[BUG] f2fs: mounting with kernel >= 4.5.0 fails

[Matthias Prager <linux@matthiasprager.de>]

Hello,

since kernel 4.5.0 I'm unable to mount f2fs filesystems with costom
section sizes.

I use 1 GiB section sizes (for smr-hdds). A filesystem created with the
command:

> mkfs.f2fs -l test2 -o 0 -a 0 -s 9 /dev/sdj

fails to mount in kernel 4.5.0 with the following dmesg output (3TB hdd):
> [  263.898973] F2FS-fs (sdj): Wrong MAIN_AREA boundary, start(1502208) end(732566528) blocks(731063808)
> [  263.898977] F2FS-fs (sdj): Can't find valid F2FS filesystem in 1th superblock
> [  263.899142] F2FS-fs (sdj): Wrong MAIN_AREA boundary, start(1502208) end(732566528) blocks(731063808)
> [  263.899144] F2FS-fs (sdj): Can't find valid F2FS filesystem in 2th superblock
> [  263.899148] F2FS-fs (sdj): Wrong MAIN_AREA boundary, start(1502208) end(732566528) blocks(731063808)
> [  263.899150] F2FS-fs (sdj): Can't find valid F2FS filesystem in 1th superblock
> [  263.899152] F2FS-fs (sdj): Wrong MAIN_AREA boundary, start(1502208) end(732566528) blocks(731063808)
> [  263.899154] F2FS-fs (sdj): Can't find valid F2FS filesystem in 2th superblock

It makes no difference whether the fs is created using kernel 4.5.0 or
earlier kernels (f2fs-tools are version 1.6.0 from gentoo stable, I
tested kernels 4.1.19, 4.3.3 and 4.4.6).

Using the command:

> mkfs.f2fs -l test2 -o 0 -a 0 /dev/sdj

instead (without the '-s 9' parameter), the fs is mountable with kernel
4.5.0 without any issues.

(The other parameters make no difference - i.e. 'mkfs.f2fs /dev/sdj' is
fine for kernel 4.5.0, 'mkfs.f2fs -s 9 /dev/sdj' is not. Different
custom sections sizes don't seem to make a difference.)

Is there a real problem with the fs or are the newly introduced checks
in kernel 4.5 simply buggy?

---
Matthias

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