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Subjectmsync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland
According to vfs.txt, ->writepage() may return AOP_WRITEPAGE_ACTIVATE back
to the VFS/VM. Indeed some filesystems such as tmpfs can return
AOP_WRITEPAGE_ACTIVATE; and stackable file systems (e.g., Unionfs) also
return AOP_WRITEPAGE_ACTIVATE if the lower f/s returned it.

Anyway, some Ubuntu users of Unionfs reported that msync(2) sometimes
returns AOP_WRITEPAGE_ACTIVATE (decimal 524288) back to userland.
Therefore, some user programs fail, esp. if they're written such as this:

err = msync(...);
if (err != 0)
// fail

They temporarily fixed the specific program in question (apt-get) to check

if (err < 0)
// fail

Is this a bug indeed, or are user programs supposed to handle
AOP_WRITEPAGE_ACTIVATE (I hope not the latter). If it's a kernel bug, what
should the kernel return: a zero, or an -errno (and which one)?

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