Re: [PATCH v1 05/10] mm/huge_memory: remove READ_ONLY_THP_FOR_FS from file_thp_enabled()

Matthew Wilcox
2026-03-30T16:19:59Z

On Tue, Mar 31, 2026 at 12:09:42AM +0800, WANG Rui wrote:
> Given the diversity of filesystems in use, each one requires dedicated
> engineering effort to implement and validate large folio support, and
> that assumes both sufficient resources and prioritization on the
> filesystem side. Even after support lands, coverage across different
> base page sizes and configurations may take additional time to mature.
>
> What I am really concerned about is the transition period: if filesystem
> support is not yet broadly ready, while we have already removed the
> fallback path, we may end up in a situation where PMD-sized mappings
> become effectively unavailable on many systems for some time.
>
> This is not about the long-term direction, but about the timing and
> practical readiness.

If we leave this fallback in place, we'll never get filesystems to move
forward. It's time to rip off this bandaid; they've got eight months
before the next stable kernel. I've talked to them about it for years

LSFMM 2022: https://lwn.net/Articles/893512/
LSFMM 2023: https://lwn.net/Articles/931794/
LSFMM 2024: https://lwn.net/Articles/973565/
LSFMM 2025: https://lwn.net/Articles/1015320/

(and earlier, but I think I've made my point)