linux-kernel - Re: [PATCH] mm: extend zero pages to same element pages for zram

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Date:   Tue, 7 Feb 2017 11:24:40 +0800
From:   zhouxianrong <zhouxianrong@...wei.com>
To:     Minchan Kim <minchan@...nel.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <akpm@...ux-foundation.org>, <sergey.senozhatsky@...il.com>,
        <willy@...radead.org>, <iamjoonsoo.kim@....com>,
        <ngupta@...are.org>, <Mi.Sophia.Wang@...wei.com>,
        <zhouxiyu@...wei.com>, <weidu.du@...wei.com>,
        <zhangshiming5@...wei.com>, <won.ho.park@...wei.com>
Subject: Re: [PATCH] mm: extend zero pages to same element pages for zram



On 2017/2/7 10:54, Minchan Kim wrote:
> On Tue, Feb 07, 2017 at 10:20:57AM +0800, zhouxianrong wrote:
>
> < snip >
>
>>>> 3. the below should be modified.
>>>>
>>>> static inline bool zram_meta_get(struct zram *zram)
>>>> @@ -495,11 +553,17 @@ static void zram_meta_free(struct zram_meta *meta, u64 disksize)
>>>>
>>>> 	/* Free all pages that are still in this zram device */
>>>> 	for (index = 0; index < num_pages; index++) {
>>>> -		unsigned long handle = meta->table[index].handle;
>>>> +		unsigned long handle;
>>>> +
>>>> +		bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
>>>> +		handle = meta->table[index].handle;
>>>>
>>>> -		if (!handle)
>>>> +		if (!handle || zram_test_flag(meta, index, ZRAM_SAME)) {
>>>> +			bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
>>>> 			continue;
>>>> +		}
>>>>
>>>> +		bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
>>>> 		zs_free(meta->mem_pool, handle);
>>>
>>> Could you explain why we need this modification?
>>>
>>>> 	}
>>>>
>>>> @@ -511,7 +575,7 @@ static void zram_meta_free(struct zram_meta *meta, u64 disksize)
>>>> static struct zram_meta *zram_meta_alloc(char *pool_name, u64 disksize)
>>>> {
>>>> 	size_t num_pages;
>>>> -	struct zram_meta *meta = kmalloc(sizeof(*meta), GFP_KERNEL);
>>>> +	struct zram_meta *meta = kzalloc(sizeof(*meta), GFP_KERNEL);
>>>
>>> Ditto
>>>
>>>>
>>>>
>>>
>>> .
>>>
>>
>> because of union of handle and element, i think a non-zero element (other than handle) is prevented from freeing.
>> if zram_meta_get was modified, zram_meta_alloc did so.
>
> Right. Thanks but I don't see why we need the locking in there and modification of
> zram_meta_alloc.
>
> Isn't it enough with this?

i am afraid someone do reset_store, so did lock.

yes, i am wrong, zram_meta_alloc should not be modified here. because meta->table has already cleared

	meta->table = vzalloc(num_pages * sizeof(*meta->table));




>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index c20b05a84f21..a25d34a8af19 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -425,8 +425,11 @@ static void zram_meta_free(struct zram_meta *meta, u64 disksize)
>  	/* Free all pages that are still in this zram device */
>  	for (index = 0; index < num_pages; index++) {
>  		unsigned long handle = meta->table[index].handle;
> -
> -		if (!handle)
> +		/*
> +		 * No memory is allocated for same element filled pages.
> +		 * Simply clear same page flag.
> +		 */
> +		if (!handle || zram_test_flag(meta, index, ZRAM_SAME))
>  			continue;
>
>  		zs_free(meta->mem_pool, handle);
>
> .
>