linux-kernel - Re: [PATCH] dcache: fix d_splice

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Date:	Fri, 17 Jan 2014 04:17:23 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
	Miklos Szeredi <mszeredi@...e.cz>
Subject: Re: [PATCH] dcache: fix d_splice_alias handling of aliases

On Wed, Jan 15, 2014 at 10:17:49AM -0500, J. Bruce Fields wrote:
> From: "J. Bruce Fields" <bfields@...hat.com>
> 
> d_splice_alias can create duplicate directory aliases (in the !new
> case), or (in the new case) d_move without holding appropriate locks.
> 
> d_materialise_unique deals with both of these problems.  (The latter
> seems to be dealt by trylocks (see __d_unalias), which look like they
> could cause spurious lookup failures--but that's at least better than
> corrupting the dcache.)

I'm a bit worried about those spurious failures, maybe we should
retry in that case?

Also looking over the changes I wonder if the explicit cecking for
aliases for every non-directory might have a major performance impact,
all the dcache growling already was a major issues in NFS workloads
years ago and I dumb it's become any better.

Also looking at this area I'd like to suggest that if you end up
merging the two I'd continue using the d_splice_alias name and
calling conventions.

Also the inode == NULL case really should be split out from
d_materialise_unique into a separate helper.  It shares almost no
code, is entirely undocumented to the point that I don't really
understand what the purpose is, and the only caller that can get
there (fuse) already branches around that case in the caller anyway.

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