Having swap "disabled" forces your system to swap out executables to disk
Read-only pages are never written to swap, because they can be retrieved as-is from the filesystem already. Binaries and libraries are accounted as buffer cache, not used memory, and under memory pressure those pages are simply dropped, not swapped out. Whether you have swap enabled or disabled doesn't change that.
Still, I hope that Debian does the sane thing and sets proper size limits. I recall having to troubleshoot memory issues on a system (Ubuntu IIRC) a decade ago where they also extensively used tmpfs: /dev, /dev/shm, /run, /tmp, /var/lock -- except that all those were mounted with the default size, which is 50% of total RAM. And the size limit is per mountpoint...
> under memory pressure those pages are simply dropped, not swapped out
This is just semantics. The pages are evicted from memory, knowing that they are backed by the disk, and can be swapped back in from disk when needed - behavior that I called "swapping out" since it's pretty similar to what happens with other memory pages in the presence of swap.
Regardless of the naming, the important part is what happens when the page is needed again. If your code page was evicted, when your thread gets scheduled again, it will ask for the page to be read back into memory, requiring a disk read; this will cause some other code page to be evicted; then a new thread will be scheduled - worse case, one that uses the exact code page that just got evicted, repeating the process. And since the scheduler will generally try to execute the thread that has been waiting the most, while the VMM will prefer to evict the oldest read pages, there is actually a decent chance that this exact worse case will happen a lot. This whole thing will completely freeze the system to a degree that is extremely unlikely for a system with a decent amount of swap space.
Read-only pages are never written to swap, because they can be retrieved as-is from the filesystem already. Binaries and libraries are accounted as buffer cache, not used memory, and under memory pressure those pages are simply dropped, not swapped out. Whether you have swap enabled or disabled doesn't change that.
Still, I hope that Debian does the sane thing and sets proper size limits. I recall having to troubleshoot memory issues on a system (Ubuntu IIRC) a decade ago where they also extensively used tmpfs: /dev, /dev/shm, /run, /tmp, /var/lock -- except that all those were mounted with the default size, which is 50% of total RAM. And the size limit is per mountpoint...