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Date: Wed, 3 Jan 2024 17:33:56 -0800
From: "Darrick J. Wong" <djwong@kernel.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH 3/3] xfile: implement write caching
Message-ID: <20240104013356.GP361584@frogsfrogsfrogs>
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In-Reply-To: <ZZUfVVJSkvDRHZsp@infradead.org>

On Wed, Jan 03, 2024 at 12:48:21AM -0800, Christoph Hellwig wrote:
> 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?

Sort of both.  For xfbtrees (or anything mapping a xfs_buftarg atop an
xfile) we can't use the cheap(er) kmap_local_page and have to use kmap,
which ... is expensive, isn't it?

Granted, forbidding highmem like you posted today makes all of this
/much/ simpler so I think it's probably worth the increased chances of
ENOMEM on i386.

That said, why not avoid a trip through shmem_get_folio_gfp aka
filemap_get_entry if we can?  Even if we can use page_address directly
now?

--D