Embedded Linux - How come virtual kernel memory is bigger than physical memory?

Question

I'm running a custom Linux distribution on my ARM-based system (it's got two A7s).

The Linux kernel version is 4.86.9.

In the process of investigating some weird graphical glitches occuring on the integrated display when running memory-intensive tasks (like tar or gzip), I executed dmesg to figure out how Linux sees system memory (which is supposed to be 620 MB out of 1 GB DDR3L, as per Yocto memory layout configuration).

The following is what dmesg showed:

Memory: 484008K/634880K available (5120K kernel code, 205K rwdata, 1476K rodata, 1024K init, 331K bss, 15704K reserved, 135168K cma-reserved, 0K highmem)
Virtual kernel memory layout:
        vector  : 0xffff0000 - 0xffff1000   (   4 kB)
        fixmap  : 0xffc00000 - 0xfff00000   (3072 kB)
        vmalloc : 0xd7000000 - 0xff800000   ( 648 MB)
        lowmem  : 0xb0000000 - 0xd6c00000   ( 620 MB)
        pkmap   : 0xafe00000 - 0xb0000000   (   2 MB)
        modules : 0xaf000000 - 0xafe00000   (  14 MB)
          .text : 0xb0008000 - 0xb0600000   (6112 kB)
          .init : 0xb0800000 - 0xb0900000   (1024 kB)
          .data : 0xb0900000 - 0xb0933468   ( 206 kB)
          .bss  : 0xb0935000 - 0xb0987e8c   ( 332 kB)

Something seems off:

1. Available memory is only 484 MB/634 MB. Neither the 484 MB nor the 634 MB make sense to me.
2. vmalloc is a whopping 648 MB.

Can you help me make sense of these values?

I point out that highmem was disabled as per Yocto configuration.

Before I disabled it, usage was still 0K (as above) but the layout displayed by dmesg is a little different:

Memory: 484720K/634880K available (5120K kernel code, 197K rwdata, 1428K rodata, 1024K init, 256K bss, 14992K reserved, 135168K cma-reserved, 0K highmem)
Virtual kernel memory layout:
        vector   : 0xffff0000 - 0xffff1000   (   4 kB)
        fixmap   : 0xffc00000 - 0xfff00000   (3072 kB)
        vmalloc  : 0xe7000000 - 0xff800000   ( 392 MB)
        lowmem   : 0xc0000000 - 0xe6c00000   ( 620 MB)
        pkmap    : 0xbfe00000 - 0xc0000000   (   2 MB)
        modules  : 0xbf000000 - 0xbfe00000   (  14 MB)
           .text : 0xc0008000 - 0xc0600000   (6112 kB)
           .init : 0xc0800000 - 0xc0900000   (1024 kB)
           .data : 0xc0900000 - 0xc09314d8   ( 198 kB)
           .bss  : 0xc0933000 - 0xc09732d8   ( 257 kB)

As you can see vmalloc is 392 MB, which is only half as odd as the 648 MB!

Thanks in advance, and please pardon me if I missed something important.

Answer 1

That is just exactly what virtual memory is: mapping of the physical memory (and other things) into a completely independent memory space.

This isn't specific to your embedded device, or to Linux; that's just what virtual memory is.

That memory space can be larger or smaller than the physical memory, no problem: Nothing says every address in the virtual memory space of each process needs to map to something.