What to do if attacked by a mountain lion?

Last night I went backpacking with a friend. We hiked about 10 miles into the wilderness before finding a nice spot to pitch tents right around dusk. Made a fire, ate dinner, all the good stuff. I walked to the creek nearby (75 ft) to get some water. And as I lean down to fill up a bag my flashlight points upwards. Out of the corner of my eye I see something bright. Centered my flashlight on it. Dang mountain lion across the creek maybe 50 feet away. I calmly decided not to get water and walked back to my friend pretty slowly where our tents were. Figured better to avoid a mountain lion attack w/ him present, and I have a little trick for hostile cats that I'd left there.

He'd already doused our fire and our hopes of relighting that were dim. So we sat there shining flashlights into the surrounding bush like sentries for awhile. We saw eyes a couple times but the vegetation was pretty heavy around us. Eventually we stopped seeing eyes. And we were planning a simple in-n-out. Plenty of batteries for that, but not enough to run flashlights all night long.

We eventually decided to call it a night. I had a lot of trouble sleeping. Every twig snapping, every little noise, kept me up. An hour and a half later I heard steps on the ground outside my tent. And then suddenly the cat meowed. I kinda poked my head up to look through the mesh screen on my tent and there's a mountain lion 5 feet away from me. Separated from me by lightweight tent fabric. I slowly laid back down and grabbed my knife and flashlight. It eventually went away. An hour later it came back and wandered around our campsite again, meowing occasionally. I didn't bother to look that time.

I know they say to get big and mean and try to scare mountain lions away, but what do you do when they're 5 feet away? That ship has already sailed. If I'd tried to scare it away it would've savaged me. My only thought was to try to aim for the throat and if that target didn't present to bury the knife in its torso. But that doesn't really save me if it decides to attack.

Pretty fearsome night. I didn't get much sleep with 3 very close (50 ft, 5 ft, could hear footsteps on grass and meowing) encounters with a mountain lion. Got a few hours of shut-eye around 9am and made the hike out. What do you do?