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.le...@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.
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.
