Re: [PATCH 00/13] Parallelizing filesystem writeback

Christoph Hellwig
2025-06-09T04:01:02Z

On Fri, Jun 06, 2025 at 10:34:42AM +0530, Kundan Kumar wrote:
> Thanks for the suggestion — I agree the default should come from a
> filesystem-level helper, not a mount option.
>
> I looked into the sysfs override idea, but one challenge is that
> nr_wb_ctx must be finalized before any writes occur. That leaves only
> a narrow window — after the bdi is registered but before any inodes
> are dirtied — where changing it is safe.
>
> This makes the sysfs knob a bit fragile unless we tightly guard it
> (e.g., mark it read-only after init). A mount option, even just as an
> override, feels simpler and more predictable, since it’s set before
> the FS becomes active.

The mount option has a few issues:

- the common VFS code only support flags, not value options, so you'd
have to wire this up in every file system
- some file system might not want to allow changing it
- changing it at runtime is actuallyt quite useful

So you'll need to quiesce writeback or maybe even do a full fs freeze
when changing it a runtime, but that seems ok for a change this invasive.