linux-kernel - Re: [PATCH] zram: idle writeback fixes and cleanup

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Message-ID: <20181227022624.GA30091@jagdpanzerIV>
Date:   Thu, 27 Dec 2018 11:26:24 +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>,
        joaodias@...gle.com, srnvs@...gle.com
Subject: Re: [PATCH] zram: idle writeback fixes and cleanup

On (12/24/18 12:35), Minchan Kim wrote:
[..]
> @@ -645,10 +680,13 @@ static ssize_t writeback_store(struct device *dev,
>  		bvec.bv_len = PAGE_SIZE;
>  		bvec.bv_offset = 0;
>  
> -		if (zram->stop_writeback) {
> +		spin_lock(&zram->wb_limit_lock);
> +		if (zram->wb_limit_enable && !zram->bd_wb_limit) {
> +			spin_unlock(&zram->wb_limit_lock);
>  			ret = -EIO;
>  			break;
>  		}
> +		spin_unlock(&zram->wb_limit_lock);
[..]
> @@ -732,11 +771,10 @@ static ssize_t writeback_store(struct device *dev,
>  		zram_set_element(zram, index, blk_idx);
>  		blk_idx = 0;
>  		atomic64_inc(&zram->stats.pages_stored);
> -		if (atomic64_add_unless(&zram->stats.bd_wb_limit,
> -					-1 << (PAGE_SHIFT - 12), 0)) {
> -			if (atomic64_read(&zram->stats.bd_wb_limit) == 0)
> -				zram->stop_writeback = true;
> -		}
> +		spin_lock(&zram->wb_limit_lock);
> +		if (zram->wb_limit_enable && zram->bd_wb_limit > 0)
> +			zram->bd_wb_limit -=  1UL << (PAGE_SHIFT - 12);
> +		spin_unlock(&zram->wb_limit_lock);

Do we really need ->wb_limit_lock spinlock? We kinda punch it twice
in this loop. If someone clears ->wb_limit_enable somewhere in between
then the worst thing to happen is that we will just write extra page
to the backing device; not a very big deal to me. Am I missing
something?

	-ss