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

[Dan Magenheimer]

> Hi, Dan.
> I reviewed quickly. So I may be wrong. :)

Hi Minchan --

Thanks for your thorough review!  I don't think anyone
else yet has examined the semantics of the cleancache
patch as deeply as you have.  Excellent!

> > +	/*
> > +	 * if we're uptodate, flush out into the cleancache, otherwise
> > +	 * invalidate any existing cleancache entries.  We can't leave
> > +	 * stale data around in the cleancache once our page is gone
> > +	 */
> > +	if (PageUptodate(page))
> > +		cleancache_put_page(page);
> > +	else
> > +		cleancache_flush_page(mapping, page);
> 
> I doubt it's right place related to PFRA.

I agree it doesn't seem to be the right place, but it does work
and there doesn't seem to be a better place.
 
> 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.

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.

> 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.

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/ 

Thanks,
Dan

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