Debian x64虚拟机配置8GB内存却仅识别4GB，求助排查原因

我在QNAP NAS上运行了一台Debian虚拟机，明明给它配置了8GB内存，但系统实际只识别到4GB，实在搞不懂哪里出问题了，麻烦大家帮忙看看！

先看硬件层面的检测结果，虚拟机确实是分配了8GB内存的：

root@backuppc:/# lshw | grep "*-memory" -A 40

*-memory

description: System Memory

physical id: 1000

size: 8GiB

但Debian系统里实际可用的内存只有4GB左右，看/proc/meminfo的输出：

root@backuppc:/etc/grub.d# cat /proc/meminfo

MemTotal:        3953672 kB
MemFree:          264084 kB
MemAvailable:    3300796 kB
Buffers:           24168 kB
Cached:          2833508 kB
SwapCached:         3724 kB
Active:          1749656 kB
Inactive:        1133408 kB
Active(anon):       5380 kB
Inactive(anon):    21856 kB
Active(file):    1744276 kB
Inactive(file):  1111552 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       1003516 kB
SwapFree:         970280 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:         22876 kB
Mapped:            29948 kB
Shmem:              1848 kB
KReclaimable:     576524 kB
Slab:             674656 kB
SReclaimable:     576524 kB
SUnreclaim:        98132 kB
KernelStack:        3200 kB
PageTables:         1904 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     2980352 kB
Committed_AS:     585300 kB
VmallocTotal:   34359738367 kB
VmallocUsed:       16580 kB
VmallocChunk:          0 kB
Percpu:             2240 kB
HardwareCorrupted:     0 kB
AnonHugePages:      4096 kB
ShmemHugePages:        0 kB
ShmemPmdMapped:        0 kB
FileHugePages:         0 kB
FilePmdMapped:         0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
Hugetlb:               0 kB
DirectMap4k:      505700 kB
DirectMap2M:     7882752 kB
DirectMap1G:     2097152 kB

而且我确认这是64位系统，排除了32位系统内存限制的问题：

root@backuppc:/etc/grub.d# uname -a

Linux backuppc41 5.10.0-19-amd64 #1 SMP Debian 5.10.149-2 (2022-10-21) x86_64 GNU/Linux

另外我也检查了内核启动参数，没有发现任何限制内存的配置：

root@backuppc:/etc/grub.d# cat /proc/cmdline

BOOT_IMAGE=/vmlinuz-5.10.0-19-amd64 root=/dev/mapper/backuppc--vg-root ro quiet zswap.enabled=1

有没有大佬知道为啥系统只用到一半内存啊？万分感谢各位的提示！

/KNEBB

备注：内容来源于stack exchange，提问作者Christian