Re: Direct I/O

Hi,

On 2023-04-10 18:55:26 -0700, Andres Freund wrote:
> On 2023-04-10 19:27:27 +1200, Thomas Munro wrote:
> > On Mon, Apr 10, 2023 at 2:57 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > Have you tried to write a reproducer for this that doesn't involve postgres?
> >
> > I tried a bit. I'll try harder soon.
> >
> > > ... What kernel version did you repro
> > > this on Thomas?
> >
> > Debian's 6.0.10-2 kernel (Debian 12 on a random laptop). Here's how I
> > set up a test btrfs in case someone else wants a head start:
> >
> > truncate -s2G 2GB.img
> > sudo losetup --show --find 2GB.img
> > sudo mkfs -t btrfs /dev/loop0 # the device name shown by losetup
> > sudo mkdir /mnt/tmp
> > sudo mount /dev/loop0 /mnt/tmp
> > sudo chown $(whoami) /mnt/tmp
> >
> > cd /mnt/tmp
> > /path/to/initdb -D pgdata
> > ... (see instructions further up for postgres command line + queries to run)
>
> I initially failed to repro the issue with these instructions. Turns out that
> the problem does not happen if huge pages are in used - I'd configured huge
> pages, so the default huge_pages=try succeeded. As soon as I disable
> huge_pages explicitly, it reproduces.
>
> Another interesting bit is that if checksums are enabled, I also can't
> reproduce the issue. Together with the huge_page issue, it does suggest that
> this is somehow related to page faults. Which fits with the thread Andrea
> referenced...

The last iteration of the fix that I could find is:
https://lore.kernel.org/linux-btrfs/20230328051957(dot)1161316-1-hch(at)lst(dot)de/T/#m1afdc3fe77e10a97302e7d80fed3efeaa297f0f7

And the fix has been merged into
https://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux.git/log/?h=for-next

I think that means it'll have to wait for 6.4 development to open (in a few
weeks), and then will be merged into the stable branches from there.

Greetings,

Andres Freund