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Re: [Xen-devel] Why frontswap and cleancache make copies in tmem?

On Mon, Feb 17, 2014 at 11:38 AM, George Dunlap
<George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Sun, Feb 16, 2014 at 10:14 PM, Jinchun Kim <cienlux@xxxxxxxxx> wrote:
>> Hi, All.
>> While I was digging tmem and its source code, I found suspicious things
>> about frontswap and cleancache.
>> When the guest OS wants to evict either a dirty or clean page, frontswap and
>> cleancache will store it in tmem.
>> The linux kernel document tells that
>> [Documentation/vm/frontswap.txt]
>> A "store" will copy the page to transcendent memory ....
>> A "load" will copy the page, if found, from transcendent memory into kernel
>> memory ...
>> [Documentation/vm/cleancache.txt]
>> A "put_page" will copy a page (presumably about-to-be-evicted) page into
>> cleancache ...
>> A "get_page" will copy the page, if found, from cleancache into kernel
>> memory ...
>> My colleagues and I think copying the page is not necessary (especially for
>> cleancache) because both kernel memory and tmem have same data. Why don't we
>> just change the pointer to the page and let it belong to tmem? We were
>> wondering if there is any specific reason not to copy the pages to tmem.
> Konrad / Boris?  (Sorry, I can't remember the other person who's been
> sending tmem patches recently.)

Oh, it was Bob Liu. :-)


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