linux-kernel - Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

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Message-Id: <20250613110415.b898c62c7c09ff6e8b0149e9@linux-foundation.org>
Date: Fri, 13 Jun 2025 11:04:15 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Robert Pang <robertpang@...gle.com>
Cc: Kuan-Wei Chiu <visitorckw@...il.com>, corbet@....net, colyli@...nel.org,
 kent.overstreet@...ux.dev, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-bcache@...r.kernel.org,
 jserv@...s.ncku.edu.tw, stable@...r.kernel.org
Subject: Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

On Fri, 13 Jun 2025 23:26:33 +0900 Robert Pang <robertpang@...gle.com> wrote:

> Hi Andrew
> 
> Bcache is designed to boost the I/O performance of slower storage
> (HDDs, network-attached storage) by leveraging fast SSDs as a block
> cache. This functionality is critical in significantly reducing I/O
> latency. Therefore, any notable increase in bcache's latency severely
> diminishes its value. For instance, our tests show a P100 (max)
> latency spike from 600 ms to 2.4 seconds every 5 minutes due to this
> regression. In real-world environments, this  increase will cause
> frequent timeouts and stalls in end-user applications that rely on
> bcache's latency improvements, highlighting the urgent need to address
> this issue.

Great, thanks.  Let's please incorporate this into the v2 changelogging.

> > > Also, if we are to address this regression in -stable kernels then
> > > reverting 866898efbb25 is an obvious way - it is far far safer.  So
> > > please also tell us why the proposed patchset is a better way for us to
> > > go.
> > >
> > I agree that reverting 866898efbb25 is a much safer and smaller change
> > for backporting. In fact, I previously raised the discussion of whether
> > we should revert the commit or instead introduce an equality-aware API
> > and use it. The bcache maintainer preferred the latter, and I also
> > believe that it is a more forward-looking approach. Given that bcache
> > has run into this issue, it's likely that other users with similar use
> > cases may encounter it as well. We wouldn't want those users to
> > continue relying on the current default heapify behavior. So, although
> > reverting may be more suitable for stable in isolation, adding an
> > equality-aware API could better serve a broader set of use cases going
> > forward.

"much safer and smaller" is very desirable for backporting, please. 
After all, 866898efbb25 didn't really fix anything and reverting that
takes us back to a known-to-work implementation.

I of course have no problem making the changes in this patchset for
"going forward"!

So if agreeable, please prepare a patch which reverts 866898efbb25. 
Robert's words above are a great basis for that patch's description.