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

[Chris Mason]

> On Jun 10, 2021, at 12:20 PM, David Sterba <dsterba@xxxxxxx> wrote:
> 
> 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@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.
> 
> 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.
> 

Right, the constraint is bigger and more complex than BTRFS_MAX_COMPRESSED.

> 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.

Sorry, how is it baked into LZO?  It definitely will have performance implications, I agree there.

-chris