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

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Message-ID: <zgaitute72xlzukunnhioqk4e2copgtkonxtn7iuvjq6gufwhr@yg4kry4wq3p2>
Date: Thu, 6 Feb 2025 17:17:41 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	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 (25/02/06 09:13), Sebastian Andrzej Siewior wrote:
> 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.

Correct.

> > > > 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.

Right, might_sleep() can be disabled, as far as I understand,
via CONFIG_DEBUG_ATOMIC_SLEEP, unlike WARN_ON_ONCE().  But I
can drop it and then just rely on might_sleep(), should be
enough.