Re: [PATCH V2 0/7] Cleancache (was Transcendent Memory): overview

[Minchan Kim]

Hi, Dan. 

On Wed, Jun 02, 2010 at 08:27:48AM -0700, Dan Magenheimer wrote:
> Hi Minchan --
> 
> > I think cleancache approach is cool. :)
> > I have some suggestions and questions.
> 
> Thanks for your interest!
> 
> > > If a get_page is successful on a non-shared pool, the page is flushed
> > (thus
> > > making cleancache an "exclusive" cache).  On a shared pool, the page
> > 
> > Do you have any reason about force "exclusive" on a non-shared pool?
> > To free memory on pesudo-RAM?
> > I want to make it "inclusive" by some reason but unfortunately I can't
> > say why I want it now.
> 
> The main reason is to free up memory in pseudo-RAM and to
> avoid unnecessary cleancache_flush calls.  If you want
> inclusive, the page can be put immediately following
> the get.  If put-after-get for inclusive becomes common,
> the interface could easily be extended to add a "get_no_flush"
> call.

Sounds good to me. 

>  
> > While you mentioned it's "exclusive", cleancache_get_page doesn't
> > flush the page at below code.
> > Is it a role of user who implement cleancache_ops->get_page?
> 
> Yes, the flush is done by the cleancache implementation.
> 
> > If backed device is ram(ie), Could we _move_ the pages from page cache
> > to cleancache?
> > I mean I don't want to copy page when get/put operation. we can just
> > move page in case of backed device "ram". Is it possible?
> 
> By "move", do you mean changing the virtual mappings?  Yes,
> this could be done as long as the source and destination are
> both directly addressable (that is, true physical RAM), but
> requires TLB manipulation and has some complicated corner
> cases.  The copy semantics simplifies the implementation on
> both the "frontend" and the "backend" and also allows the
> backend to do fancy things on-the-fly like page compression
> and page deduplication.

Agree. But I don't mean it. 
If I use brd as backend, i want to do it follwing as. 

put_page :

remove_from_page_cache(page);
brd_insert_page(page);

get_page :

brd_lookup_page(page);
add_to_page_cache(page);

Of course, I know it's impossible without new metadata and modification of 
page cache handling and it makes front and backend's good layered design. 

What I want is to remove copy overhead when backend is ram and it's also
part of main memory(ie, we have page descriptor). 

Do you have an idea?

> 
> > You send the patches which is core of cleancache but I don't see any
> > use case.
> > Could you send use case patches with this series?
> > It could help understand cleancache's benefit.
> 
> Do you mean the Xen Transcendent Memory ("tmem") implementation?
> If so, this is four files in the Xen source tree (common/tmem.c,
> common/tmem_xen.c, include/xen/tmem.h, include/xen/tmem_xen.h).
> There is also an html document in the Xen source tree, which can
> be viewed here:
> http://oss.oracle.com/projects/tmem/dist/documentation/internals/xen4-internals-v01.html 
> 
> Or did you mean a cleancache_ops "backend"?  For tmem, there
> is one file linux/drivers/xen/tmem.c and it interfaces between
> the cleancache_ops calls and Xen hypercalls.  It should be in
> a Xenlinux pv_ops tree soon, or I can email it sooner.

I mean "backend". :) 

> 
> I am also eagerly awaiting Nitin Gupta's cleancache backend
> and implementation to do in-kernel page cache compression.

Do Nitin say he will make backend of cleancache for
page cache compression? 

It would be good feature. 
I have a interest, too. :)

Thanks, Dan. 

> 
> Thanks,
> Dan

-- 
Kind regards,
Minchan Kim
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