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

From: Al Viro
Date: Thu Mar 12 2015 - 05:52:36 EST

On Tue, Mar 10, 2015 at 03:45:17PM -0400, Josef Bacik wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 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))

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