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

Le 10/06/2021 à 15:54, Chris Mason a écrit :

quoted

On Jun 10, 2021, at 1:23 AM, Christophe Leroy [off-list ref] 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:

quoted

Le 09/06/2021 à 15:55, kernel test robot a écrit :

quoted

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).