> > If so, why does the kernel need these areas of memory mapped into its > > virtual address space? It seems like the kernel could keep a list of > > all page frames that are available and then use this to map in page > > frames for userspace memory without ever having it mapped into kernel > > memory space. > > Besides the "kernel's own data structures" argument, there are > security reasons why the kernel has to clear out a page's data > before mapping the page into user space. > > This is more efficiently done if the page is already mapped into > kernel address space, and does not have to be mapped in especially > for the purpose. > > The kernel will leave the memory mapped into kernel address space > after it has been given to a user process. This makes it easier > to process the page in the future, eg. after the process has stopped > using the page. > > > Then, lets say we have a machine with 1.5GB of memory. The kernel > > linearly maps 896MB of space into kernel space and the leaves teh > > remaning 128MB for temporary mappings. When are these temporary > > mappings done, and for what purpose. Would these temporary mappings > > take place when user processes require space? > > The temporary mappings are done when the kernel needs to access > memory above the 896MB boundary. The kernel needs to do accesses > when it has to clear out the data in a page before it is given to > userspace, or when userspace calls sendfile(2) and the kernel has > to copy data into or out of the page. There probably are other > occasions too. So after the light of infos above, is my general summary correct: a useful idea, one-to-one mapping, had some troubles when physical memory past the 896MB/1GB barrier but then resolved by highmem even if not exactly(Full solution is 64 bit)? -- Bora SAHIN -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
