On Mon, Nov 05, 2018 at 11:02:47PM +0100, Samuel Thibault wrote: > proxy-defpager is typically set on /servers/default-pager, but its > permissions are by default 644, which makes it unusable by normal users, > it'd need to be 755 (see the x check in the defpager source). > > Apart from allowing users to eat memory, which they currently already > can do anyway, is there any downside to making this 755 so people can > mount their own tmpfs?
I personally wondered why it wasn't the case from the start. That being said, I'll use this as an opportunity to restate a core problem of Mach memory management, as I couldn't find it on the wiki. This problem may or may not be even more triggered by using unprivileged tmpfs instances. Thu Dec 29 2016 : < braunr> i've identified a fundamental flaw with the default pager < braunr> and actually, with mach in general i suppose < braunr> i assumed that it was necessary to trust the server only < braunr> that a server didn't need to trust its client < braunr> but mach messages carry memory that is potentially mapped from unprivileged pagers < braunr> which means faulting on that memory effectively makes the faulting process a client to the unprivileged pager < braunr> and that's something that can happen to the default pager during heavy memory pressure < braunr> in which case it deadlocks on itself because the copyout hangs on a fault, waiting for the unprivileged pager to provide the data < braunr> (which it can't because of heavy memory pressure and because it's unprivileged, it's blocked, waiting until allocations resume) < braunr> the pageout daemon will keep paging out to the default pager in the hope those pages get freed < braunr> but sending to the default pager is now impossible because its map is locked on the never-ending fault -- Richard Braun