[PATCH] zram: panic when use ext4 over zram

[caiqingfu <baicaiaichibaicai@xxxxxxxxx>]

> What's your use-case, what apps you run and what data is stored
> on zram disk?

The steps to reproduce are as follows:

1. force slow path,then re-build kernel

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 0ca6d55c9917..29ac52a4f2e7 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1683,12 +1683,14 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
         * if we have a 'non-null' handle here then we are coming
         * from the slow path and handle has already been allocated.
         */
+    /*
        if (IS_ERR_VALUE(handle))
                handle = zs_malloc(zram->mem_pool, comp_len,
                                __GFP_KSWAPD_RECLAIM |
                                __GFP_NOWARN |
                                __GFP_HIGHMEM |
                                __GFP_MOVABLE);
+        */
        if (IS_ERR_VALUE(handle)) {
                zcomp_stream_put(zram->comps[ZRAM_PRIMARY_COMP]);
                atomic64_inc(&zram->stats.writestall);

2. create an arm64 virtual machine

qemu-system-aarch64 -M virt -cpu cortex-a57 -smp 4 -m 1024 -kernel kernel-build/arch/arm64/boot/Image \ 
    -nographic -append "console=ttyAMA0,115200 root=/dev/vda rw" \ 
    -drive file=$(IMGFILE),format=raw -nic user,hostfwd=tcp::60022-:22

3. after the virtual machine is started, create zram

cat /sys/class/zram-control/hot_add
echo 524288000 > /sys/devices/virtual/block/zram0/disksize
echo 524288000 > /sys/devices/virtual/block/zram1/disksize
mkfs.ext4 -O ^has_journal -b 4096 -F -L TEMP -m 0 /dev/zram0
mkdir /tmp/zram
mount -t ext4 -o errors=continue,nosuid,nodev,noatime /dev/zram0 /tmp/zram
mkswap /dev/zram1
swapon /dev/zram1
echo 100 > /proc/sys/vm/swappiness
mkdir -p /tmp/zram/stressTest


4. build demo app and run again and again

demo app :

#include <stdio.h>
#include <sys/mman.h>
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <pthread.h>
#include <errno.h>
#include <string.h>
#include <syscall.h>

#define THREAD_NUM      400
#define BUFF_SIZE       128*1024

const static char * basePath = "/tmp/zram/stressTest";
static unsigned char buf[BUFF_SIZE];

void * cb(void * arg){

    char path[256];
    unsigned int tid = (unsigned int)(syscall(SYS_gettid));
    unsigned int start = ((unsigned int)random())%(BUFF_SIZE - 1);
    unsigned int msleep = (((unsigned int)random())%10) * 1000;
    void * p;
    int i;

    snprintf(path,sizeof(path) - 1,"%s/%u",basePath,tid);

    FILE * fp = fopen(path,"a");

    if (NULL == fp) {
        fprintf(stderr,"%u open file '%s' failure '%s'...\n",tid,path,strerror(errno));
        return NULL;
    }

    start = ((unsigned int)random())%(BUFF_SIZE - 1);
    fwrite(&buf[start],1,BUFF_SIZE - start,fp);
    fflush(fp);
    fclose(fp);
    sync();
    usleep(msleep);

    return NULL;
}

int main(int argc, char *argv[]) {

    pthread_t pid[THREAD_NUM];
    int i;

    fprintf(stderr,"build @ %s\n",__TIME__);

    FILE * fp = fopen("/dev/urandom","ro");
    fread(buf,BUFF_SIZE,1,fp);
    fclose(fp);

    fprintf(stderr,"read urandom size '%d'\n",BUFF_SIZE);
    srandom((unsigned)time(NULL));

    for (i = 0;i < THREAD_NUM;i++) {
        pthread_create(&pid[i],NULL,cb,NULL);
    }

    fprintf(stderr,"create over\n");

    for (i = 0;i < THREAD_NUM;i++) {
        pthread_join(pid[i],NULL);
    }

    return 0;
}


run it

while true;do rm /tmp/zram_vsd/stressTest/* &> /dev/null;./a.out &> /dev/null ;echo again...;done