linux-kernel - Re: [PATCH 2/9] inode: add IOP_NOTHASHED to avoid inode hash lock in evict

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Date:	Thu, 12 Mar 2015 09:52:07 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Josef Bacik <jbacik@...com>
Cc:	linux-fsdevel@...r.kernel.org, david@...morbit.com, jack@...e.cz,
	linux-kernel@...r.kernel.org, Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH 2/9] inode: add IOP_NOTHASHED to avoid inode hash lock in
 evict

On Tue, Mar 10, 2015 at 03:45:17PM -0400, Josef Bacik wrote:
> From: Dave Chinner <dchinner@...hat.com>
> 
> Some filesystems don't use the VFS inode hash and fake the fact they
> are hashed so that all the writeback code works correctly. However,
> this means the evict() path still tries to remove the inode from the
> hash, meaning that the inode_hash_lock() needs to be taken
> unnecessarily. Hence under certain workloads the inode_hash_lock can
> be contended even if the inode is never actually hashed.
> 
> To avoid this, add an inode opflag to allow inode_hash_remove() to
> avoid taking the hash lock on inodes have never actually been
> hashed.

Why bother with flags, etc. when we could just do

static inline bool hlist_fake(struct hlist_node *h) 
{
	return h->pprev == &h->next;
}

> -	if (!inode_unhashed(inode))
> +	if (!((inode->i_opflags & IOP_NOTHASHED) || inode_unhashed(inode)))

and turn this check into
	if (!inode_unhashed(inode) && !hlist_fake(&inode->i_hash))

instead?
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