我正在尝试理解“内存是如何工作的”。据我所知，在调用 mmap 创建 MAP_ANONYMOUS 映射时，操作系统(在我的例子中是 Linux):
mmap()creates a new mapping in the virtual address space of the calling process
据我所知，进程的虚拟地址空间可能会超过可用的实际物理内存。
据我所知，当 CPU 尝试访问尚未在页表中的内存页时触发页面错误时，会发生实际映射到物理内存的情况。
操作系统捕获页面错误并在页面目录中创建一个条目。
如果我 mmap 了一些匿名内存(但没有触及任何页面)，然后其他进程耗尽了所有物理内存，然后我尝试使用其中一个页面 mmaped(我已禁用交换)？
CPU 应该触发页面错误，然后尝试在页面目录中创建一个条目。但是由于没有留下任何物理内存，它将无法这样做......
最佳答案
使用 mmap (MAP_ANONYMOUS) 或 malloc 不会改变你的情况，如果你没有足够的可用内存 mmap 返回 MAP_FAILED 和 malloc 返回 NULL
如果我使用那个程序:
#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char ** argv)
{
int n = atoi(argv[1]);
void * m;
if (argc == 1) {
m = mmap(NULL, n*1024*1024, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
if (m == MAP_FAILED) {
puts("ko");
return 0;
}
}
else {
m = malloc(n*1024*1024);
if (m == 0) {
puts("ko");
return 0;
}
}
puts("ok");
getchar();
char * p = (char *) m;
char * sup = p + n*1024*1024;
while (p < sup) {
*p = 0;
p += 512;
}
puts("done");
getchar();
return 0;
}
我在一个有 1Gb 内存和 100Mo 交换空间的树莓派上，内存已经被 chromium 使用，因为我在 SO
proc/meminfo 给出:
MemTotal: 949448 kB
MemFree: 295008 kB
MemAvailable: 633560 kB
Buffers: 39296 kB
Cached: 360372 kB
SwapCached: 0 kB
Active: 350416 kB
Inactive: 260960 kB
Active(anon): 191976 kB
Inactive(anon): 41908 kB
Active(file): 158440 kB
Inactive(file): 219052 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 102396 kB
SwapFree: 102396 kB
Dirty: 352 kB
Writeback: 0 kB
AnonPages: 211704 kB
Mapped: 215924 kB
Shmem: 42304 kB
Slab: 24528 kB
SReclaimable: 12108 kB
SUnreclaim: 12420 kB
KernelStack: 2128 kB
PageTables: 5676 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 577120 kB
Committed_AS: 1675164 kB
VmallocTotal: 1114112 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
CmaTotal: 8192 kB
CmaFree: 6796 kB
如果我这样做:
pi@raspberrypi:/tmp $ ./a.out 750
ko
750 太大了，但是
pi@raspberrypi:/tmp $ ./a.out 600 &
[1] 1525
pi@raspberrypi:/tmp $ ok
已用内存(top 等)不反射(reflect) 600Mo，因为我不在其中读/写
proc/meminfo 给出:
MemTotal: 949448 kB
MemFree: 282860 kB
MemAvailable: 626016 kB
Buffers: 39432 kB
Cached: 362860 kB
SwapCached: 0 kB
Active: 362696 kB
Inactive: 260580 kB
Active(anon): 199880 kB
Inactive(anon): 41392 kB
Active(file): 162816 kB
Inactive(file): 219188 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 102396 kB
SwapFree: 102396 kB
Dirty: 624 kB
Writeback: 0 kB
AnonPages: 220988 kB
Mapped: 215672 kB
Shmem: 41788 kB
Slab: 24788 kB
SReclaimable: 12296 kB
SUnreclaim: 12492 kB
KernelStack: 2136 kB
PageTables: 5692 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 577120 kB
Committed_AS: 2288564 kB
VmallocTotal: 1114112 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
CmaTotal: 8192 kB
CmaFree: 6796 kB
我又可以了
pi@raspberrypi:/tmp $ ./a.out 600 &
[2] 7088
pi@raspberrypi:/tmp $ ok
pi@raspberrypi:/tmp $ jobs
[1]- stopped ./a.out 600
[2]+ stopped ./a.out 600
pi@raspberrypi:/tmp $
即使对于内存 + 交换空间来说总数太大了，/proc/meminfo 给出:
MemTotal: 949448 kB
MemFree: 282532 kB
MemAvailable: 626112 kB
Buffers: 39432 kB
Cached: 359980 kB
SwapCached: 0 kB
Active: 365200 kB
Inactive: 257736 kB
Active(anon): 202280 kB
Inactive(anon): 38320 kB
Active(file): 162920 kB
Inactive(file): 219416 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 102396 kB
SwapFree: 102396 kB
Dirty: 52 kB
Writeback: 0 kB
AnonPages: 223520 kB
Mapped: 212600 kB
Shmem: 38716 kB
Slab: 24956 kB
SReclaimable: 12476 kB
SUnreclaim: 12480 kB
KernelStack: 2120 kB
PageTables: 5736 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 577120 kB
Committed_AS: 2876612 kB
VmallocTotal: 1114112 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
CmaTotal: 8192 kB
CmaFree: 6796 kB
如果我写入 %1 的内存然后停止它我在闪存上完成了很多交换
pi@raspberrypi:/tmp $ %1
./a.out 600
done
^Z
[1]+ stopped ./a.out 600
现在几乎没有空闲交换空间，也几乎没有空闲内存，/proc/meminfo 给出
MemTotal: 949448 kB
MemFree: 33884 kB
MemAvailable: 32544 kB
Buffers: 796 kB
Cached: 66032 kB
SwapCached: 66608 kB
Active: 483668 kB
Inactive: 390360 kB
Active(anon): 462456 kB
Inactive(anon): 374188 kB
Active(file): 21212 kB
Inactive(file): 16172 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 102396 kB
SwapFree: 3080 kB
Dirty: 96 kB
Writeback: 0 kB
AnonPages: 740984 kB
Mapped: 61176 kB
Shmem: 29288 kB
Slab: 21932 kB
SReclaimable: 9084 kB
SUnreclaim: 12848 kB
KernelStack: 2064 kB
PageTables: 7012 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 577120 kB
Committed_AS: 2873112 kB
VmallocTotal: 1114112 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
CmaTotal: 8192 kB
CmaFree: 6796 kB
%1 仍在等待 getchar，如果我对 %2 执行相同的操作，它会起作用，但实际上是因为进程 %1 消失了(shell 上没有消息)
如果我malloc(为程序提供第二个参数)，行为是相同的
另见 What is the purpose of MAP_ANONYMOUS flag in mmap system call?
关于c - 了解内存分配，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/54696382/