On Thu, 6 Feb 2014, Andrew Morton wrote:
> On Thu, 6 Feb 2014 14:58:21 -0800 (PST) David Rientjes <rientjes@xxxxxxxxxx> 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?
>
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().
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