linux-kernel - Re: [PATCH v7 11/17] zsmalloc: make zspage lock preemptible

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Message-ID: <Z7jZfIaqMRTLYp23@google.com>
Date: Fri, 21 Feb 2025 19:52:28 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Hillf Danton <hdanton@...a.com>, Kairui Song <ryncsn@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 11/17] zsmalloc: make zspage lock preemptible

On Fri, Feb 21, 2025 at 06:38:04PM +0900, Sergey Senozhatsky wrote:
> In order to implement preemptible object mapping we need a zspage lock
> that satisfies several preconditions:
> - it should be reader-write type of a lock
> - it should be possible to hold it from any context, but also being
>   preemptible if the context allows it
> - we never sleep while acquiring but can sleep while holding in read
>   mode
> 
> An rwsemaphore doesn't suffice, due to atomicity requirements, rwlock
> doesn't satisfy due to reader-preemptability requirement.  It's also
> worth to mention, that per-zspage rwsem is a little too memory heavy
> (we can easily have double digits megabytes used only on rwsemaphores).
> 
> Switch over from rwlock_t to a atomic_t-based implementation of a
> reader-writer semaphore that satisfies all of the preconditions.
> 
> The spin-lock based zspage_lock is suggested by Hillf Danton.
> 
> Suggested-by: Hillf Danton <hdanton@...a.com>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>

FWIW the code looks correct to me, but I don't trust my locking
knowledge enough to give a Reviewed-by :)