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Date:   Fri, 9 Jul 2021 07:46:17 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Neal Gompa <ngompa13@gmail.com>, David Sterba <dsterba@suse.com>,
        Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH 0/6] Remove highmem allocations, kmap/kunmap
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On Fri, Jul 09, 2021 at 07:53:39AM +0800, Qu Wenruo wrote:
> Sorry, I can't see the reason why it would cause performance drop or
> higher memory usage.
> 
> The point of HIGHMEM is to work on archs where system can only access
> memory below 4G reliably, any memory above 4G must be manually mapped
> into the 4G range before access.
> 
> AFAIK it's only x86 using PAE needs this, and none of the ARM SoC uses
> such feature.

Arm calls it LPAE, but otherwise it is the same.