linux-kernel - Re: [PATCH 16/24] link_path

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Date:	Mon, 20 Apr 2015 22:32:47 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Neil Brown <neilb@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 16/24] link_path_walk: kill the recursion

On Mon, Apr 20, 2015 at 02:04:53PM -0700, Linus Torvalds wrote:

> That said, you then introduce a stack-allocated "struct saved stack[]"
> in path_mountpoint[] instead, *and* nameidata is saved on stack, so
> this all ends up being very stack-intensive anyway.
> 
> I might have missed some patch here, but would it be possible to just
> allocate a single per-thread nameidata, and just leave it at that?
> Because allocating that thing on the stack when it contains what is
> now one kilobyte of array data is *not* acceptable.

What kilobyte?  It's 9*4 pointers, IOW, 288 bytes total (assuming 64bit box).
And nd->saved_names[] goes away, so scratch 9 pointers we used to have.  Sure,
we can allocate that dynamically (or hold a couple of elements on stack and
allocate when/if we overgrow that), but it's not particularly large win.

Breakeven point is circa the second level of nesting - symlink met when
traversing a symlink...  That - on amd64; on something with fatter stack
frames I would expect the comparison to be even worse for mainline...

We need to preserve 4 pointers on stack per level of nesting.  Seeing that
single link_path_walk() stack frame in mainline is about 5-6 times bigger
than that, "just put enough for all levels into auto array" is an obvious
approach - a couple of link_path_walk() stack frames will be heavier than
that.  For renameat() (the worst-case user of link_path_walk() - there are
two struct nameidata on stack) we end up with breakeven at *one* level of
nesting, what with getting rid of 2*9 pointers in ->saved_names[] of those
nameidata.  And yes, I've measured the actual stack use before and after...

A kilobyte would suffice for 32 levels.  _IF_ we go for "lift the restrictions
on nesting completely", sure, we want to switch to (on-demand) dynamic
allocation.  It's not particularly hard to add and it might be worth doing,
but it's a separate story.  This series leaves the set of accepted pathnames
as-is...
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