"Invalid Argument" when attempting to remount /system as RW

While I was trying to install the "GSAM Root Companion" app and having it fail, and I discovered that the problem was that /system is still mounted RO. No problem, says I, and fired up my terminal app, pop into a root prompt with su, and issue the old invocation:

Code:

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#mount -o remount,rw /system
 mount: Invalid argument

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I did some googling, and tried various different ways of laying out the command that other people have had luck with. I didn't have any

Code:

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# /system/bin/mount -o remount,rw /system
# /system/bin/mount -o remount,rw /system /system
# /system/bin/mount -o rw,remount,rw /system
# /system/bin/mount -o remount,rw /dev/block/platform/mtk-msdc.0/by-name/system
# /system/bin/mount -o remount,rw /dev/block/platform/mtk-msdc.0/by-name/system /system
..etc..

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My device is properly rooted (typing 'su' in the terminal prompt gives me my '#' prompt), selinux is set to 'permissive' (per the output of 'getenforce'), but all of them just keep giving me "Invalid Argument". At this point, the only thing I can think of is that it's actually blocked in the kernel, but as I'm hardly an expert, I figured I should put it up here before I gave up for good.

Anyone have a line on this?

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