Re: [RFC] cleanup of use-once

[Nikita Danilov <nikita@clusterfs.com>]

Rik van Riel writes:
 > The special cases in the use-once code have annoyed me for a while,
 > and I'd like to see if replacing them with something simpler could
 > be worthwhile.
 > 
 > I haven't actually benchmarked (or even tested) this code yet, but
 > the basic idea is that we want to ignore multiple references to the
 > same page if they happen really close to each other, and only keep
 > a page on the active list if it got accessed again on a time scale
 > that matters to the pageout code.  In other words, filtering out
 > correlated references in a simpler way.
 > 
 > Opinions ?

[...]

 > - * active,unreferenced		->	active,referenced
 >   */
 >  void fastcall mark_page_accessed(struct page *page)
 >  {
 > -	if (!PageActive(page) && PageReferenced(page) && PageLRU(page)) {
 > -		activate_page(page);
 > -		ClearPageReferenced(page);
 > -	} else if (!PageReferenced(page)) {
 > +	if (!PageReferenced(page))
 >  		SetPageReferenced(page);
 > -	}
 >  }

So file system pages never get to the active list? Hmm... this doesn't
sound right.

 >  
 >  EXPORT_SYMBOL(mark_page_accessed);
 > @@ -157,6 +149,7 @@ void fastcall lru_cache_add_active(struc
 >  	if (!pagevec_add(pvec, page))

[...]

 >  			goto keep_locked;
 >  
 >  		referenced = page_referenced(page, 1, sc->priority <= 0);
 > -		/* In active use or really unfreeable?  Activate it. */
 > -		if (referenced && page_mapping_inuse(page))
 > +
 > +		if (referenced) {
 > +			/* New page. Let's see if it'll get used again... */
 > +			if (TestClearPageNew(page))
 > +				goto keep_locked;
 >  			goto activate_locked;
 > +		}

This will screw scanning most likely: no referenced page is ever
reclaimed unless lowest scanning priority is reached---this looks like
sure way to oom and has capacity to increase CPU consumption
significantly.

 >  
 >  #ifdef CONFIG_SWAP

Nikita.