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

On Fri, 1 Aug 2025 at 10:37, Kent Overstreet <[email protected]> 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.