Linux's contemporary filesystem mount API went without documentation for six years — latest man-page package finally adds content for 2019 code

Core system code doesn't move at quite the same pace as the latest vibe coders, and thankfully so, as it needs to be stable, efficient, and battle-tested. However, sometimes delays can get a little ridiculous, as was the case with the documentation for Linux's contemporary filesystem API. As noted by Phoronix, it was over a six-year wait for the info to appear in the standard man-page (manual page) documentation.

The tale goes like this: in 2019, Linux got the minty-fresh new filesystem mounting API, with the main "fsconfig," "fsmount," and "fsopen" calls replacing the old, monolithic "mount." This made it a lot easier for developers to use the new functionality, enjoying cleaner code, better error handling, and the ability to output proper error messages, rather than the cryptic "mount failed."

Developer David Howells wrote drafts of the man-pages in 2020, but those were never merged to the man-pages package, as the maintainers didn't want to include potentially incomplete or imprecise drafts. Howells apparently couldn't complete the work, and nobody else stepped up.

During the following years, developers wanting to use the new system would have to dig through an ugly file in the kernel source code, find discussions in the Linux kernel mailing list, or enjoy reading through dozens of patch notes. At the best of times, external websites actually contained a how-to of sorts, given the situation.

Another reason for nobody stepping up to write new man-pages was that the standardized format is Groff, which is now 35 years old and is so human-unreadable that it might as well have been written by Cthulhu.

Documentation master Christian Brauner got fed up in 2024, stating "years of writing Groff have made me tired," and he created his own repository with Markdown version of the pages, finally giving developers the one spot for finding the necessary information. Finally, as of October 2025, the official man-pages package now contains these pages.

It's somewhat plausible that the almost-missing documentation might have contributed to the long delays of multiple filesystem handlers using the new API. Good ol' Ext4 only used it in 2022, CIFS (SMB) did it in 2023, Btrfs was late to the party in 2024, and F2FS's (flash filesystem) implementation is still in progress. The new man-pages have yet to find their way into common distributions, so only rolling-release types will have them.

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