问题描述
一般遇到的OOM，那就是OOM，确实可用内存剩余很少，申请不到触发的。
有一次的OOM，从应用角度看明明还有好几百M，但就是分配失败了触发系统OOM异常。
root:/$ free -m
total used free shared buffers cached
Mem: 1949 126 1823 70 0 70 // 刚启动时空闲内存1823M
-/+ buffers/cache: 56 1893
Swap: 0 0 0
root:/$
root:/$ oncealloc2 & // 每次执行用户态程序 malloc分配128M，然后memset清零。
root:/$
root:/$ free -m // 最后一次成功申请到128M，剩余空闲664M
total used free shared buffers cached
Mem: 1949 1285 664 70 0 70
-/+ buffers/cache: 1215 734
Swap: 0 0 0
再一次申请128M时，触发OOM，其中内存统计信息如下
[ 89.906443] Node 0 DMA32 free:613120kB min:91584kB low:114432kB high:137280kB active_anon:1315328kB
inactive_anon:6208kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:2031616kB
managed:1996672kB mlocked:0kB kernel_stack:5760kB pagetables:2624kB bounce:0kB free_pcp:2496kB local_pcp:1088kB free_cma:523776kB
可以看到: free:613120kB，系统剩余可用内存600M； free_cma:523776kB，cma区域预留了512M
613120 - 523776 = 89344 / 1024 = 87.25M < 128M
dmesg里也有明确打印
[ 0.000000] cma: Reserved 512 MiB at 0x00000000c0000000
CMA区域预留内存是给驱动通过DMA方式申请大块连续内存时用的，必须通过dma_alloc_xxx这样固定的接口。
别说用户态进程 malloc了，就是内核态kmalloc都不能从这里分配。
预留方式
1) arm架构dts文件中指定
linux,cma {
compatible = "shared-dma-pool";
inactive;
reusable;
reg = <0x00 0x10000000 0x04 0x000000>;
linux,cma-default;
};
2) 内核传递参数指定
cma=nn[MG]@[start[MG][-end[MG]]]
比如cma=128M@1G，从1G物理地址开始预留128M。如果传递cmd=0，表示禁止系统预留CMA内存。
一般不建议自己指定cma范围，留给系统启动阶段自行选择
因为一旦cma区域和其它专用区域重叠，很容易出现奇怪的问题。
完整打印
root:/$
[ 89.758736] oncealloc2 invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0, oom_score_adj=0
[ 89.768559] CPU: 2 PID: 1440 Comm: oncealloc2 Tainted: G S 5.4.74 #14
[ 89.776288] Hardware name: E2000Q DEMO DDR4 (DT)
[ 89.780892] Call trace:
[ 89.783414] dump_backtrace+0x0/0x170
[ 89.787064] show_stack+0x24/0x30
[ 89.790367] dump_stack+0xe8/0x168
[ 89.793756] dump_header+0x48/0x200
[ 89.797232] out_of_memory+0x3d8/0x510
[ 89.800968] __alloc_pages_slowpath+0xb9c/0xe60
[ 89.805486] __alloc_pages_nodemask+0x2cc/0x360
[ 89.810004] alloc_pages_vma+0x90/0x228
[ 89.813827] __handle_mm_fault+0x558/0xeb0
[ 89.817911] handle_mm_fault+0xe8/0x1a8
[ 89.821734] do_page_fault+0x1c0/0x480
[ 89.825471] do_translation_fault+0x9c/0xb0
[ 89.829641] do_mem_abort+0x50/0xb0
[ 89.833117] el0_da+0x20/0x24
[ 89.836071] User Call trace:
[ 89.838939] ===> 0xffffa533f9bc
[ 89.842154] ===> 0xaaaac8c20774
[ 89.845378] Mem-Info:
[ 89.847650] active_anon:20533 inactive_anon:97 isolated_anon:0
[ 89.847650] active_file:0 inactive_file:0 isolated_file:0
[ 89.847650] unevictable:0 dirty:0 writeback:0 unstable:0
[ 89.847650] slab_reclaimable:113 slab_unreclaimable:386
[ 89.847650] mapped:30 shmem:1121 pagetables:41 bounce:0
[ 89.847650] free:9580 free_pcp:39 free_cma:8184
[ 89.879522] Node 0 active_anon:1314112kB inactive_anon:6208kB active_file:0kB inactive_file:0kB unevictable:0kB
isolated(anon):0kB isolated(file):0kB mapped:1920kB dirty:0kB writeback:0kB shmem:71744kB shmem_thp: 0kB
shmem_pmdmapped: 0kB anon_thp: 0kB writeback_tmp:0kB unstable:0kB all_unreclaimable? yes
[ 89.906443] Node 0 DMA32 free:613120kB min:91584kB low:114432kB high:137280kB active_anon:1315328kB
inactive_anon:6208kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:2031616kB
managed:1996672kB mlocked:0kB kernel_stack:5760kB pagetables:2624kB bounce:0kB free_pcp:2496kB
local_pcp:1088kB free_cma:523776kB
[ 89.935971] lowmem_reserve[]: 0 0 0
[ 89.939459] Node 0 DMA32: 12*64kB (UM) 10*128kB (UM) 5*256kB (UM) 5*512kB (MC) 5*1024kB (UMC) 4*2048kB (MC) 3*4096kB (UMC)
1*8192kB (C) 3*16384kB (UMC) 2*32768kB (MC) 1*65536kB (C) 1*131072kB (C) 1*262144kB (C) 0*524288kB = 613120kB
[ 89.960072] Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=16777216kB
[ 89.968852] Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=524288kB
