Josef “Jeff” Sipek

Segment Drivers

Lately, I started poking around the Illumos memory management code. As I’ve done in the past, I decided to use this blahg as a place to document some of my discoveries.

Memory Layout

In Illumos (and Solaris), address spaces are managed as sets of segments. Each segment has a base address, length, and a number of other properties. This is true for both process memory as well as kernel memory. Do not confuse these segments with Wikipedia article: memory segmentation that processors like Wikipedia article: x86 provide.

Each process has its own struct as:

> ::pgrep vim
S    PID   PPID   PGID    SID    UID      FLAGS             ADDR NAME
R  10852  10777  10850  10777    101 0x4a004000 ffffff0411e1c0a0 vim
> ffffff0411e1c0a0::print proc_t p_as | ::print struct as a_segtree
a_segtree = {
    a_segtree.avl_root = 0xffffff03f7c62ea8
    a_segtree.avl_compar = as_segcompar
    a_segtree.avl_offset = 0x20
    a_segtree.avl_numnodes = 0x18
    a_segtree.avl_size = 0x60

The kernel address space is maintained in the kas global:

> kas::print a_segtree
a_segtree = {
    a_segtree.avl_root = kvseg+0x20
    a_segtree.avl_compar = as_segcompar
    a_segtree.avl_offset = 0x20
    a_segtree.avl_numnodes = 0x9
    a_segtree.avl_size = 0x60

(Once upon a time this set of segments was a linked list, but for a long while now it has been an AVL tree indexed by the base address.)

Regardless of which address space we’re dealing with, the same rules apply: segments represent contiguous regions within the address space. Each segment can represent a different type of memory. For example, walking the kernel address space segment tree yields nine different segments of four different types (kpm, kmem, kp, and map):

> kas::print a_segtree | ::walk avl | ::printf "%p.%016x %a\n" "struct seg" s_base s_size s_ops
fffffe0000000000.000000031e000000 segkpm_ops
ffffff0000000000.0000000017000000 segkmem_ops
ffffff0017000000.0000000080000000 segkp_ops
ffffff0097000000.00000002fca00000 segkmem_ops
ffffff03d3a00000.0000000004000000 segmap_ops
ffffff03d7a00000.000000fbe8600000 segkmem_ops
ffffffffc0000000.000000003b7fb000 segkmem_ops
fffffffffb800000.0000000000550000 segkmem_ops
ffffffffff800000.0000000000400000 segkmem_ops

Segment Drivers

Illumos comes with seven different architecture- and platform-independent segment drivers. A segment driver is a “driver” that implements a couple of functions to manage a segment of memory. That is, each segment type can handle page faults, page locking, sync operations, etc. differently.

For example, suppose that a page fault occurs because a process tried to load a value from a page that lacks a page table entry. The platform specific (assembly) fault handling code gets invoked by the processor. After doing a little bit of work, it calls into the generic (C) fault handling code, as_fault. There, the segtree AVL tree is consulted and the corresponding segment’s fault operation gets invoked.

(Solaris Internals lists 12 and 11 segment drivers, respectively, in the two editions.) In Illumos, the seven common segment drivers are:

Most of the time, userspace processes do not need to map devices into their address space. In the rare case when a process does want a device mapped (e.g., Xorg), the dev segment driver maintains that mapping.
This segment driver maps the kernel heap, module text, and all early boot memory. (code)
In general, kernel memory is not pageable. In the rare case that something can be in kernel pageable memory, this segment is what maintains the anonymous page mappings.
If possible (you’re on a 64-bit system), the kpm segment driver maps all physical memory into the kernel’s address space. This allows the kernel to not have to set up temporary mappings to operate on physical memory. (code)
The map segment driver is a kernel-only higher performance version of the vn segment driver. (See below.)
This segment driver is responsible for maintaining SysV shared memory segments. (Not to be confused with POSIX shared memory.)
Memory mapped files are handled by the vn segment driver. This includes both regular files as well as anonymous memory.

There are also two platform specific segment drivers:

seg_mf (i86xpv only)
This segment driver is only used by dom0 processes (read: Xen) to map pages from other domains.
seg_nf (sparc v9 only)
The header for the file says that it is for non-faulting loads. I don’t actually know what exactly it is for. (And I don’t care enough to dig deeper given that it is Sparc specific.)

The Reality

This is a lot of different segment drivers. Are all of them used all the time? Well, sort of. The mdb output earlier shows that the (amd64) kernel uses only four different segment drivers (kpm, kmem, kp, and map). A typical userspace process is very boring — it is only made up of vn segments. There are, however, exceptions. For instance, Xorg uses vn and dev. This accounts for six of the seven drivers. The last common segment driver is spt, which provides System V shared memory. (I talked about SysV shared memory previously.) So, on a 64-bit x86 system, all seven common segment drivers are in use.

The story is a bit different on 32-bit kernels. Since a 32-bit system has much smaller address space, the kernel tries to eliminate a number of mappings. Here is the list of segments in a 32-bit kernel:

> kas::print a_segtree | ::walk avl | ::printf "%p %a\n" "struct seg" s_base s_ops
b5802000 segmap_ops
b6800000 segkmem_ops
ef400000 segkmem_ops
fe800000 segkmem_ops
ff000000 segkmem_ops

As you can see, the kp and kpm segments went away. While at first this is surprising, it actually makes perfect sense. When thinking about memory there are two “types” to consider: physical and virtual. In theory, one can have more virtual than physical thanks to the MMU but in reality this is only true on 64-bit systems. The physical memory sizes have outgrown 4 GB a number of years ago and therefore a 32-bit address space can trivially be 100% backed by physical memory. In other words, 32-bit address spaces are tight on virtual memory, while 64-bit address spaces are “tight” on physical memory.

Let’s consider the disappearance of the kp segment on 32-bits. What does kp let us do? It lets us oversubscribe physical memory by backing some virtual memory with disk space. On 32-bit systems we have enough physical memory to back all the virtual memory in the kernel so we don’t need to back some of it by disk. So we have no use for it. (Yes, the kernel still could have paged parts of itself out, but kernel text and data is generally considered important enough to keep it in non-pageable memory. The memory utilization will more than pay for itself by the performance improvement of not having the kernel paged out.)

As I stated before, kpm segments map physical memory into the kernel’s address space for performance reasons (without it the kernel would have to temporarily map a page to access the contents). Therefore, they are good candidates for removal when it comes to slimming down the kernel’s address space demands. (Well, the actual story is the other way… the introduction of 64-bit capable hardware allowed kpm segments to exist to improve kernel performance.)

Virtual Machines

Mendel Rosenblum, a Stanford University associate professor, VMWare co-founder, became an ACM Fellow, in 2008. The page about his fellow citation reads:

For contributions to reinventing virtual machines.

Very amusing.

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