zram: introduce compressed data writeback

zram: introduce compressed data writeback

Patch series "zram: introduce compressed data writeback", v2.

As writeback becomes more common there is another shortcoming that needs
to be addressed - compressed data writeback.  Currently zram does
uncompressed data writeback which is not optimal due to potential CPU and
battery wastage.  This series changes suboptimal uncompressed writeback to
a more optimal compressed data writeback.

This patch (of 7):

zram stores all written back slots raw, which implies that during
writeback zram first has to decompress slots (except for ZRAM_HUGE slots,
which are raw already).  The problem with this approach is that not every
written back page gets read back (either via read() or via page-fault),
which means that zram basically wastes CPU cycles and battery
decompressing such slots.  This changes with introduction of decompression
on demand, in other words decompression on read()/page-fault.

One caveat of decompression on demand is that async read is completed in
IRQ context, while zram decompression is sleepable.  To workaround this,
read-back decompression is offloaded to a preemptible context - system
high-prio work-queue.

At this point compressed writeback is still disabled, a follow up patch
will introduce a new device attribute which will make it possible to
toggle compressed writeback per-device.

[senozhatsky@chromium.org: rewrote original implementation]
Link: https://lkml.kernel.org/r/20251201094754.4149975-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20251201094754.4149975-2-senozhatsky@chromium.org
Signed-off-by: Richard Chang <richardycc@google.com>
Co-developed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Minchan Kim <minchan@google.com>
Suggested-by: Brian Geffon <bgeffon@google.com>
Cc: David Stevens <stevensd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>