[ 89.977458] Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=2048kB
[ 89.985890] 1121 total pagecache pages
[ 89.989634] 0 pages in swap cache
[ 89.992946] Swap cache stats: add 0, delete 0, find 0/0
[ 89.998168] Free swap = 0kB
[ 90.001044] Total swap = 0kB
[ 90.003923] 31744 pages RAM
[ 90.006713] 0 pages HighMem/MovableOnly
[ 90.010544] 546 pages reserved
[ 90.013594] 8192 pages cma reserved
[ 90.017078] 0 pages hwpoisoned
[ 90.020127] Tasks state (memory values in pages):
[ 90.024827] [ pid ] uid tgid total_vm rss pgtables_bytes swapents oom_score_adj name
[ 90.033440] [ 1415] 0 1415 60 36 393216 0 0 ash
[ 90.041439] [ 1428] 0 1428 2082 2053 327680 0 0 oncealloc2
[ 90.050046] [ 1429] 0 1429 2082 2053 327680 0 0 oncealloc2
[ 90.058655] [ 1430] 0 1430 2082 2053 327680 0 0 oncealloc2
[ 90.067263] [ 1431] 0 1431 2082 2053 327680 0 0 oncealloc2
[ 90.075869] [ 1432] 0 1432 2082 2053 393216 0 0 oncealloc2
[ 90.084479] [ 1433] 0 1433 2082 2053 327680 0 0 oncealloc2
[ 90.093086] [ 1434] 0 1434 2082 2053 393216 0 0 oncealloc2
[ 90.101694] [ 1435] 0 1435 2082 2053 327680 0 0 oncealloc2
[ 90.110301] [ 1436] 0 1436 2082 2053 327680 0 0 oncealloc2
[ 90.118908] [ 1440] 0 1440 2081 998 393216 0 0 oncealloc2
[ 90.127515] Kernel panic - not syncing: Out of memory: compulsory panic_on_oom is enabled
[ 90.135678] Modules linked in:
[ 90.138723] CPU: 2 PID: 1440 Comm: oncealloc2 Tainted: G S 5.4.74 #14
[ 90.146451] Hardware name: E2000Q DEMO DDR4 (DT)
[ 90.151057] pstate: 00000085 (nzcv daIf -PAN -UAO)
[ 90.155836] pc : arch_get_regs+0x11c/0x140
[ 90.159919] lr : arch_get_regs+0xb4/0x140
[ 90.163915] sp : ffff80001736f6e0
[ 90.167217] x29: ffff80001736f6e0 x28: 0000000000000000
[ 90.172517] x27: 0000000000000000 x26: ffff80001736fc60
[ 90.177816] x25: ffff800011b95700 x24: ffff00007bf73410
[ 90.183115] x23: 0000000000000000 x22: ffff8000113d6460
[ 90.188414] x21: ffff80001736f8c0 x20: ffff80001736f8e8
[ 90.193713] x19: ffff800011b94000 x18: ffffffffffffffff
[ 90.199013] x17: 6f5f6e6f5f63696e x16: 61702079726f736c
[ 90.204312] x15: ffff800011b94d08 x14: 0000000000000000
[ 90.209611] x13: 3a6e692064656b6e x12: 696c2073656c7564
[ 90.214910] x11: 0000000005f5e0ff x10: ffff800011b95558
[ 90.220209] x9 : 0000000000000000 x8 : ffff80001736f858
[ 90.225508] x7 : 0000000000000000 x6 : 000000000000003f
[ 90.230807] x5 : 0000000000000040 x4 : ffffffffffffffe0
[ 90.236106] x3 : ffff80001736f718 x2 : 0000000000000008
[ 90.241405] x1 : 0000000000000000 x0 : ffff80001736f718
[ 90.246704] Call trace:
[ 90.249225] User Call trace:
[ 90.252093] ===> 0xffffa533f9bc
[ 90.255308] ===> 0xaaaac8c20774
[ 90.258524] monitor note oops info
[ 90.262035] Monitor save item info
[ 90.265424] Monitor notify normal exception cpu=2
[ 90.270116] CPU: 2 PID: 1440 Comm: oncealloc2 Tainted: G S 5.4.74 #14
[ 90.277845] Hardware name: E2000Q DEMO DDR4 (DT)
[ 90.282449] Call trace:
[ 90.284970] dump_backtrace+0x0/0x170
[ 90.288620] show_stack+0x24/0x30
[ 90.291923] dump_stack+0xe8/0x168
[ 90.295313] panic+0x1cc/0x36c
[ 90.298356] out_of_memory+0x400/0x510
[ 90.302092] __alloc_pages_slowpath+0xb9c/0xe60
[ 90.306610] __alloc_pages_nodemask+0x2cc/0x360
[ 90.311128] alloc_pages_vma+0x90/0x228
[ 90.314951] __handle_mm_fault+0x558/0xeb0
[ 90.319035] handle_mm_fault+0xe8/0x1a8
[ 90.322858] do_page_fault+0x1c0/0x480
[ 90.326595] do_translation_fault+0x9c/0xb0
[ 90.330765] do_mem_abort+0x50/0xb0
[ 90.334241] el0_da+0x20/0x24
[ 90.337195] User Call trace:
[ 90.340063] ===> 0xffffa533f9bc
[ 90.343278] ===> 0xaaaac8c20774
PANIC: Out of memory: compulsory panic_on_oom is enabled
Entering kdb (current=0xffff000020e12b80, pid 1440) on processor 2 due to Keyboard Entry
[2]kdb>
