Re:Introducing PackageManagement in Windows 10
by Freaky on Thursday April 30, 2015 @09:41AM
(#9464966)
This is no different from the situation in Linux, BSD etc, where executables and libraries are simply mapped into the process with mmap() - they're backed by the files and the system is free to discard those clean memory pages as it sees fit.
When you install(1) a new version of a file, the old one is deleted and the new one written in its place. The file handle in existing processes continues to point to the original, and the filesystem will wait until everything's closed it before actually discarding the file data.
It's a choice. Windows specifically uses DLLs as non-dirty swap, i.e. if the memory manager want's to free up pages it may free memory pages used for mapping a least recently used DLL because it will not require it to write any "dirty" memory. A DLL page can never be dirty. Windows knows that it really easy to read in the image again (from the DLL) - so it can just kill the page and reuse it immediately. But then it has to know that it can get the exact same memory page back from the DLL. If the DLL has been deleted or changed that would lead to some strange errors.