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

[WANG Rui <r@hev.cc>]

Hi Zi,

>> Now without READ_ONLY_THP_FOR_FS you're going to:
>>
>>                        |    PF     | MADV_COLLAPSE | khugepaged |
>> 		       |-----------|---------------|------------|
>>  large folio fs        |     ✓     |       x       |      x     |
>>  large folio + r/o     |     ✓     |       ✓       |      ✓     |
>>
>> And intentionally leaving behind the 'not large folio fs, r/o' case because
>> those file systems need to implement large folio support.
>>
>> I guess we'll regress those users but we don't care?
>
> Yes. This also motivates FSes without large folio support to add large folio
> support instead of relying on READ_ONLY_THP_FOR_FS hack.

Interesting, thanks for making this feature unconditional.

From my experiments, this is going to be a performance regression.

Before this patch, even when the filesystem (e.g. btrfs without experimental)
didn't support large folios, READ_ONLY_THP_FOR_FS still allowed read-only
file-backed code segments to be collapsed into huge page mappings via khugepaged.

After this patch, FilePmdMapped will always be 0 unless the filesystem supports
large folios up to PMD order, and it doesn't look like that support will arrive
anytime soon [1].

Is there a reason we can't keep this hack while continuing to push filesystems
toward proper large folio support?

I'm currently working on making the ELF loader more THP-friendly by adjusting
the virtual address alignment of read-only code segments [2]. The data shows a
noticeable drop in iTLB misses, especially for programs whose text size is just
slightly larger than PMD_SIZE. That size profile is actually quite common for
real-world binaries when using 2M huge pages. This optimization relies on
READ_ONLY_THP_FOR_FS. If the availability of huge page mappings for code segments
ends up depending on filesystem support, it will be much harder to take advantage
of this in practice. [3]

[1] https://lore.kernel.org/linux-fsdevel/ab2IIwKzmK9qwIlZ@casper.infradead.org/
[2] https://lore.kernel.org/linux-fsdevel/20260313005211.882831-1-r@hev.cc/
[3] https://lore.kernel.org/linux-fsdevel/20260320160519.80962-1-r@hev.cc/

Thanks,
Rui