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

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Date:   Thu, 24 Nov 2022 12:32:45 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH] zswap: do not allocate from atomic pool

On (22/11/22 12:30), Sergey Senozhatsky 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. The same applies to zswap_writeback_entry().
> 
> Use GFP_KERNEL for temporary buffer allocation in both
> cases.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Nhat Pham <nphamcs@...il.com>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> ---

Folks, how do we want to proceed with this? One of the hunks here
conflicts with https://lore.kernel.org/lkml/20221119001536.2086599-2-nphamcs@gmail.com/

Do we want to remove conflicting hunk from "[PATCH 1/6] zswap: fix writeback
lock ordering for zsmalloc" and pick this patch up?

> diff --git a/mm/zpool.c b/mm/zpool.c
> index 68facc193496..f46c0d5e766c 100644
> --- a/mm/zpool.c
> +++ b/mm/zpool.c
> @@ -387,6 +387,13 @@ bool zpool_evictable(struct zpool *zpool)
>   * zpool_can_sleep_mapped - Test if zpool can sleep when do mapped.
>   * @zpool:	The zpool to test
>   *
> + * Some allocators enter non-preemptible context in ->map() callback (e.g.
> + * disable pagefaults) and exit that context in ->unmap(), which limits what
> + * we can do with the mapped object. For instance, we cannot wait for
> + * asynchronous crypto API to decompress such an object or take mutexes
> + * since those will call into the scheduler. This function tells us whether
> + * we use such an allocator.
> + *
>   * Returns: true if zpool can sleep; false otherwise.
>   */
>  bool zpool_can_sleep_mapped(struct zpool *zpool)
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 2d48fd59cc7a..3019f0bde194 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -958,7 +958,7 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
>  	};
>  
>  	if (!zpool_can_sleep_mapped(pool)) {
> -		tmp = kmalloc(PAGE_SIZE, GFP_ATOMIC);
> +		tmp = kmalloc(PAGE_SIZE, GFP_KERNEL);
>  		if (!tmp)
>  			return -ENOMEM;
>  	}
> @@ -1311,7 +1311,7 @@ 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;
> -- 
> 2.38.1.584.g0f3c55d4c2-goog
>