linux-kernel - Re: [PATCH] zram/zcomp: use GFP

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Date:	Mon, 23 Nov 2015 15:18:12 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org,
	sergey.senozhatsky.work@...il.com
Subject: Re: [PATCH] zram/zcomp: use GFP_NOIO to allocate streams

On Mon, 23 Nov 2015 22:27:59 +0900 Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:

> We can end up allocating a new compression stream with GFP_KERNEL
> from within the IO path, which may result is nested (recursive) IO
> operations. That can introduce problems if the IO path in question
> is a reclaimer, holding some locks that will deadlock nested IOs.
> 
> Allocate streams and working memory using GFP_NOIO flag, forbidding
> recursive IO and FS operations.
> 
> ...
>
> --- a/drivers/block/zram/zcomp.c
> +++ b/drivers/block/zram/zcomp.c
> @@ -76,7 +76,7 @@ static void zcomp_strm_free(struct zcomp *comp, struct zcomp_strm *zstrm)
>   */
>  static struct zcomp_strm *zcomp_strm_alloc(struct zcomp *comp)
>  {
> -	struct zcomp_strm *zstrm = kmalloc(sizeof(*zstrm), GFP_KERNEL);
> +	struct zcomp_strm *zstrm = kmalloc(sizeof(*zstrm), GFP_NOIO);
>  	if (!zstrm)
>  		return NULL;
>  
> @@ -85,7 +85,7 @@ static struct zcomp_strm *zcomp_strm_alloc(struct zcomp *comp)
>  	 * allocate 2 pages. 1 for compressed data, plus 1 extra for the
>  	 * case when compressed size is larger than the original one
>  	 */
> -	zstrm->buffer = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, 1);
> +	zstrm->buffer = (void *)__get_free_pages(GFP_NOIO | __GFP_ZERO, 1);

OK.

> --- a/drivers/block/zram/zcomp_lz4.c
> +++ b/drivers/block/zram/zcomp_lz4.c
> @@ -20,10 +20,13 @@ static void *zcomp_lz4_create(void)
>  	void *ret;
>  
>  	ret = kzalloc(LZ4_MEM_COMPRESS,
> -			__GFP_NORETRY|__GFP_NOWARN|__GFP_NOMEMALLOC);
> -	if (!ret)
> -		ret = vzalloc(LZ4_MEM_COMPRESS);
> -	return ret;
> +			__GFP_NORETRY | __GFP_NOWARN | __GFP_NOMEMALLOC);

But here we've still lost __GFP_RECLAIM, unnecessarily.  And it's quite
unclear why __GFP_NORETRY and __GFP_NOMEMALLOC are being used.

IOW, why not simply use (GFP_NOIO|__GFP_NOWARN)?


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