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

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Date:	Thu, 6 Feb 2014 15:58:17 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Damien Ramonda <damien.ramonda@...el.com>,
	Jan Kara <jack@...e.cz>, Linus <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V5] mm readahead: Fix readahead fail for no local
 memory and limit readahead pages

On Thu, 6 Feb 2014, David Rientjes wrote:

> > > > > +#define MAX_REMOTE_READAHEAD   4096UL

> Normally it wouldn't matter because there's no significant downside to it 
> racing, things like mempolicies which use numa_node_id() extensively would 
> result in, oops, a page allocation on the wrong node.
> 
> This stands out to me, though, because you're expecting the calculation to 
> be correct for a specific node.
> 
> The patch is still wrong, though, it should just do
> 
> 	int node = ACCESS_ONCE(numa_mem_id());
> 	return min(nr, (node_page_state(node, NR_INACTIVE_FILE) +
> 		        node_page_state(node, NR_FREE_PAGES)) / 2);
> 
> since we want to readahead based on the cpu's local node, the comment 
> saying we're reading ahead onto "remote memory" is wrong since a 
> memoryless node has local affinity to numa_mem_id().
> 

Oops, forgot about the MAX_REMOTE_READAHEAD which needs to be factored in 
as well, but this handles the bound on local node's statistics.
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