linux-kernel - Re: [RFC][PATCHSET v3] non-recursive pathname resolution & RCU symlinks

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Message-ID: <20150516141811.GT7232@ZenIV.linux.org.uk>
Date:	Sat, 16 May 2015 15:18:11 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	NeilBrown <neilb@...e.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andreas Dilger <adilger@...ger.ca>,
	Dave Chinner <david@...morbit.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC][PATCHSET v3] non-recursive pathname resolution & RCU
 symlinks

On Sat, May 16, 2015 at 06:46:26AM +0100, Al Viro wrote:

> Dealing with multi-component lookups isn't impossible and might be a good
> idea, but only if all intermediates are populated.  What information does
> NFSv4 multi-component lookup give you?  9p one gives an array of FIDs,
> one per component, and that is best used as multi-component revalidate
> on hot dcache...

	Having reread the RFC...  What's the problem with intermediates?
Just put GETFH and GETATTR between the LOOKUP for each component in
the same compound and be done with that - you've got yourself everything
you might possibly need for populating them.  Confused...

	BTW, I would still very much prefer to allocate a chain of
dentries in fs/namei.c (yes, marking them "in-lookup"), then gave an
array of pointers (or beginning and end of the chain, but that can
be more delicate due to dentry tree topology changes from e.g.
d_materialize_unique(), aka d_splice_alias() these days).  With
the requirement being "populate them in root-to-leaves order, do nothing
for ones that had in-lookup flag already cleared".

	Another fun possibility (but that would take somewhat more
restructuring in fs/namei.c) would be to have (on hot cache) a path
traced for several components, seeing that they are all on the same
fs and delaying revalidation for a while.  With bulk revalidate covering
all the chain when we stumble across .., mountpoint or something we believe
to be a symlink, or when the chain reaches fs-specified limit.  Said bulk
revalidate should tell how long a prefix had been OK.  Permission change
handling would be the painful part here...
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