different behaviour once ENOSPC is hit withi this patchset.
> > > 
> > > It ends up with an endless stream of errors like this:
> > 
> > I've spent some time trying to reproduce this.  I'm actually getting
> > the OOM killer almost reproducible for for-next without the iomap
> > patches as well when just using 1GB of mem.  1400 MB is the minimum
> > I can reproducibly finish the test with either code base.
> > 
> > But with the 1400 MB setup I see a few interesting things.  Even
> > with the baseline, no-iomap case I see a few errors in the log:
> > 
> > [   70.407465] Filesystem "vdc": reserve blocks depleted! Consider increasing
> > reserve pool
> > size.
> > [   70.195645] XFS (vdc): page discard on page ffff88005682a988, inode 0xd3, offset 761856.
> > [   70.408079] Buffer I/O error on dev vdc, logical block 1048513, lost async
> > page write
> > [   70.408598] Buffer I/O error on dev vdc, logical block 1048514, lost async
> > page write
> >  27s
> > 
> > With iomap I also see the spew of page discard errors your see, but while
> > I see a lot of them, the rest still finishes after a reasonable time,
> > just a few seconds more than the pre-iomap baseline.  I also see the
> > reserve block depleted message in this case.
> > 
> > Digging into the reserve block depleted message - it seems we have
> > too many parallel iomap_allocate transactions going on.  I suspect
> > this might be because the writeback code will not finish a writeback
> > context if we have multiple blocks inside a page, which can
> > happen easily for this 1k ENOSPC setup.  I've not had time to fully
> > check if this is what really happens, but I did a quick hack (see below)
> > to only allocate 1k at a time in iomap_begin, and with that generic/224
> > finishes without the warning spew.  Of course this isn't a real fix,
> > and I need to fully understand what's going on in writeback due to
> > different allocation / dirtying patterns from the iomap change.
> 
> Any progress here, Christoph? The current test run has been running
> generic/224 on the 1GB mem test Vm for almost 6 hours now, and it's
> still discarding pages. This doesn't always happen - sometimes it
> takes the normal amount of time to run, but every so often it falls
> into this "discard every page" loop and it takes hours to
> complete...

.... and I've now got a 16p/16GB RAM VM stuck in this loop in
generic/224, so it's not limited to low memory machines....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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