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Documentation states that zRAM writeback feature supports only swap partitions as it's backing_dev. But I've successfully used a swap file too, by attaching it to a loop device

losetup /dev/loop0 /swapfile
cd /sys/block/zram0
echo /dev/loop0 > backing_dev
echo 8G > disksize
mkswap /dev/zram0
swapon /dev/zram0

After this, swapon -s shows zRAM device enabled, cat /sys/block/zram0/backing_dev returns /dev/loop0 and echo huge > /sys/block/zram0/writeback works with no problem, with cat /sys/block/zram0/bd_stats confirming that the write indeed succeeded

Can this be used, or does this method have some kind of nasty drawbacks?

Роман Мавроян
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1 Answers1

0

This is fine. On Android the writeback feature is enabled using loopback file:

static bool PrepareZramBackingDevice(off64_t size) {

    constexpr const char* file_path = "/data/per_boot/zram_swap";
    if (size == 0) return true;

    // Prepare target path
    unique_fd target_fd(TEMP_FAILURE_RETRY(open(file_path, O_RDWR | O_CREAT | O_CLOEXEC, 0600)));
    if (target_fd.get() == -1) {
        PERROR << "Cannot open target path: " << file_path;
        return false;
    }
    if (fallocate(target_fd.get(), 0, 0, size) < 0) {
        PERROR << "Cannot truncate target path: " << file_path;
        return false;
    }

    // Allocate loop device and attach it to file_path.
    LoopControl loop_control;
    std::string loop_device;
    if (!loop_control.Attach(target_fd.get(), 5s, &loop_device)) {
        return false;
    }

    ConfigureIoScheduler(loop_device);

    ConfigureQueueDepth(loop_device, "/");

    // set block size & direct IO
    unique_fd loop_fd(TEMP_FAILURE_RETRY(open(loop_device.c_str(), O_RDWR | O_CLOEXEC)));
    if (loop_fd.get() == -1) {
        PERROR << "Cannot open " << loop_device;
        return false;
    }
    if (!LoopControl::EnableDirectIo(loop_fd.get())) {
        return false;
    }

    return InstallZramDevice(loop_device);
}

Source: https://android.googlesource.com/platform/system/core/+/refs/tags/android-12.1.0_r2/fs_mgr/fs_mgr.cpp#2051

Max_Payne
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