Re: [PATCH] mm/readahead: Fix large folio support in async readahead

[Yafang Shao <laoar.shao@gmail.com>]

On Thu, Nov 7, 2024 at 5:03 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed,  6 Nov 2024 17:21:14 +0800 Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > When large folio support is enabled and read_ahead_kb is set to a smaller
> > value, ra->size (4MB) may exceed the maximum allowed size (e.g., 128KB). To
> > address this, we need to add a conditional check for such cases. However,
> > this alone is insufficient, as users might set read_ahead_kb to a larger,
> > non-hugepage-aligned value (e.g., 4MB + 128KB). In these instances, it is
> > essential to explicitly align ra->size with the hugepage size.
>
> How much performance improvement is this likely to offer our users?

The performance boost comes from enabling the use of hugepages
directly. Previously, users were unable to leverage large folios as
expected. With this change, however, large folios are now usable as
intended.

This improvement addresses a critical need in services like AI
inference, which benefit substantially from hugetlbfs. However, using
hugetlbfs effectively within containerized environments can be
challenging. To overcome this limitation, we explored large folios as
a more flexible and production-friendly alternative.

> IOW, should we consider backporting it?

We should consider backporting this change. We've already backported
it to our local 6.1.y kernel, where it's performing well.
The Fixes tag should ensure it will be included in the stable kernel, right?

>
> (I bet anyone who comes across this will say "oh goody" and backport it
> anyway, so why not do this for them?)
>

--
Regards

Yafang