Re: [PATCH V2 3/7] Cleancache (was Transcendent Memory): VFS hooks

[Minchan Kim]

On Fri, Jun 04, 2010 at 08:13:14AM -0700, Dan Magenheimer wrote:
> > 1)
> > You mentiond PFRA in you description and I understood cleancache has
> > a cold clean page which is evicted by reclaimer.
> > But __remove_from_page_cache can be called by other call sites.
> > 
> > For example, shmem_write page calls it for moving the page from page
> > cache
> > to swap cache. Although there isn't the page in page cache, it is in
> > swap cache.
> > So next read/write of shmem until swapout happens can be read/write in
> > swap cache.
> > 
> > I didn't looked into whole of callsites. But please review again them.
> 
> I think the "if (PageUptodate(page))" eliminates all the cases
> where bad things can happen.

I missed it. my fisrt concern has gone. :)

> 
> Note that there may be cases where some unnecessary puts/flushes
> occur.  The focus of the patch is on correctness first; it may
> be possible to increase performance (marginally) in the future by
> reducing unnecessary cases.

I think it wouldn't be marginally. It depends on implementation
of backend. 
I think frontend would be better to notify to backend in 
only exact place. As your descrption, we can call it in shrink_page_list
with some check or change __remove_mapping which adding a argument to tell
"this is calling of reclaim path". 

> 
> > 3) Please consider system memory pressure.
> > And I hope Nitin consider this, too.
> 
> This is definitely very important but remember that cleancache
> provides a great deal of flexibility:  Any page in cleancache
> can be thrown away at any time as every page is clean!  It
> can even accept a page and throw it away immediately.  Clearly
> the backend needs to do this intelligently so this will
> take some policy work.

I admit design goal of cleancache is to give a greate deal of flexibility. 
But I think system memory pressure(ie, direct reclaim and even OOM) is 
exceptional. Whenever we implement various backend, every backend(non-virtual
environemnt)have to implement policy which deal with system memory 
pressure emergency to prevent system hang, I think. 

And backend might need some hack to know the situation. It's horrible.
So I hope frontend gives little information to backend, at least. 

If some backend don't need it, it can just ignore. 
But if some backend need it, it can be a big deal. :)

> 
> Since I saw you sent a separate response to Nitin, I'll
> let him answer for his in-kernel page cache compression
> work.  The solution to the similar problem for Xen is
> described in the tmem internals document that I think
> I pointed to earlier here:
> http://oss.oracle.com/projects/tmem/documentation/internals/ 

I will read it when I have a time. 
Thanks for quick reply but I can't. 
It's time to sleep and weekend. 
See you soon and have a nice weekend. 

> 
> Thanks,
> Dan
> 

-- 
Kind regards,
Minchan Kim
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