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Subject: Re: [PATCH] btrfs: Disable BTRFS on platforms having 256K pages
To:     Chris Mason <clm@fb.com>
Cc:     Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>,
        "linux-hexagon@vger.kernel.org" <linux-hexagon@vger.kernel.org>
References: <a16c31f3caf448dda5d9315e056585b6fafc22c5.1623302442.git.christophe.leroy@csgroup.eu>
 <185278AF-1D87-432D-87E9-C86B3223113E@fb.com>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <cdadf66e-0a6e-4efe-0326-7236c43b2735@csgroup.eu>
Date:   Thu, 10 Jun 2021 16:50:09 +0200
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Le 10/06/2021 à 15:54, Chris Mason a écrit :
> 
>> On Jun 10, 2021, at 1:23 AM, Christophe Leroy <christophe.leroy@csgroup.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.


https://lkml.org/lkml/2021/6/9/978

Le 09/06/2021 à 17:22, David Sterba a écrit :
 > On Wed, Jun 09, 2021 at 04:01:20PM +0200, Christophe Leroy wrote:
 >> Le 09/06/2021 à 15:55, kernel test robot a écrit :
 >>> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
 >>> head:   368094df48e680fa51cedb68537408cfa64b788e
 >>> commit: 4eeef098b43242ed145c83fba9989d586d707589 powerpc/44x: Remove STDBINUTILS kconfig option
 >>> date:   4 months ago
 >>> config: powerpc-randconfig-r012-20210609 (attached as .config)
 >>> compiler: powerpc-linux-gcc (GCC) 9.3.0
 >>
 >> That's a BTRFS issue, and not directly linked to the above mentioned commit. Before that commit the
 >> problem was already present.
 >>
 >> Problem is that with 256k PAGE_SIZE, following BUILD_BUG() pops up:
 >>
 >> BUILD_BUG_ON((BTRFS_MAX_COMPRESSED % PAGE_SIZE) != 0)
 >
 > A 256K page is a problem for btrfs, until now I was not even aware
 > there's an architecture supporting that so. That the build fails is
 > probably best thing. Maximum metadata nodesize supported is 64K and
 > having that on a 256K page would need deeper changes, no top of the
 > currently developed subpage changes (that do 4K blocks on 64K pages).
 >