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

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Date:	Fri, 07 Feb 2014 16:12:44 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	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 02/07/2014 05:28 AM, David Rientjes wrote:
> 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.
>

So following discussion TODO for my patch is:

1) Update the changelog with user visible impact of the patch.
(Andrew's suggestion)
2) Add ACCESS_ONCE to numa_node_id().
3) Change the "readahead into remote memory" part of the documentation
which is misleading.

( I feel no need to add numa_mem_id() since we would specifically limit
the readahead with MAX_REMOTE_READAHEAD in memoryless cpu cases).

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