Re: [RFC PATCH V5] mm readahead: Fix readahead fail for no local memory and limit readahead pages

[David Rientjes <rientjes@google.com>]

On Thu, 6 Feb 2014, Andrew Morton wrote:

> > --- a/mm/readahead.c
> > +++ b/mm/readahead.c
> > @@ -237,14 +237,32 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
> >  	return ret;
> >  }
> >  
> > +#define MAX_REMOTE_READAHEAD   4096UL
> >  /*
> >   * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
> >   * sensible upper limit.
> >   */
> >  unsigned long max_sane_readahead(unsigned long nr)
> >  {
> > -	return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
> > -		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
> > +	unsigned long local_free_page;
> > +	int nid;
> > +
> > +	nid = numa_node_id();

If you're intending this to be cached for your calls into 
node_page_state() you need nid = ACCESS_ONCE(numa_node_id()).

What's the downside of just using numa_mem_id() here instead which is 
usually "local memory to this memoryless node cpu" and forget about 
testing node_present_pages(nid)?

> > +	if (node_present_pages(nid)) {
> > +		/*
> > +		 * We sanitize readahead size depending on free memory in
> > +		 * the local node.
> > +		 */
> > +		local_free_page = node_page_state(nid, NR_INACTIVE_FILE)
> > +				 + node_page_state(nid, NR_FREE_PAGES);
> > +		return min(nr, local_free_page / 2);
> > +	}
> > +	/*
> > +	 * Readahead onto remote memory is better than no readahead when local
> > +	 * numa node does not have memory. We limit the readahead to 4k
> > +	 * pages though to avoid trashing page cache.
> > +	 */
> > +	return min(nr, MAX_REMOTE_READAHEAD);
> >  }