linux-kernel - Re: [PATCH 3/6] zram: introduce ZRAM

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Date:   Tue, 20 Nov 2018 11:46:59 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 3/6] zram: introduce ZRAM_IDLE flag

Hello,

On (11/16/18 16:20), Minchan Kim wrote:
[..]
> +static ssize_t idle_store(struct device *dev,
> +		struct device_attribute *attr, const char *buf, size_t len)
> +{
> +	struct zram *zram = dev_to_zram(dev);
> +	unsigned long nr_pages = zram->disksize >> PAGE_SHIFT;
> +	int index;
> +
> +	down_read(&zram->init_lock);
> +	if (!init_done(zram)) {
> +		up_read(&zram->init_lock);
> +		return -EINVAL;
> +	}
> +
> +	for (index = 0; index < nr_pages; index++) {
> +		zram_slot_lock(zram, index);
> +		if (!zram_allocated(zram, index))
> +			goto next;
> +
> +		zram_set_flag(zram, index, ZRAM_IDLE);
> +next:
> +		zram_slot_unlock(zram, index);
> +	}
> +
> +	up_read(&zram->init_lock);
> +
> +	return len;
> +}

This is one way of doing it.

The other one could, probabaly, be a bit more friendly to the cache
lines and CPU cycles. Basically, have a static timestamp variable,
which would keep the timestamp of last idle_store().

static idle_snapshot_ts;

static ssize_t idle_store(struct device *dev,
			  struct device_attribute *attr,
			  const char *buf, size_t len)
{
	idle_snapshot_ts = ktime();
}

And then in read_block_state() compare handle access time and
idle_snapshot_ts (if it's not 0). If the page was not modified/access
since the last idle_snapshot_ts (handle access time <= idle_snapshot_ts),
then it's idle, otherwise (handle access time > idle_snapshot_ts) it's
not idle.

Would this do the trick?

	-ss