Loss of several MB of run-time memory

[Patrick Venture]

Just jumped from 4.7 kernel to 4.18 running the latest openbmc image
on the quanta-q71l board.  And I see now I have ~20MiB of RAM free for
stuff once things are settled, whereas before I could have up to
35MiB.

Here are some dumps:

Now:
[    0.000000] CPU: ARM926EJ-S [41069265] revision 5 (ARMv5TEJ), cr=0005317f
[    0.000000] CPU: VIVT data cache, VIVT instruction cache
[    0.000000] OF: fdt: Machine model: Quanta Q71L BMC
[    0.000000] Memory policy: Data cache writeback
[    0.000000] On node 0 totalpages: 30720
[    0.000000]   Normal zone: 240 pages used for memmap
[    0.000000]   Normal zone: 0 pages reserved
[    0.000000]   Normal zone: 30720 pages, LIFO batch:7
[    0.000000] random: get_random_bytes called from
start_kernel+0x8c/0x4c0 with crng_init=0
[    0.000000] pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768
[    0.000000] pcpu-alloc: [0] 0
[    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 30480
[    0.000000] Kernel command line: console=ttyS4,115200n8
root=/dev/ram rw clk_ignore_unused
[    0.000000] Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
[    0.000000] Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
[    0.000000] Memory: 111076K/122880K available (5120K kernel code,
365K rwdata, 1104K rodata, 1024K init, 143K bss, 11804K reserved, 0K
cma-reserved)
[    0.000000] Virtual kernel memory layout:
[    0.000000]     vector  : 0xffff0000 - 0xffff1000   (   4 kB)
[    0.000000]     fixmap  : 0xffc00000 - 0xfff00000   (3072 kB)
[    0.000000]     vmalloc : 0x88000000 - 0xff800000   (1912 MB)
[    0.000000]     lowmem  : 0x80000000 - 0x87800000   ( 120 MB)
[    0.000000]       .text : 0x(ptrval) - 0x(ptrval)   (6112 kB)
[    0.000000]       .init : 0x(ptrval) - 0x(ptrval)   (1024 kB)
[    0.000000]       .data : 0x(ptrval) - 0x(ptrval)   ( 366 kB)
[    0.000000]        .bss : 0x(ptrval) - 0x(ptrval)   ( 144 kB)
[    0.000000] ftrace: allocating 18546 entries in 55 pages

cat /proc/meminfo
MemTotal:         113952 kB
MemFree:           19944 kB
MemAvailable:      62432 kB
Buffers:           11032 kB
Cached:            48732 kB
SwapCached:            0 kB
Active:            40940 kB
Inactive:          26728 kB
Active(anon):      17068 kB
Inactive(anon):     9316 kB
Active(file):      23872 kB
Inactive(file):    17412 kB
Unevictable:        9088 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:         17008 kB
Mapped:            21120 kB
Shmem:              9392 kB
Slab:              11956 kB
SReclaimable:       6472 kB
SUnreclaim:         5484 kB
KernelStack:         560 kB
PageTables:         1384 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:       56976 kB
Committed_AS:     124224 kB
VmallocTotal:    1957888 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB

Before:
dmesg
  Normal zone: 30720 pages, LIFO batch:7
pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768
pcpu-alloc: [0] 0
Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 30480
Kernel command line: console=ttyS4,115200n8 root=/dev/ram rw
PID hash table entries: 512 (order: -1, 2048 bytes)
Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
Memory: 113644K/122880K available (4206K kernel code, 150K rwdata,
860K rodata, 1024K init, 111K bss, 9236K reserved, 0K cma-reserved)
Virtual kernel memory layout:
    vector  : 0xffff0000 - 0xffff1000   (   4 kB)
    fixmap  : 0xffc00000 - 0xfff00000   (3072 kB)
    vmalloc : 0xc8000000 - 0xff800000   ( 888 MB)
    lowmem  : 0xc0000000 - 0xc7800000   ( 120 MB)
      .text : 0xc0008000 - 0xc05f28ec   (6059 kB)
      .init : 0xc0600000 - 0xc0700000   (1024 kB)
      .data : 0xc0700000 - 0xc0725be8   ( 151 kB)
       .bss : 0xc0725be8 - 0xc0741a38   ( 112 kB)

cat /proc/meminfo
MemTotal:         116224 kB
MemFree:           35832 kB
MemAvailable:      76952 kB
Buffers:            9596 kB
Cached:            39776 kB
SwapCached:            0 kB
Active:            40516 kB
Inactive:          25432 kB
Active(anon):      17004 kB
Inactive(anon):     6968 kB
Active(file):      23512 kB
Inactive(file):    18464 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:         16588 kB
Mapped:            20064 kB
Shmem:              7396 kB
Slab:               9424 kB
SReclaimable:       4532 kB
SUnreclaim:         4892 kB
KernelStack:         720 kB
PageTables:         1328 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:       58112 kB
Committed_AS:     142324 kB
VmallocTotal:     909312 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB

This matters for a few reasons:
1) my memory chip is too small to be practical and I need all the
bytes I can get.
2) I need at least 32MiB to load a new firmware image.

I dropped all the python except the mapper, and I dropped the newer
daemons from my build to clear out that difference.  It was originally
about 16MiB difference, so I was thinking that something is now being
mapped by default that wasn't before, such as part of a flash image.

Patrick