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Date:   Thu, 10 Jun 2021 18:20:46 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Chris Mason <clm@...com>, 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 Thu, Jun 10, 2021 at 04:50:09PM +0200, Christophe Leroy wrote:
> 
> 
> Le 10/06/2021 à 15:54, Chris Mason a écrit :
> > 
> >> 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.
> > 
> 
> But if 256K is problematic in other ways, is it worth bumping BTRFS_MAX_COMPRESSED to 256K ?
> 
> David, in below mail, said that 256K support would require deaper changes. So disabling BTRFS 
> support seems the easiest solution for the time being, at least for Stable (I forgot the Fixes: tag 
> and the CC: to stable).
> 
> On powerpc, 256k pages is a corner case, it requires customised binutils, so I don't think disabling 
> BTRFS is a issue there. For hexagon I don't know.

That it blew up due to the max compressed size is a coincidence. We
could have explicit BUILD_BUG_ONs for page size or other constraints
derived from the page size like INLINE_EXTENT_BUFFER_PAGES.

And there's no such thing like "just bump BTRFS_MAX_COMPRESSED to 256K".
The constant is part of on-disk format for lzo and otherwise changing it
would impact performance so this would need proper evaluation.

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