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

[Joonsoo Kim]

On Mon, Jan 23, 2017 at 01:03:47PM +0900, Sergey Senozhatsky wrote:
> On (01/23/17 11:58), Joonsoo Kim wrote:
> > Hello,
> > 
> > On Sun, Jan 22, 2017 at 10:58:38AM +0800, zhouxianrong wrote:
> > > 1. memset is just set a int value but i want to set a long value.
> > 
> > Sorry for late review.
> > 
> > Do we really need to set a long value? I cannot believe that
> > long value is repeated in the page. Value repeatition is
> > usually done by value 0 or 1 and it's enough to use int. And, I heard
> > that value 0 or 1 is repeated in Android. Could you check the distribution
> > of the value in the same page?
> 
> Hello Joonsoo,
> 
> thanks for taking a look and for bringing this question up.
> so I kinda wanted to propose union of `ulong handle' with `uint element'
> and switching to memset(), but I couldn't figure out if that change would
> break detection of some patterns.
> 
>  /* Allocated for each disk page */
>  struct zram_table_entry {
> -       unsigned long handle;
> +       union {
> +               unsigned long handle;
> +               unsigned int element;
> +       };
>         unsigned long value;
>  };

Hello,

Think about following case in 64 bits kernel.

If value pattern in the page is like as following, we cannot detect
the same page with 'unsigned int' element.

AAAAAAAABBBBBBBBAAAAAAAABBBBBBBB...

4 bytes is 0xAAAAAAAA and next 4 bytes is 0xBBBBBBBB and so on.

However, as I said before, I think that it is uncommon case.

Thanks.