Re: mm: CMA reservations require 32MiB alignment in 16KiB page size kernels instead of 8MiB in 4KiB page size kernel.

["Zi Yan" <ziy@nvidia.com>]

On Sun Jan 19, 2025 at 6:55 PM EST, Barry Song wrote:
<snip>
>> >>>
>> >>>
>> >>> However, with this workaround, we can't use transparent huge pages.
>> >>>
>> >>> Is the CMA_MIN_ALIGNMENT_BYTES requirement alignment only to support huge pages?
>> No. CMA_MIN_ALIGNMENT_BYTES is limited by CMA_MIN_ALIGNMENT_PAGES, which
>> is equal to pageblock size. Enabling THP just bumps the pageblock size.
>
> Currently, THP might be mTHP, which can have a significantly smaller
> size than 32MB. For
> example, on arm64 systems with a 16KiB page size, a 2MB CONT-PTE mTHP
> is possible.
> Additionally, mTHP relies on the CONFIG_TRANSPARENT_HUGEPAGE configuration.
>
> I wonder if it's possible to enable CONFIG_TRANSPARENT_HUGEPAGE
> without necessarily
> using 32MiB THP. If we use other sizes, such as 64KiB, perhaps a large
> pageblock size wouldn't
> be necessary?

I think this should work by reducing MAX_PAGE_ORDER like Juan did for
the experiment. But MAX_PAGE_ORDER is a macro right now, Kconfig needs
to be changed and kernel needs to be recompiled. Not sure if it is OK
for Juan's use case.

-- 
Best Regards,
Yan, Zi