Linux memory mapping

2
votes

I got few questions about linux memory management(assume x86 32bit platform)

  1. By default for all processes the top 1Gig of virtual address is mapped to kernel area. Theoretically the Kernel can map additional memory from high memory using vmalloc. My question is what happens with the page tables of all the user processes , I assume that they should get updates about the kernel memory allocation?( may be that memory will get used when the kernel is in process context).

  2. Can someone explain from where The X86 logical address mapping limitation comes from? in "linux device drivers" chapter 15 it is said that there is a limitation on mapping logical address but with no deep explanation:

    in many cases, even 32-bit processors can address more than 4 GB of physical memory. The limitation on how much memory can be directly mapped with logical addresses remains, however. Only the lowest portion of memory (up to 1 or 2 GB, depending on the hardware and the kernel configuration) has logical addresses; the rest (high memory) does not.

  3. When does the kernel switch to its own page table(not including boot time)?. When its in process context, and interrupt context it uses the user mode process page table. The kernel threads use the process page table as well.

1
memory-managementlinux-kernelx86

1 Answers

0
votes

1.) There is only one set of 256 page tables that map the kernel's 1GiB region. The top 256 entries of each user space page directory point to these page tables. Thus, if the kernel changes a virtual mapping, all user space processes get the update as well.

2.) I'm not sure which limitations you mean, can you quote some text so I can find the passage in the book.

3.) When a process, like QEMU, starts a virtual CPU with kvm, the kernel swaps out the page table of the process, even though it doesn't yield to a different process. There may be more places like this, but in general, I don't think there is such a thing as a "kernel page table". All process page tables already map kernel memory, and it would thus seem wasteful to switch them out.

"Linux Device Drivers" is a great reference, but I can also recommend "Understanding the Linux Virtual Memory Manager", and of course, the kernel's source code.