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...
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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