This isn't actually true. The kernel has two giant tricks up its sleeve when it comes to serving zero pages: the kernel gets to decide what happens when a page

This isn't actually true. The kernel has two giant tricks up its sleeve when it comes to serving zero pages: the kernel gets to decide what happens when a page fault happens, and it gets to decide what to tell the mmu. At boot, the kernel allocates a physical page of RAM, zeroes it, and marks it read only. Some time later, a user process requests a zeroed page. The kernel finds a new address in virtual memory, and tells the mmu to point that address to the zeroed page. The user process reads from the page, and gets zeroes back, because that's what's in the zero page. And there's nothing wrong with reading a read only page. Then the user tries to write into the zero page, and a page fault happens, because the page is read only. The kernel allocates new physical RAM, zeroes it, and updates the mmu mapping. This doesn't appear to do any less work, but gigabytes of virtual memory can in 4kB of cache, because it's all the same page. So it's way faster. One of my favorite benchmarks of all time is creating an identity matrix by using malloc, memset(0) it, then assign ones to the diagonal, vs using calloc, (which as per its specification zeroes it) and assigning ones to the diagonal. Then comparing the run time of multiplying the two identity matrices by some other matrix. The calloc identity matrix is multiple orders of magnitude faster.