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

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Date:   Thu, 29 Nov 2018 11:43:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Joey Pabalinas <joeypabalinas@...il.com>
Subject: Re: [PATCH v3 1/7] zram: fix lockdep warning of free block handling

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@...nel.org>

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

	-ss