QUESTION: Any way to release OS memory after files getting removed from the zram devices?

[amit-gshe]

I removed all files from the zram device, but it still occupying muth of memory. How to release these memory back to OS?

[nabijaczleweli]

Probably blkdiscard for the raw blockdev and fstrim for the filesystem? Try running fstrim -v /mnt/zram2.

[amit-gshe]

I tried triming but it seems not work:

[nabijaczleweli]

I can repro this, fstrim and blkdiscard don't seem to do anything (except zeroing the data):

/sys/block/zram0# tee disksize
4000000000
4000000000
/sys/block/zram0# mkfs.ext4 /dev/zram0
mke2fs 1.47.0 (5-Feb-2023)
Discarding device blocks: done
Creating filesystem with 976563 4k blocks and 244320 inodes
Filesystem UUID: a00a7e82-d052-4ce9-9196-4db4539c560f
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

/sys/block/zram0# mount /dev/zram0 trace
/sys/block/zram0$ cd trace/
/sys/block/zram0/trace$ l
total 16k
drwx------ 2 root root 16k 12-24 15:28 lost+found
/sys/block/zram0/trace$ zramctl --output-all
NAME       DISKSIZE DATA COMPR ALGORITHM STREAMS ZERO-PAGES TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT
/dev/zram0     3.7G   2M 39.9K lzo-rle        24          0  100K        0B     100K       0B /sys/devices/virtual/block/zram0/trace
/sys/block/zram0/trace# head -c 500M < /dev/urandom | sudo tee rand > /dev/null
/sys/block/zram0/trace$ zramctl --output-all
NAME       DISKSIZE  DATA COMPR ALGORITHM STREAMS ZERO-PAGES  TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT
/dev/zram0     3.7G  430M  428M lzo-rle        24          0 428.1M        0B   428.1M       0B /sys/devices/virtual/block/zram0/trace
/sys/block/zram0/trace# rm rand
/sys/block/zram0/trace$ zramctl --output-all
NAME       DISKSIZE  DATA COMPR ALGORITHM STREAMS ZERO-PAGES  TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT
/dev/zram0     3.7G  430M  428M lzo-rle        24          1 428.2M        0B   428.2M       0B /sys/devices/virtual/block/zram0/trace
/sys/block/zram0/trace# fstrim -v $PWD
/sys/block/zram0/trace: 3.2 GiB (3405623296 bytes) trimmed
/sys/block/zram0/trace$ zramctl --output-all
NAME       DISKSIZE  DATA COMPR ALGORITHM STREAMS ZERO-PAGES  TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT
/dev/zram0     3.7G  430M  428M lzo-rle        24          1 428.2M        0B   428.2M       0B /sys/devices/virtual/block/zram0/trace
/sys/block/zram0/trace$ cd ..
/sys/block/zram0# umount trace
/sys/block/zram0$ zramctl --output-all
NAME       DISKSIZE   DATA COMPR ALGORITHM STREAMS ZERO-PAGES  TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT
/dev/zram0     3.7G 430.1M  428M lzo-rle        24          3 428.2M        0B   428.2M       0B
/sys/block/zram0# blkdiscard /dev/zram0
blkdiscard: /dev/zram0 contains existing file system (ext4).
blkdiscard: This is destructive operation, data will be lost! Use the -f option to override.
/sys/block/zram0# blkdiscard /dev/zram0 -f
blkdiscard: Operation forced, data will be lost!
/sys/block/zram0$ zramctl --output-all
NAME       DISKSIZE DATA COMPR ALGORITHM STREAMS ZERO-PAGES TOTAL MEM-LIMIT MEM-USED MIGRATED MOUNTPOINT
/dev/zram0     3.7G   0B    0B lzo-rle        24          0    0B        0B   428.2M       0B

Except blkdiscard changes TOTAL... but doesn't change MEM-USED. So. idk. This seems like a linux bug.