[$] Patching until the COWs come home (part 1)

The kernel's memory-management subsystem is built upon many concepts, oneof which is called "copy on write", or "COW".The idea behind COW is conceptually simple, but itsdetails are tricky and its past is troublesome. Any change to itsimplementation can have unexpected consequences and cause subtle breakagefor existing workloads. So it is somewhat surprising that last year we sawtwo major changes the kernel's COW code; less surprising is the fact that,both times, these changes had unexpected consequences and broke things. Some of the resulting problems are still not fixedtoday, almost ten months after the first change, while the original reasonfor the changes - a security vulnerability - is also not fully fixed. Readon for a description of COW, the vulnerability, and the initial fix; theconcluding article in the series will describe the complications that arosethereafter.