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 15:22:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
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 14:58:21 -0800 (PST) David Rientjes <rientjes@...gle.com> wrote:

> > > +#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()).

ugh.  That's too subtle and we didn't even document it.

We could put the ACCESS_ONCE inside numa_node_id() I assume but we
still have the same problem as smp_processor_id(): the numa_node_id()
return value is wrong as soon as you obtain it if running preemptibly. 

We could plaster Big Fat Warnings all over the place or we could treat
numa_node_id() and derivatives in the same way as smp_processor_id()
(which is a huge pain).  Or something else, but we've left a big hand
grenade here and Raghavendra won't be the last one to pull the pin?
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