linux-kernel - Re: [PATCH 04/19] fs: Implement lazy LRU updates for inodes.

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Date:	Sun, 17 Oct 2010 04:34:22 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/19] fs: Implement lazy LRU updates for inodes.

On Sun, Oct 17, 2010 at 04:29:24AM +1100, Nick Piggin wrote:
> On Sat, Oct 16, 2010 at 12:59:30PM -0400, Christoph Hellwig wrote:
> > I don't think the pointer check will work either.  By the time we retake
> > the lru lock the inode might already have been reaped through a call
> > to invalidate_inodes.  There's no way we can do anything with it after
> 
> I don't think you're right. If we re take inode_lock, ensure it is on
> the LRU, and call the can_unuse checks, there is no more problem than
> the regular loop taking items from the LRU, AFAIKS.
> 
> > iput.  What we could do is using variant of can_unuse to decide to move
> > the inode to the front of the lru before doing the iput.  That way
> > we'll get to it next after retaking the lru lock if it's still there.
> 
> This might actually be the better approach anyway (even for upstream)
> -- it means we don't have to worry about the "check head element"
> heuristic of the LRU check which could get false negatives if there is
> a lot of concurrency on the LRU.

Oh hmm, but then you do have the double lock of the LRU lock.

if (can_unuse_after_iput(inode)) {
  spin_lock(&inode_lock);
  list_move(inode, list tail)
  spin_unlock(&inode_lock);
}
iput(inode);
spin_lock(&inode_lock);

Is that worth it?

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