linux-kernel - Re: [PATCH v8 14/17] zram: permit reclaim in zstd custom allocator

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Message-ID: <vh3uzody42dfcfduwrhnha3wrjm6wi7awwferzed35zuodn4zu@txo4zpftf2uq>
Date: Tue, 25 Feb 2025 13:42:55 +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>, 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 14/17] zram: permit reclaim in zstd custom allocator

On (25/02/24 10:10), Sebastian Andrzej Siewior wrote:
> On 2025-02-22 07:25:45 [+0900], Sergey Senozhatsky wrote:
> >  static void *zstd_custom_alloc(void *opaque, size_t size)
> >  {
> > -	if (!preemptible())
> > +	/* Technically this should not happen */
> > +	if (WARN_ON_ONCE(!preemptible()))
> >  		return kvzalloc(size, GFP_ATOMIC);
> 
> This check works only on preemptible kernels.

I'm not sure this is true.

> If you run this on !PREEMPTIBLE kernels, preemptible() reports always 0
> so that WARNING will always trigger there.

I thought that preemptible() depends on PREEMPT_COUNT, not on
PREEMPTIBLE, because even on !PREEMPTIBLE preempt-count still
holds hard/soft irq counts, etc.

I ran CONFIG_PREEMPT_NONE=y zram-zstd tests and didn't see any
warnings.