Re: bcachefs dropped writes with lockless buffered io path, COMPACTION/MIGRATION=y

[Kent Overstreet <kent.overstreet@linux.dev>]

On Tue, Aug 27, 2024 at 04:36:33AM GMT, Matthew Wilcox wrote:
> On Mon, Aug 26, 2024 at 11:29:52PM -0400, Kent Overstreet wrote:
> > We had a report of corruption on nixos, on tests that build a system
> > image, it bisected to the patch that enabled buffered writes without
> > taking the inode lock:
> > 
> > https://evilpiepirate.org/git/bcachefs.git/commit/?id=7e64c86cdc6c
> > 
> > It appears that dirty folios are being dropped somehow; corrupt files,
> > when checked against good copies, have ranges of 0s that are 4k aligned
> > (modulo 2k, likely a misaligned partition).
> > 
> > Interestingly, it only triggers for QEMU - the test fails pretty
> > consistently and we have a lot of nixos users, we'd notice (via nix
> > store verifies) if the corruption was more widespread. We believe it
> > only triggers with QEMU's snapshots mode (but don't quote me on that).
> 
> Just to be crystal clear here, the corruption happens while running
> bcachefs in the qemu guest, and it doesn't matter what the host
> filesystem is?
> 
> Or did I misunderstand, and it occurs while running anything inside qemu
> on top of a bcachefs host?

The host is running bcachefs, backing qemu's disk image.

(And I'm using nested virtualization for bisecting, it's been a lot to
keep straight).