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	Wed, 03 Jan 2024 08:48:21 +0000
Date: Wed, 3 Jan 2024 00:48:21 -0800
From: Christoph Hellwig <hch@infradead.org>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH 3/3] xfile: implement write caching
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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?