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

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

On Mon, Aug 26, 2024 at 11:40:57PM GMT, Kent Overstreet wrote:
> 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).

Also, the size of the missing data is not a power of two - it's not a
single folio.