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

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Message-Id: <20140206145105.27dec37b16f24e4ac5fd90ce@linux-foundation.org>
Date:	Thu, 6 Feb 2014 14:51:05 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
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 Wed, 22 Jan 2014 16:23:45 +0530 Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> wrote:

> max_sane_readahead returns zero on the cpu having no local memory
> node. Fix that by returning a sanitized number of pages viz.,
> minimum of (requested pages, 4k)

um, fix what?  The changelog should describe the user-visible impact of
the current implementation.  There are a whole bunch of reasons for
this, but I tire of typing them in day after day after day.

> --- 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 (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);
>  }

Looks reasonable to me.  Please send along a fixed up changelog.
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