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Message-Id: <20140206152219.45c2039e5092c8ea1c31fd38@linux-foundation.org>
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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