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linux kernel page allocation failure and tuning of page cache




Hi 

We are using Renesas RZ/A1 processor based custom target board . linux kernel version is 4.9.123.

1) the platform is low memory platform having memory 64MB.

2)  we are doing around 45MB TCP data transfer from PC to target using netcat utility .On Target , a process receives data over socket and writes the data to flash disk .

3) At the start of data transfer , we explicitly clear linux kernel cached memory by  calling echo 3 > /proc/sys/vm/drop_caches .

4) during TCP data transfer , we could see free -m showing "free" getting dropped to almost 1MB and most of the memory appearing as "cached" 

# free -m
                                            total         used   free     shared   buffers   cached
Mem:                                  57            56         1                 0            2           42
-/+ buffers/cache:                          12        45
Swap:                                   0              0           0

5) sometimes , we observed kernel memory getting exhausted as page allocation failure happens in kernel  with the backtrace is printed below :
# [  775.947949] nc.traditional: page allocation failure: order:0, mode:0x2080020(GFP_ATOMIC)
[  775.956362] CPU: 0 PID: 1288 Comm: nc.traditional Tainted: G           O    4.9.123-pic6-g31a13de-dirty #19
[  775.966085] Hardware name: Generic R7S72100 (Flattened Device Tree)
[  775.972501] [<c0109829>] (unwind_backtrace) from [<c010796f>] (show_stack+0xb/0xc)
[  775.980118] [<c010796f>] (show_stack) from [<c0151de3>] (warn_alloc+0x89/0xba)
[  775.987361] [<c0151de3>] (warn_alloc) from [<c0152043>] (__alloc_pages_nodemask+0x1eb/0x634)
[  775.995790] [<c0152043>] (__alloc_pages_nodemask) from [<c0152523>] (__alloc_page_frag+0x39/0xde)
[  776.004685] [<c0152523>] (__alloc_page_frag) from [<c03190f1>] (__netdev_alloc_skb+0x51/0xb0)
[  776.013217] [<c03190f1>] (__netdev_alloc_skb) from [<c02c1b6f>] (sh_eth_poll+0xbf/0x3c0)
[  776.021342] [<c02c1b6f>] (sh_eth_poll) from [<c031fd8f>] (net_rx_action+0x77/0x170)
[  776.029051] [<c031fd8f>] (net_rx_action) from [<c011238f>] (__do_softirq+0x107/0x160)
[  776.036896] [<c011238f>] (__do_softirq) from [<c0112589>] (irq_exit+0x5d/0x80)
[  776.044165] [<c0112589>] (irq_exit) from [<c012f4db>] (__handle_domain_irq+0x57/0x8c)
[  776.052007] [<c012f4db>] (__handle_domain_irq) from [<c01012e1>] (gic_handle_irq+0x31/0x48)
[  776.060362] [<c01012e1>] (gic_handle_irq) from [<c0108025>] (__irq_svc+0x65/0xac)
[  776.067835] Exception stack(0xc1cafd70 to 0xc1cafdb8)
[  776.072876] fd60:                                     0002751c c1dec6a0 0000000c 521c3be5
[  776.081042] fd80: 56feb08e f64823a6 ffb35f7b feab513d f9cb0643 0000056c c1caff10 ffffe000
[  776.089204] fda0: b1f49160 c1cafdc4 c180c677 c0234ace 200e0033 ffffffff
[  776.095816] [<c0108025>] (__irq_svc) from [<c0234ace>] (__copy_to_user_std+0x7e/0x430)
[  776.103796] [<c0234ace>] (__copy_to_user_std) from [<c0241715>] (copy_page_to_iter+0x105/0x250)
[  776.112503] [<c0241715>] (copy_page_to_iter) from [<c0319aeb>] (skb_copy_datagram_iter+0xa3/0x108)
[  776.121469] [<c0319aeb>] (skb_copy_datagram_iter) from [<c03443a7>] (tcp_recvmsg+0x3ab/0x5f4)
[  776.130045] [<c03443a7>] (tcp_recvmsg) from [<c035e249>] (inet_recvmsg+0x21/0x2c)
[  776.137576] [<c035e249>] (inet_recvmsg) from [<c031009f>] (sock_read_iter+0x51/0x6e)
[  776.145384] [<c031009f>] (sock_read_iter) from [<c017795d>] (__vfs_read+0x97/0xb0)
[  776.152967] [<c017795d>] (__vfs_read) from [<c01781d9>] (vfs_read+0x51/0xb0)
[  776.159983] [<c01781d9>] (vfs_read) from [<c0178aab>] (SyS_read+0x27/0x52)
[  776.166837] [<c0178aab>] (SyS_read) from [<c0105261>] (ret_fast_syscall+0x1/0x54)
[  776.174308] Mem-Info:
[  776.176650] active_anon:2037 inactive_anon:23 isolated_anon:0
[  776.176650]  active_file:2636 inactive_file:7391 isolated_file:32
[  776.176650]  unevictable:0 dirty:1366 writeback:1281 unstable:0
[  776.176650]  slab_reclaimable:719 slab_unreclaimable:724
[  776.176650]  mapped:1990 shmem:26 pagetables:159 bounce:0
[  776.176650]  free:373 free_pcp:6 free_cma:0
[  776.209062] Node 0 active_anon:8148kB inactive_anon:92kB active_file:10544kB inactive_file:29564kB unevictable:0kB isolated(anon):0kB isolated(file):128kB mapped:7960kB dirty:5464kB writeback:5124kB shmem:104kB writeback_tmp:0kB unstable:0kB pages_scanned:0 all_unreclaimable? no
[  776.233602] Normal free:1492kB min:964kB low:1204kB high:1444kB active_anon:8148kB inactive_anon:92kB active_file:10544kB inactive_file:29564kB unevictable:0kB writepending:10588kB present:65536kB managed:59304kB mlocked:0kB slab_reclaimable:2876kB slab_unreclaimable:2896kB kernel_stack:1152kB pagetables:636kB bounce:0kB free_pcp:24kB local_pcp:24kB free_cma:0kB
[  776.265406] lowmem_reserve[]: 0 0
[  776.268761] Normal: 7*4kB (H) 5*8kB (H) 7*16kB (H) 5*32kB (H) 6*64kB (H) 2*128kB (H) 2*256kB (H) 0*512kB 0*1024kB 0*2048kB 0*4096kB = 1492kB
10071 total pagecache pages
[  776.284124] 0 pages in swap cache
[  776.287446] Swap cache stats: add 0, delete 0, find 0/0
[  776.292645] Free swap  = 0kB
[  776.295532] Total swap = 0kB
[  776.298421] 16384 pages RAM
[  776.301224] 0 pages HighMem/MovableOnly
[  776.305052] 1558 pages reserved

6) we have certain questions as below :
a) how the kernel memory got exhausted ? at the time of low memory conditions in kernel , are the kernel page flusher threads , which should have written dirty pages from page cache to flash disk , not executing at right time ? is the kernel page reclaim mechanism not executing at right time ? 

b) are there any parameters available within the linux memory subsystem with which the reclaim procedure can be monitored and  fine tuned ?

c) can  some amount of free memory be reserved so that linux kernel does not caches it and kernel can use it for its other required page allocation ( particularly gfp_atomic ) as needed above on behalf of netcat nc process ? can some tuning be done in linux memory subsystem eg by using /proc/sys/vm/min_free_kbytes  to achieve this objective .

d) can we be provided with further clues on how to debug this issue further for out of memory condition in kernel  ?


Regards
Amit