Message-ID: <dkrr6obybksiq6p2purht5tvhktau2l2syemnvmtrpxlwfxmir@vca5yahzvniu>
Date: Sat, 2 Aug 2025 14:49:05 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Coly Li <colyli@...nel.org>
Cc: Zhou Jifeng <zhoujifeng@...inos.com.cn>, 
	linux-bcache <linux-bcache@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bcache: enhancing the security of dirty data writeback

On Sun, Aug 03, 2025 at 01:29:55AM +0800, Coly Li wrote:
> On Fri, Aug 01, 2025 at 02:10:12PM +0800, Zhou Jifeng wrote:
> > On Fri, 1 Aug 2025 at 11:42, Kent Overstreet <kent.overstreet@...ux.dev> wrote:
> > >
> > > 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@...ux.dev> 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?
> > >
> > 
> > I checked and found that there was no code for sending a flush request
> > to the backend device before the execution log was written. Additionally,
> > in the callback function after the dirty data was written back, when it
> > updated the bkey, it did not insert this update into the log.
> >
> 
> It doesn't have to. If the previous dirty version of the key is on cache device
> already, and power off happens, even the clean version of the key is gone, the
> dirty version and its data are all valid. If the LBA range of this key is
> allocated to a new key, a GC must have alrady happened, and the dirty key is
> invalid due to bucket generation increased. So don't worry, the clean key is
> unncessary to go into journal in the writeback scenario.
>  
> IMHO, you may try to flush backing device in a kworker before calling
> set_gc_sectors() in bch_gc_thread(). The disk cache can be flushed in time
> before the still-dirty-on-disk keys are invalidated by increase bucket key
> gen. And also flushing backing device after searched_full_index becomes
> true in the writeback thread main loop (as you did now).
> 
> Flushing backing device after read_dirty() returns is too heavy, event with
> the flush hint keys. And the flushing operations are unnecessary before the
> keys are reclaimed by garbage collaction.

No, a flush is necessary.

And it's not after read_dirty, the flush just needs to happen before
bcache does its journal commit.

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