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

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Date:	Tue, 24 Nov 2015 16:56:33 +0900
From:	Kyeongdon Kim <kyeongdon.kim@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	minchan@...nel.org, ngupta@...are.org,
	sergey.senozhatsky.work@...il.com, linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH v3] zram: try vmalloc() after kmalloc()

Hello Andrew,
On 2015-11-24 오전 7:52, Andrew Morton wrote:
> 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.

Sorry for the delay in replying,
I just tried to remove that warning message. If there are more rightable
gfp flags(like a code in Minchan's patch), we can use it.

Thanks,
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