Super slow subpage IO on zram block devices?

[Qu Wenruo <wqu@suse.com>]

Hi,

Not sure if it's a regression or not, but on the latest 7.0-rc kernels, 
the discard is unacceptable slow on zram devices, if the block size is 
smaller than page size:

  # uname -a
  Linux btrfs-aarch64 7.0.0-rc5-custom-64k+ #7 SMP PREEMPT_DYNAMIC Tue 
Mar 24 13:38:13 ACDT 2026 aarch64 GNU/Linux

  # getconf PAGESIZE
  65536

  # zramctl
  NAME       ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
  /dev/zram0 zstd          128M   0B    0B    0B

  # time blkdiscard -p 4k /dev/zram0
  [ It takes too long time that exceed my patient ]

After a reboot, if discarding using page size (64K), it finishes in seconds:

  # time blkdiscard -p 64k /dev/zram0

  real	0m0.024s
  user	0m0.004s
  sys	0m0.011s

I understand that using block size smaller than page size would cause 
some overhead, but seemingly eternal is beyond my expectation of "overhead".

Is this a known bug or something else?

Thanks,
Qu