Re: [PATCH v3 1/7] zram: fix lockdep warning of free block handling

From: Sergey Senozhatsky
Date: Wed Nov 28 2018 - 21:43:41 EST

On (11/27/18 14:54), Minchan Kim wrote:
> With writeback feature, zram_slot_free_notify could be called
> in softirq context by end_swap_bio_read. However, bitmap_lock
> is not aware of that so lockdep yell out. Thanks.
> get_entry_bdev
> spin_lock(bitmap->lock);
> irq
> softirq
> end_swap_bio_read
> zram_slot_free_notify
> zram_slot_lock <-- deadlock prone
> zram_free_page
> put_entry_bdev
> spin_lock(bitmap->lock); <-- deadlock prone
> With akpm's suggestion(i.e. bitmap operation is already atomic),
> we could remove bitmap lock. It might fail to find a empty slot
> if serious contention happens. However, it's not severe problem
> because huge page writeback has already possiblity to fail if there
> is severe memory pressure. Worst case is just keeping
> the incompressible in memory, not storage.
> The other problem is zram_slot_lock in zram_slot_slot_free_notify.
> To make it safe is this patch introduces zram_slot_trylock where
> zram_slot_free_notify uses it. Although it's rare to be contented,
> this patch adds new debug stat "miss_free" to keep monitoring
> how often it happens.
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>