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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

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.)

 -George

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