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    SubjectRe: [PATCH 06/12] inode: Make unused inode LRU per superblock
    On Sat, Jun 04, 2011 at 01:25:52AM +0100, Al Viro wrote:
    > On Thu, Jun 02, 2011 at 05:01:01PM +1000, Dave Chinner wrote:
    > > From: Dave Chinner <dchinner@redhat.com>
    > >
    > > The inode unused list is currently a global LRU. This does not match
    > > the other global filesystem cache - the dentry cache - which uses
    > > per-superblock LRU lists. Hence we have related filesystem object
    > > types using different LRU reclaimation schemes.
    > >
    > > To enable a per-superblock filesystem cache shrinker, both of these
    > > caches need to have per-sb unused object LRU lists. Hence this patch
    > > converts the global inode LRU to per-sb LRUs.
    > >
    > > The patch only does rudimentary per-sb propotioning in the shrinker
    > > infrastructure, as this gets removed when the per-sb shrinker
    > > callouts are introduced later on.
    >
    > What protects s_nr_inodes_unused?

    For this patch, the modifications are protected by the
    inode_lru_lock, but the reads are unprotected. That's the same
    protection as the inode_stat.nr_unused field, and the same as the
    existing dentry cache per-sb LRU accounting. In the next patch
    modifcations are moved under the sb->s_inode_lru_lock, but reads
    still remain unprotected.

    I can see how the multiple reads in shrink_icache_sb() could each
    return a different value during the proportioning, but I don't think
    that is a big problem. That proportioning code goes away in the next
    patch and is replaced by different code in prune_super(), so if you
    want the reads protected by locks or a single snapshot used for the
    proportioning calculations I'll do it in the new code in
    prune_super().

    Cheers,

    Dave.
    --
    Dave Chinner
    david@fromorbit.com


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