Possible mem cgroup bug in kernels between 4.18.0 and 5.3-rc1.

[Masoud Sharbiani]

Hey folks,
Iâve come across an issue that affects most of 4.19, 4.20 and 5.2 linux-stable kernels that has only been fixed in 5.3-rc1.
It was introduced by

29ef680 memcg, oom: move out_of_memory back to the charge path 

The gist of it is that if you have a memory control group for a process that repeatedly maps all of the pages of a file with  repeated calls to:

   mmap(NULL, pages * PAGE_SIZE, PROT_WRITE|PROT_READ, MAP_FILE|MAP_PRIVATE, fd, 0)

The memory cg eventually runs out of memory, as it should. However, prior to the 29ef680 commit, it would kill the running process with OOM; After that commit ( and until 5.3-rc1; Havenât pinpointed the exact commit in between 5.2.0 and 5.3-rc1) the offending process goes into %100 CPU usage, and doesnât die (prior behavior) or fail the mmap call (which is what happens if one runs the test program with a low ulimit -v value).

Any ideas on how to chase this down further?

(Test program and script have been pasted below)
Thanks,
Masoud


//âââ leaker.c ââ
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <stdio.h>
#include <errno.h>
#include <signal.h>

#ifndef PAGE_SIZE
#define PAGE_SIZE 4096
#endif

void sighandler(int x) {
   printf("SIGNAL %d received. Quitting\n", x);
   exit(2);
}

int main(int ac, char*av[])
{
   int i;
   int fd;
   int pages = 4096;
   char buf[PAGE_SIZE];
   char *d;
   int sum = 0, loop_cnt = 0;
   int max_loops = 100000;
   // For getopt(3) stuff:
   int opt;

   while ((opt = getopt(ac, av, "p:c:")) != -1) {
       switch (opt) {
           case 'p':
               pages = atoi(optarg);
               break;
           case 'c':
               max_loops = atoi(optarg);
               break;
           default:
               fprintf(stderr, "Wrong usage:\n");
               fprintf(stderr, "%s -p <pages> -c <loop_count>\n", av[0]);
               exit(-1);
       }

   }
   signal(SIGTERM, sighandler);
   printf("Mapping %d pages anonymously %d times.\n", pages, max_loops);
   printf("File size will be %ld\n", pages * (long)PAGE_SIZE);
   printf("max memory usage size will be %ld\n", (long) max_loops * pages * PAGE_SIZE);

   memset(buf, 0, PAGE_SIZE);

   fd = open("big-data-file.bin", O_CREAT|O_WRONLY|O_TRUNC , S_IRUSR | S_IWUSR);
   if (fd == -1) {
       printf("open failed: %d - %s\n", errno, strerror(errno));
       return -1;
   }
   for (i=0; i < pages; i++) {
       write(fd, buf, PAGE_SIZE);
   }
   close(fd);
   fd = open("big-data-file.bin", O_RDWR);
   printf("fd is %d\n", fd);
   while (loop_cnt < max_loops) {
       d = mmap(NULL, pages * PAGE_SIZE, PROT_WRITE|PROT_READ, MAP_FILE|MAP_PRIVATE, fd, 0);
       if (d == MAP_FAILED) {
           printf("mmap failed: %d - %s\n", errno, strerror(errno));
           return -1;
       }
       printf("Buffer is @ %p\n", d);
       for (i = 0; i < pages * PAGE_SIZE; i++) {
           sum += d[i];
           if ((i & (PAGE_SIZE-1)) == 0)
               d[i] = 42;
       }
       printf("Todal sum was %d. Loop count is %d\n", sum, loop_cnt++);
   }
   close(fd);
   return 0;
}

///ââ test script launching itâ
#!/bin/sh

if [ `id -u` -ne 0 ]; then
       echo NEED TO RUN THIS AS ROOT.; exit 1
fi
PID=$(echo $$)
echo PID detected as: $PID
mkdir /sys/fs/cgroup/memory/leaker
echo 536870912 > /sys/fs/cgroup/memory/leaker/memory.limit_in_bytes

echo leaker mem cgroup created, with `cat /sys/fs/cgroup/memory/leaker/memory.limit_in_bytes` bytes.
echo $PID > /sys/fs/cgroup/memory/leaker/cgroup.procs
echo Moved into the leaker cgroup.
ps -o cgroup $PID
sleep 15
echo Starting...
./leaker -p 10240 -c 100000

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