linux-kernel - Re: [PATCH] zswap: do not allocate from atomic pool

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Date:   Mon, 21 Nov 2022 17:56:19 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        Nhat Pham <nphamcs@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] zswap: do not allocate from atomic pool

On Tue, 22 Nov 2022 10:33:38 +0900 Sergey Senozhatsky <senozhatsky@...omium.org> wrote:

> zswap_frontswap_load() should be called from preemptible
> context (we even call mutex_lock() there) and it does not
> look like we need to do GFP_ATOMIC allocaion for temp
> buffer there. Use GFP_KERNEL instead.
> 
> ...
>
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1314,7 +1314,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>  	}
>  
>  	if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> -		tmp = kmalloc(entry->length, GFP_ATOMIC);
> +		tmp = kmalloc(entry->length, GFP_KERNEL);
>  		if (!tmp) {
>  			ret = -ENOMEM;
>  			goto freeentry;

It seems strange to do

	if (! can sleep)
		do something which can sleep

or am I misreading the intent of zpool_driver.sleep_mapped?  If so,
perhaps some explanatory code comments will help.