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

[Christophe Leroy]

Le 10/06/2021 à 15:54, Chris Mason a écrit :
> > On Jun 10, 2021, at 1:23 AM, Christophe Leroy <christophe.leroy@xxxxxxxxxx> 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).
>