Re: [PATCH] zswap: Same-filled pages handling

[Srividya Desireddy]

On Wed, Oct 18, 2017 at 7:41 PM, Matthew Wilcox wrote: 
> On Wed, Oct 18, 2017 at 04:33:43PM +0300, Timofey Titovets wrote:
>> 2017-10-18 15:34 GMT+03:00 Matthew Wilcox <willy@xxxxxxxxxxxxx>:
>> > On Wed, Oct 18, 2017 at 10:48:32AM +0000, Srividya Desireddy wrote:
>> >> +static void zswap_fill_page(void *ptr, unsigned long value)
>> >> +{
>> >> +     unsigned int pos;
>> >> +     unsigned long *page;
>> >> +
>> >> +     page = (unsigned long *)ptr;
>> >> +     if (value == 0)
>> >> +             memset(page, 0, PAGE_SIZE);
>> >> +     else {
>> >> +             for (pos = 0; pos < PAGE_SIZE / sizeof(*page); pos++)
>> >> +                     page[pos] = value;
>> >> +     }
>> >> +}
>> >
>> > I think you meant:
>> >
>> > static void zswap_fill_page(void *ptr, unsigned long value)
>> > {
>> >         memset_l(ptr, value, PAGE_SIZE / sizeof(unsigned long));
>> > }
>> 
>> IIRC kernel have special zero page, and if i understand correctly.
>> You can map all zero pages to that zero page and not touch zswap completely.
>> (Your situation look like some KSM case (i.e. KSM can handle pages
>> with same content), but i'm not sure if that applicable there)
> 
>You're confused by the word "same".  What Srividya meant was that the
>page is filled with a pattern, eg 0xfffefffefffefffe..., not that it is
>the same as any other page.

In kernel there is a special zero page or empty_zero_page which is in
general allocated in paging_init() function, to map all zero pages. But,
same-value-filled pages including zero pages exist in memory because
applications may be initializing the allocated pages with a value and not
using them; or the actual content written to the memory pages during 
execution itself is same-value, in case of multimedia data for example.

I had earlier posted a patch with similar implementaion of KSM concept 
for Zswap:
https://lkml.org/lkml/2016/8/17/171
https://lkml.org/lkml/2017/2/17/612

- Srividya