linux-kernel - Re: [PATCH v8 01/17] zram: sleepable entry locking

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Message-ID: <20250227131253.T_S_Icyt@linutronix.de>
Date: Thu, 27 Feb 2025 14:12:53 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Yosry Ahmed <yosry.ahmed@...ux.dev>,
	Hillf Danton <hdanton@...a.com>, Kairui Song <ryncsn@...il.com>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 01/17] zram: sleepable entry locking

On 2025-02-27 22:04:16 [+0900], Sergey Senozhatsky wrote:
> On (25/02/27 21:42), Sergey Senozhatsky wrote:
> > > ach. Got it. What about
> > > 
> > > | static void zram_slot_lock_init(struct zram *zram, u32 index)
> > > | {
> > > | 	static struct lock_class_key __key;
> > > | 
> > > | 	lockdep_init_map(slot_dep_map(zram, index),
> > > | 			 "zram->table[index].lock",
> > > | 			 &__key, 0);
> > > | }
> > > 
> > > So every lock coming from zram belongs to the same class. Otherwise each
> > > lock coming from zram_slot_lock_init() would belong to a different class
> > > and for lockdep it would look like they are different locks. But they
> > > are used always in the same way.
> > 
> > I see.  I thought that they key was "shared" between zram meta table
> > entries because the key is per-zram device, which sort of made sense
> > (we can have different zram devices in a system - one swap, a bunch
> > mounted with various file-systems on them).

Yes. So usually you do spin_lock_init() and this creates a key at _this_
very position. So every lock initialized at this position shares the
same class/ the same pattern.

> So the lock class is registered dynamically for each zram device
> 
> zram_add()
> 	lockdep_register_key(&zram->lock_class);
> 
> and then we use that zram->lock_class to init zram->table entries.
> 
> We unregister the lock_class during each zram device destruction
> 
> zram_remove()
> 	lockdep_unregister_key(&zram->lock_class);
> 
> Does this still put zram->table entries into different lock classes?

You shouldn't need to register and unregister the lock_class. What you
do should match for instance j_trans_commit_map in fs/jbd2/journal.c or
__key in include/linux/rhashtable.h & lib/rhashtable.c.

At least based on my understanding so far.

Sebastian