On Fri, Jun 06, 2025 at 10:34:42AM +0530, Kundan Kumar wrote:
> Thanks for the suggestion β€” 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 β€” after the bdi is registered but before any inodes
> are dirtied β€” 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.


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