Re: [PATCH 16/24] link_path_walk: kill the recursion

Al Viro
2015-04-20T21:32:57Z

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...