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    SubjectRe: [PATCH 3/12] ksm: pages_unshared and pages_volatile
    On Tue, 4 Aug 2009, Andrew Morton wrote:
    > On Mon, 3 Aug 2009 13:11:53 +0100 (BST)
    > Hugh Dickins <> wrote:
    > > pages_volatile is harder to define. It includes those pages changing
    > > too fast to get into the unstable tree, but also whatever other edge
    > > conditions prevent a page getting into the trees: a high value may
    > > deserve investigation. Don't try to calculate it from the various
    > > conditions: it's the total of rmap_items less those accounted for.
    > > static inline struct rmap_item *alloc_rmap_item(void)
    > > {
    > > - return kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
    > > + struct rmap_item *rmap_item;
    > > +
    > > + rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
    > > + if (rmap_item)
    > > + ksm_rmap_items++;
    > > + return rmap_item;
    > > }
    > ksm_rmap_items was already available via /proc/slabinfo. I guess that
    > wasn't a particularly nice user interface ;)

    procfs is not a nice interface for sysfs to be reading
    when it's asked to show pages_volatile!

    And not even always available, I think: SLOB wouldn't be able to report
    the number of objects of any particular type, SLUB would need slub_nomerge
    (or a debug flag) to keep the kmem_cache separate; and even SLAB would
    have to assemble numbers from different cpus and queues, I guess.
    Easier and more reliable for KSM to do its own thing here.

    I do agree that slabinfo or slqbinfo or /proc/slabinfo is a good enough
    interface for checking up on the number of rmap_items in use; it's what
    I was using most of the time. I did once fleetingly wonder whether to
    show the count of rmap_items under /sys/kernel/mm/ksm, but rejected it
    as not interesting enough to deserve more than what slabinfo tells.

    But here the rmap_item count is being used to deduce something more
    interesting (though more obscure), and I don't want SL?B divergences
    to perturb the resulting number more than it is already. I did start
    out without the rmap_items count, incrementing and decrementing
    pages_volatile all over the place; but soon abandoned that in
    favour of the difference calculation.

    But I think I'm taking you more seriously than you intended,
    sorry for my humourlessness!


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