linux-kernel - Re: [PATCH v3] zram: try vmalloc() after kmalloc()

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Date:	Mon, 23 Nov 2015 14:52:26 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kyeongdon Kim <kyeongdon.kim@....com>
Cc:	<minchan@...nel.org>, <ngupta@...are.org>,
	<sergey.senozhatsky.work@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] zram: try vmalloc() after kmalloc()

On Mon, 23 Nov 2015 15:21:15 +0900 Kyeongdon Kim <kyeongdon.kim@....com> wrote:

> When we're using LZ4 multi compression streams for zram swap,
> we found out page allocation failure message in system running test.
> That was not only once, but a few(2 - 5 times per test).
> Also, some failure cases were continually occurring to try allocation
> order 3.
> 
> In order to make parallel compression private data, we should call
> kzalloc() with order 2/3 in runtime(lzo/lz4). But if there is no order
>  2/3 size memory to allocate in that time, page allocation fails.
> This patch makes to use vmalloc() as fallback of kmalloc(), this
> prevents page alloc failure warning.
> 
> After using this, we never found warning message in running test, also
> It could reduce process startup latency about 60-120ms in each case.
> 
> ...
>
> --- a/drivers/block/zram/zcomp_lz4.c
> +++ b/drivers/block/zram/zcomp_lz4.c
> @@ -10,17 +10,25 @@
>  #include <linux/kernel.h>
>  #include <linux/slab.h>
>  #include <linux/lz4.h>
> +#include <linux/vmalloc.h>
> +#include <linux/mm.h>
>  
>  #include "zcomp_lz4.h"
>  
>  static void *zcomp_lz4_create(void)
>  {
> -	return kzalloc(LZ4_MEM_COMPRESS, GFP_KERNEL);
> +	void *ret;
> +
> +	ret = kzalloc(LZ4_MEM_COMPRESS,
> +			__GFP_NORETRY|__GFP_NOWARN|__GFP_NOMEMALLOC);
> +	if (!ret)
> +		ret = vzalloc(LZ4_MEM_COMPRESS);
> +	return ret;
>  }

What's the reasoning behind the modification to the gfp flags?

It clears __GFP_FS, __GFP_IO and even __GFP_WAIT.  I suspect the latter
two (at least) can be retained.  And given that vmalloc() uses
GFP_KERNEL, what's the point in clearing those flags for the kmalloc()
case?

If this change (or something like it) remains in place, it should have
a comment which fully explains the reasons, please.

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