linux-kernel - Re: [PATCHv4 01/17] zram: switch to non-atomic entry locking

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Message-ID: <20250206081326.xMa5Z5oJ@linutronix.de>
Date: Thu, 6 Feb 2025 09:13:26 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Hillf Danton <hdanton@...a.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCHv4 01/17] zram: switch to non-atomic entry locking

On 2025-02-06 16:47:02 [+0900], Sergey Senozhatsky wrote:
> zram is atomic right now, e.g.
> 
> zram_read()
> 	lock entry by index   # disables preemption
> 	map zsmalloc entry    # possibly memcpy
> 	decompress
> 	unmap zsmalloc
> 	unlock entry          # enables preemption
> 
> That's a pretty long time to keep preemption disabled (e.g. using slow
> algorithm like zstd or deflate configured with high compression levels).
> Apart from that that, difficult to use async algorithms, which can
> e.g. wait for a H/W to become available, or algorithms that might want
> to allocate memory internally during compression/decompression, e.g.
> zstd).
> 
> Entry lock is not the only lock in zram currently that makes it
> atomic, just one of.

Okay. So there are requirements for the sleeping lock. A mutex isn't
fitting the requirement because it is too large I guess.

> > > static void zram_slot_lock(struct zram *zram, u32 index)
> > > {
> > >         unsigned long *lock = &zram->table[index].flags;
> > > 
> > >         WARN_ON_ONCE(!preemptible());
> > 
> > you want might_sleep() here instead. preemptible() works only on
> > preemptible kernels. And might_sleep() is already provided by
> > wait_on_bit_lock(). So this can go.
> 
> wait_on_bit_lock() has might_sleep().

My point exactly. This makes the WARN_ON_ONCE() obsolete.

Sebastian