linux-kernel - Re: [PATCH] btrfs: Disable BTRFS on platforms having 256K pages

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Date:   Thu, 10 Jun 2021 13:54:28 +0000
From:   Chris Mason <clm@...com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
CC:     Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        "linux-hexagon@...r.kernel.org" <linux-hexagon@...r.kernel.org>
Subject: Re: [PATCH] btrfs: Disable BTRFS on platforms having 256K pages


> On Jun 10, 2021, at 1:23 AM, Christophe Leroy <christophe.leroy@...roup.eu> wrote:
> 
> With a config having PAGE_SIZE set to 256K, BTRFS build fails
> with the following message
> 
> include/linux/compiler_types.h:326:38: error: call to '__compiletime_assert_791' declared with attribute error: BUILD_BUG_ON failed: (BTRFS_MAX_COMPRESSED % PAGE_SIZE) != 0
> 
> BTRFS_MAX_COMPRESSED being 128K, BTRFS cannot support platforms with
> 256K pages at the time being.
> 
> There are two platforms that can select 256K pages:
> - hexagon
> - powerpc
> 
> Disable BTRFS when 256K page size is selected.
> 

We’ll have other subpage blocksize concerns with 256K pages, but this BTRFS_MAX_COMPRESSED #define is arbitrary.  It’s just trying to have an upper bound on the amount of memory we’ll need to uncompress a single page’s worth of random reads.

We could change it to max(PAGE_SIZE, 128K) or just bump to 256K.

-chris