Re: iomap infrastructure and multipage writes V5

[Dave Chinner <david@fromorbit.com>]

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.

The reserve block pool depleted message is normal for me in this
test.  We're throwing a thousand concurrent processes at the
filesystem at ENOSPC, and so the metadata reservation for the
delayed allocation totals quite a lot. We only reserve 8192 blocks
for the reserve pool, so a delalloc reservation for one page on each
file (4 blocks per page, which means a couple of blocks for the
metadata reservation via the indlen calculation) is going to consume
the reserve pool quite quickly if the up front reservation
overshoots the XFS_ALLOC_SET_ASIDE() ENOSPC threshold.

> 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.

Right - this test has regularly triggered that warning on this
particular test setup for me - it's not something new to the iomap
patchset.

> 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.

Which tends to indicate that the multi-block allocation has a larger
indlen reservation, and that's what is causing the code to hit
whatever edge-case that is leading to it not recovering. However,
I'm still wondering how we are not throwing ENOSPC back to userspace
at XFS_ALLOC_SET_ASIDE limits.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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