On Tue, Jun 3, 2025 at 7:35 PM Christoph Hellwig <hch@xxxxxx> wrote: > > On Tue, Jun 03, 2025 at 04:04:45PM +0200, Christoph Hellwig wrote: > > On Tue, Jun 03, 2025 at 07:22:18PM +0530, Anuj gupta wrote: > > > > A mount option is about the worst possible interface for behavior > > > > that depends on file system implementation and possibly hardware > > > > chacteristics. This needs to be set by the file systems, possibly > > > > using generic helpers using hardware information. > > > > > > Right, that makes sense. Instead of using a mount option, we can > > > introduce generic helpers to initialize multiple writeback contexts > > > based on underlying hardware characteristics — e.g., number of CPUs or > > > NUMA topology. Filesystems like XFS and EXT4 can then call these helpers > > > during mount to opt into parallel writeback in a controlled way. > > > > Yes. A mount option might still be useful to override this default, > > but it should not be needed for the normal use case. > > .. actually a sysfs file on the bdi is probably the better interface > for the override than a mount option. Hi Christoph, 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.
