Re: [PATCH] bcache: enhancing the security of dirty data writeback

[Kent Overstreet]

On Fri, Aug 01, 2025 at 11:30:43AM +0800, Zhou Jifeng wrote:
> On Fri, 1 Aug 2025 at 10:37, Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote:
> >
> > On Fri, Aug 01, 2025 at 10:27:21AM +0800, Zhou Jifeng wrote:
> > > In the writeback mode, the current bcache code uses the
> > > REQ_OP_WRITE operation to handle dirty data, and clears the bkey
> > > dirty flag in the btree during the bio completion callback. I think
> > > there might be a potential risk: if in the event of an unexpected
> > > power outage, the data in the HDD hardware cache may not have
> > > had time to be persisted, then the data in the HDD hardware cache
> > > that is pending processing may be lost. Since at this time the bkey
> > > dirty flag in the btree has been cleared, the data status recorded
> > > by the bkey does not match the actual situation of the SSD and
> > > HDD.
> > > Am I understanding this correctly?
> >
> > For what you're describing, we need to make sure the backing device is
> > flushed when we're flushing the journal.
> >
> > It's possible that this isn't handled correctly in bcache; bcachefs
> > does, and I wrote that code after bcache - but the bcache version would
> > look quite different.
> >
> > You've read that code more recently than I have - have you checked for
> > that?
> 
> In the `write_dirty_finish` function, there is an attempt to update the
> `bkey` status, but I did not observe any logging writing process. In the
> core function `journal_write_unlocked` of bcache for writing logs, I
> also couldn't find the code logic for sending a FLUSH command to the
> backend HDD.

The right place for it would be in the journal code: before doing a
journal write, issue flushes to the backing devices.

Can you check for that?