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    SubjectRe: [RFC PATCH V5] mm readahead: Fix readahead fail for no local memory and limit readahead pages
    On Thu, 6 Feb 2014, Andrew Morton wrote:

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

    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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