linux-kernel - Re: [PATCHv1 3/6] zsmalloc: make zspage lock preemptible

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Message-ID: <vrcthiy3pkwpl56a56f4bepiafrc6mlok42u4micvcfztnoubk@t5q3gsjreqb6>
Date: Wed, 29 Jan 2025 20:25:27 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Minchan Kim <minchan@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, Nhat Pham <nphamcs@...il.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCHv1 3/6] zsmalloc: make zspage lock preemptible

JFI

On (25/01/29 15:43), Sergey Senozhatsky wrote:
> +static void zspage_lock_init(struct zspage *zspage)
> +{
> +	atomic_set(&zspage->lock, ZS_PAGE_UNLOCKED);
> +}
> +
> +/*
> + * zspage lock permits preemption on the reader-side (there can be multiple
> + * readers).  Writers (exclusive zspage ownership), on the other hand, are
> + * always run in atomic context and cannot spin waiting for a (potentially
> + * preempted) reader to unlock zspage.  This, basically, means that writers
> + * can only call write-try-lock and must bail out if it didn't succeed.
> + *
> + * At the same time, writers cannot reschedule under zspage write-lock,
> + * so readers can spin waiting for the writer to unlock zspage.
> + */
> +static void zspage_read_lock(struct zspage *zspage)
> +{
> +	atomic_t *lock = &zspage->lock;
> +	int old;
> +
> +	while (1) {
> +		old = atomic_read(lock);
> +		if (old == ZS_PAGE_WRLOCKED) {
> +			cpu_relax();
> +			continue;
> +		}
> +
> +		if (atomic_try_cmpxchg(lock, &old, old + 1))
> +			return;
> +
> +		cpu_relax();
> +	}
> +}
> +
> +static void zspage_read_unlock(struct zspage *zspage)
> +{
> +	atomic_dec(&zspage->lock);
> +}
> +
> +static int zspage_try_write_lock(struct zspage *zspage)
> +{
> +	atomic_t *lock = &zspage->lock;
> +	int old = ZS_PAGE_UNLOCKED;
> +
> +	preempt_disable();
> +	if (atomic_try_cmpxchg(lock, &old, ZS_PAGE_WRLOCKED))
> +		return 1;
> +
> +	preempt_enable();
> +	return 0;
> +}
> +
> +static void zspage_write_unlock(struct zspage *zspage)
> +{
> +	atomic_set(&zspage->lock, ZS_PAGE_UNLOCKED);
> +	preempt_enable();
> +}

Below is what I currently have based on a (private) feedback from
Uros.  No functional changes.

I think I'll wait before sending out v2, based on the fact that
this is not a fix or a functional change (WR-locked case is relatively
uncommon; it's only when we map an object from a page that is currently
either under migration or compaction.)

---

static void zspage_lock_init(struct zspage *zspage)
{
	atomic_set(&zspage->lock, ZS_PAGE_UNLOCKED);
}

static void zspage_read_lock(struct zspage *zspage)
{
	atomic_t *lock = &zspage->lock;
	int old = atomic_read(lock);

	do {
		if (old == ZS_PAGE_WRLOCKED) {
			cpu_relax();
			old = atomic_read(lock);
			continue;
		}
	} while (!atomic_try_cmpxchg(lock, &old, old + 1));
}

static void zspage_read_unlock(struct zspage *zspage)
{
	atomic_dec(&zspage->lock);
}

static bool zspage_try_write_lock(struct zspage *zspage)
{
	atomic_t *lock = &zspage->lock;
	int old = ZS_PAGE_UNLOCKED;

	preempt_disable();
	if (atomic_try_cmpxchg(lock, &old, ZS_PAGE_WRLOCKED))
		return true;

	preempt_enable();
	return false;
}

static void zspage_write_unlock(struct zspage *zspage)
{
	atomic_set(&zspage->lock, ZS_PAGE_UNLOCKED);
	preempt_enable();
}