slot_free() basically completely resets the slots by clearing all of
its flags and attributes. While zram_writeback_complete() restores
some of flags back (those that are necessary for async read
decompression) we still lose a lot of slot's metadata. For example,
slot's ac-time, or ZRAM_INCOMPRESSIBLE.
More importantly, restoring flags/attrs requires extra attention as
some of the flags are directly affecting zram device stats. And the
original code did not pay that attention. Namely ZRAM_HUGE slots
handling in zram_writeback_complete(). The call to slot_free() would
decrement ->huge_pages, however when zram_writeback_complete() restored
the slot's ZRAM_HUGE flag, it would not get reflected in an incremented
->huge_pages. So when the slot would finally get freed, slot_free()
would decrement ->huge_pages again, leading to underflow.
Fix this by open-coding the required memory free and stats updates in
zram_writeback_complete(), rather than calling the destructive
slot_free(). Since we now preserve the ZRAM_HUGE flag on written-back
slots (for the deferred decompression path), we also update slot_free()
to skip decrementing ->huge_pages if ZRAM_WB is set.
Link: https://lkml.kernel.org/r/20260320023143.2372879-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20260319034912.1894770-1-senozhatsky@chromium.org
Fixes: d38fab605c ("zram: introduce compressed data writeback")
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Richard Chang <richardycc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
b0377ee804