how to enable and disable dm verity on android devices?

In short I can't give you a solution to this yet.

However here are some useful hints: That's the error I got:

C:\Users\Test>adb remount
dm_verity is enabled on the system and vendor partitions.
Use "adb disable-verity" to disable verity.
If you do not, remount may succeed, however, you will still not be able to write to these volumes.
remount of system failed: Permission denied
remount failed

(^Some exact text is also important for people that search in google to find here^)

When reverse engineering/Decompiling '\sbin\adbd' with IDA Hex-rays I notice That the relevant adbd source code that outputs this error is in net:

void remount_service(int fd, void *cookie)
{
    char buffer[200];
    char prop_buf[PROPERTY_VALUE_MAX];

    bool system_verified = false, vendor_verified = false;
    property_get("partition.system.verified", prop_buf, "0");
    if (!strcmp(prop_buf, "1")) {
        system_verified = true;
    }

    property_get("partition.vendor.verified", prop_buf, "0");
    if (!strcmp(prop_buf, "1")) {
        vendor_verified = true;
    }

    if (system_verified || vendor_verified) {
        // Allow remount but warn of likely bad effects
        bool both = system_verified && vendor_verified;
        snprintf(buffer, sizeof(buffer),
                 "dm_verity is enabled on the %s%s%s partition%s.\n",
                 system_verified ? "system" : "",
                 both ? " and " : "",
                 vendor_verified ? "vendor" : "",
                 both ? "s" : "");
        write_string(fd, buffer);
        snprintf(buffer, sizeof(buffer),
                 "Use \"adb disable-verity\" to disable verity.\n"
                 "If you do not, remount may succeed, however, you will still "
                 "not be able to write to these volumes.\n");
        write_string(fd, buffer);
    }

    if (remount("/system", &system_ro)) {
        snprintf(buffer, sizeof(buffer), "remount of system failed: %s\n",strerror(errno));
        write_string(fd, buffer);
    }

    if (hasVendorPartition()) {
        if (remount("/vendor", &vendor_ro)) {
            snprintf(buffer, sizeof(buffer), "remount of vendor failed: %s\n",strerror(errno));
            write_string(fd, buffer);
        }
    }

    if (!system_ro && (!vendor_ro || !hasVendorPartition()))
        write_string(fd, "remount succeeded\n");
    else {
        write_string(fd, "remount failed\n");
    }

    adb_close(fd);
}

http://www.contrib.andrew.cmu.edu/~rjkohler/android-tools-5.0.1+git20141213/core/adb/remount_service.c btw the adb deamon I used to decomile is from Android 5.1.1.

So the essential points here are the partition.vendor.verified and partition.system.verified. If they set to "1" you'll get the error.

Well next will be to hunt down the why and how these are set ... and how to prevent that.

However all what adb remount does is remounting /system (and maybe /vendor). You can also do this your own:

adb shell su mount -o remount /system

That little line usually help me out and accomplished the same. The su - that'll do the trick. (But yes the su command will only be there if your device is 'rooted'.)

adb disable-verity will work only with adb version 1.0.33 and above. so upgrade your adb version