msync(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)?

Thanks,
Erez.
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