PAE还有4GB吗？

Question

试图在32位CPU上获得超过4GB内存的PAE，但不能。

~# grep -o pae /proc/cpuinfo 
pae
pae
pae
pae
~# uname -a
Linux debian 4.9.0-6-686-pae #1 SMP Debian 4.9.82-1+deb9u3 (2018-03-02) i686 GNU/Linux
~# grep HIGHMEM /boot/config-4.9.0-6-686-pae 
# CONFIG_NOHIGHMEM is not set
# CONFIG_HIGHMEM4G is not set
CONFIG_HIGHMEM64G=y
CONFIG_HIGHMEM=y
# CONFIG_DEBUG_HIGHMEM is not set
~# dmidecode -t memory | grep Enabled
    Enabled Error Correcting Capabilities:
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)

仍有

~# free -h
              total        used        free      shared  buff/cache   available
Mem:           3.8G         29M        3.6G         10M        244M        3.5G
Swap:            9G          0B          9G

也许有人能帮上忙，我去哪找？

Answer 1

要诊断内存问题，最好的起点是内核引导消息；内核打印出与系统内存有关的所有内容。从dmesg的输出开始，寻找“内存”。

正如你所发现的，在你的例子中

[ 0.000000] WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 2176MB of RAM.
[ 0.000000] WARNING: CPU: 0 PID: 0 at /build/linux-rtbsTs/linux-4.9.82/arch/x86/kernel/cpu/mtrr/cleanup.c:978 mtrr_trim_uncached_memory+0x2f5/0x326

这表明BIOS没有为6 6GiB系统正确设置。