MFC r305378,r305379,r305386,r305684,r306224,r306608,r306803,r307650,r307685,

MFC r305378,r305379,r305386,r305684,r306224,r306608,r306803,r307650,r307685,
r308407,r308665,r308667,r309067:

    cache: put all negative entry management code into dedicated functions

==
    cache: manage negative entry list with a dedicated lock

    Since negative entries are managed with a LRU list, a hit requires a
    modificaton.

    Currently the code tries to upgrade the global lock if needed and is
    forced to retry the lookup if it fails.

    Provide a dedicated lock for use when the cache is only shared-locked.

==

    cache: defer freeing entries until after the global lock is dropped

    This also defers vdrop for held vnodes.

==

    cache: improve scalability by introducing bucket locks

    An array of bucket locks is added.

    All modifications still require the global cache_lock to be held for
    writing. However, most readers only need the relevant bucket lock and in
    effect can run concurrently to the writer as long as they use a
    different lock. See the added comment for more details.

    This is an intermediate step towards removal of the global lock.

==

    cache: get rid of the global lock

    Add a table of vnode locks and use them along with bucketlocks to provide
    concurrent modification support. The approach taken is to preserve the
    current behaviour of the namecache and just lock all relevant parts before
    any changes are made.

    Lookups still require the relevant bucket to be locked.

==

    cache: ignore purgevfs requests for filesystems with few vnodes

    purgevfs is purely optional and induces lock contention in workloads
    which frequently mount and unmount filesystems.

    In particular, poudriere will do this for filesystems with 4 vnodes or
    less. Full cache scan is clearly wasteful.

    Since there is no explicit counter for namecache entries, the number of
    vnodes used by the target fs is checked.

    The default limit is the number of bucket locks.

== (by kib)

    Limit scope of the optimization in r306608 to dounmount() caller only.
    Other uses of cache_purgevfs() do rely on the cache purge for correct
    operations, when paths are invalidated without unmount.

==

    cache: split negative entry LRU into multiple lists

    This splits the ncneg_mtx lock while preserving the hit ratio at least
    during buildworld.

    Create N dedicated lists for new negative entries.

    Entries with at least one hit get promoted to the hot list, where they
    get requeued every M hits.

    Shrinking demotes one hot entry and performs a round-robin shrinking of
    regular lists.

==

    cache: fix up a corner case in r307650

    If no negative entry is found on the last list, the ncp pointer will be
    left uninitialized and a non-null value will make the function assume an
    entry was found.

    Fix the problem by initializing to NULL on entry.

== (by kib)

    vn_fullpath1() checked VV_ROOT and then unreferenced
    vp->v_mount->mnt_vnodecovered unlocked.  This allowed unmount to race.
    Lock vnode after we noticed the VV_ROOT flag.  See comments for
    explanation why unlocked check for the flag is considered safe.

==

    cache: fix a race between entry removal and demotion

    The negative list shrinker can demote an entry with only hotlist + neglist
    locks held. On the other hand entry removal possibly sets the NCF_DVDROP
    without aformentioned locks held prior to detaching it from the respective
    netlist., which can lose the update made by the shrinker.

==

    cache: plug a write-only variable in cache_negative_zap_one

==

    cache: ensure that the number of bucket locks does not exceed hash size

    The size can be changed by side effect of modifying kern.maxvnodes.

    Since numbucketlocks was not modified, setting a sufficiently low value
    would give more locks than actual buckets, which would then lead to
    corruption.

    Force the number of buckets to be not smaller.

    Note this should not matter for real world cases.