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

[Andres Freund]

Hi,

On 2026-03-26 21:42:50 -0400, Zi Yan wrote:
> Replace it with a check on the max folio order of the file's address space
> mapping, making sure PMD_ORDER is supported.

Question from a user space developer (who learned of this via LWN [1]):

Right now MADV_COlLAPSE on file mappings appears to only only work with
CONFIG_READ_ONLY_THP_FOR_FS.

That's pretty sad, because I see quite consistent gains (~15%) from doing
madvise(MADV_COLLAPSE) on e.g. postgres' .text - but because
CONFIG_READ_ONLY_THP_FOR_FS is not enabled default (or at least not commonly,
I think?) in distro kernels, it never quite seemed worth the fiddliness.


Do I understand correctly that by replacing the check here (and the
predecessor commits), madvise(MADV_COLLAPSE) on file mappings might actually
start working in more situations on typical distro kernels?

Greetings,

Andres

[1]  https://lwn.net/SubscriberLink/1066582/b92de1d7595a01e4/