Meltdown缓解措施结合calloc()的CoW延迟分配是否影响内存分配性能？

Hey there! Let’s break down the inner workings of calloc() step by step—this is a great deep dive into how operating systems and hardware collaborate on memory management:

• Virtual Memory Request: When you call calloc(), it doesn’t immediately carve out physical memory for you. Instead, it sends a request to the OS for a block of virtual memory.
• CoW Zero Page Trick: The OS teams up with the Memory Management Unit (MMU) to hand back a virtual address that maps to a read-only zero page using Copy-on-Write (CoW) logic. This is super efficient because multiple processes can share that single physical zero page initially, saving tons of memory.
• Write-Triggered Page Fault: When your program tries to write to any spot in that virtual page, a page fault fires up—since the underlying physical page is locked as read-only. The OS then jumps in: it creates a private copy of the zero page, updates your process’s virtual memory mapping to point to this new, writable zero page, and lets the write operation continue like nothing happened.

After the Meltdown vulnerability came to light, operating systems had to overhaul this memory handling to patch side-channel attacks that exploited speculative execution. Most OSes tweaked how they manage page table entries and CoW zero pages, adding safeguards that block unauthorized access to kernel memory via speculative reads. This did add a tiny bit of overhead to memory allocation calls like calloc(), but it was a necessary trade-off to keep systems secure.

内容的提问来源于stack exchange，提问作者Tullo_x86