Re: [PATCH RFC] Btrfs: only subtract from len_to_oe_boundary when it is tracking an extent

[Chris Mason <clm@meta.com>]

On 7/31/23 3:00 AM, Christoph Hellwig wrote:
> On Sun, Jul 30, 2023 at 12:02:26PM -0700, Chris Mason wrote:
>> [ This is an RFC because Christoph switched us to almost always set
>> len_to_oe_boundary in a patch in for-next  I think we still need this
>> commit for strange corners, but it's already pretty hard to hit reliably
>> so I wanted to toss it out for discussion. We should consider either
>> Christoph's "btrfs: limit write bios to a single ordered extent" or this
>> commit for 6.4 stable as well ]
> 
> I'm torn.  On the one hand "btrfs: limit write bios to a single ordered
> extent" is a pretty significant behavior change, on the other hand
> stable-only patches with totally different behavior are always a bit
> strange.

When are we creating bios without bio_ctrl->wbc set?  I think reads will
do this?

> 
> Note that with my entire pending queue, len_to_oe_boundary goes away
> entirely, but with the current speed of patch application it might take
> another 6 to 8 month to get there.
> 
>> This is hard to trigger because bio_add_page() isn't going to make a bio
>> of U32_MAX size unless you give it a perfect set of pages and fully
>> contiguous extents on disk.  We can hit it pretty reliably while making
>> large swapfiles during provisioning because the machine is freshly
>> booted, mostly idle, and the disk is freshly formatted.
> 
> It might be useful to create and xfstests for that, even if it only
> hits on a freshly booted machine, although we'll need some reordering
> in the xfstests sequence to make sure it gets run early..
> 

The test below works for me on 6.4 but not Linus git.  In theory the
swapfile component is entirely unrelated, but I haven't been able to
trigger without it.

It usually takes about 5 or 6 loops on a virtual machine with 32 cpus,
32GB of ram, and an ssd that can push 800MB/s streaming writes on
/dev/vdb.  I'm happy to add an xfstest, but the failure rate is low
enough that I'm not sure it'll catch anything.

As part of testing 6.4, we booted 100 machines and put them through
provisioning.  17 made it out the other end, and the rest hit a message
similar to this one.  Some had a one byte IO, some had a 4095 byte IO,
it was just whichever bio of our split page happened to finish first:

BTRFS error (device nvme0n1p2): partial page write in btrfs with offset
4095 and length 1

followed by a pretty unsurprising:

kernel BUG at mm/filemap.c:1622!

        if (!__folio_end_writeback(folio))
                BUG();

#!/bin/bash

SUBVOL=/btrfs/swapvol
SWAPFILE=$SUBVOL/swapfile
SZMB=8192

mkfs.btrfs -f /dev/vdb
mount /dev/vdb /btrfs

btrfs subvol create $SUBVOL
sync
chattr +C $SUBVOL

while(true) ; do
    swapoff -a
    dd if=/dev/zero of=$SWAPFILE bs=1M count=$SZMB
    mkswap $SWAPFILE
    swapon $SWAPFILE
done