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    SubjectRe: [PATCH 1/5] inode: Make unused inode LRU per superblock
    On Thu, May 27, 2010 at 01:32:30PM -0700, Andrew Morton wrote:
    > On Tue, 25 May 2010 18:53:04 +1000
    > Dave Chinner <david@fromorbit.com> 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 reclaimatin 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.
    > >
    > > ...
    > >
    > > + list_move(&inode->i_list, &inode->i_sb->s_inode_lru);
    >
    > It's a shape that s_inode_lru is still protected by inode_lock. One
    > day we're going to get in trouble over that lock. Migrating to a
    > per-sb lock would be logical and might help.
    >
    > Did you look into this?

    Yes, I have. Yes, it's possible. It's solving a different problem,
    so I figured it can be done in a different patch set.

    > I expect we'd end up taking both inode_lock
    > and the new sb->lru_lock in several places, which wouldn't be of any
    > help, at least in the interim. Long-term, the locking for
    > fs-writeback.c should move to the per-superblock one also, at which
    > time this problem largely goes away I think. Unfortunately the
    > writeback inode lists got moved into the backing_dev_info, whcih messes
    > things up a bit.

    *nod*

    >
    > > inodes_stat.nr_unused--;
    > > + inode->i_sb->s_nr_inodes_unused--;
    >
    > It's regrettable to be counting the same thing twice. Did you look
    > into removing (or no longer using) inodes_stat.nr_unused?

    Sort of. The complexity is the stats are userspace visible, so they
    can't just be removed. Replacing the current stats means that when
    they are read from /proc we would need to walk all the superblocks
    to aggregate them. The bit I haven't looked at yet is whether
    walking superblocks is allowed in a proc handler.

    So in the mean time, I just copied what was done for the
    dentry_stats. If it's ok to do this walk, then we can change both
    the dentry and inode stats at the same time.

    > > + /* Now, we reclaim unused dentrins with fairness.
    >
    > May as well fix the typo while we're there.
    >
    > Please review all these comments to ensure that they are still accurate
    > and complete.

    Will do.

    Cheers,

    Dave.
    --
    Dave Chinner
    david@fromorbit.com


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