Re: [PATCH 3/3] xfile: implement write caching

Christoph Hellwig
2024-01-03T08:48:21Z

On Sun, Dec 31, 2023 at 12:40:40PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> Mapping a page into the kernel's address space is expensive.

What do you mean with mapping into the kernel's address space?

Mormally that owuld point to the kmap* family of helpers, but those
are complete no-ops on the typical xfs setups without highmem. But
even with highmem at least kmap_local_page isn't too expensive.

My xfile diet patches actually change the xfile mapping to never
allocate highmem, which simplifies things a bit (and fixes a bug
in the xfs_buf use that just uses page_address instead of a kmap).

So I suspect this is something else and more about looking up pages?