linux-kernel - Re: [PATCH 00/13] Support follow_link in RCU-walk.

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Message-ID: <20150316191444.GZ29656@ZenIV.linux.org.uk>
Date:	Mon, 16 Mar 2015 19:14:45 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	NeilBrown <neilb@...e.de>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/13] Support follow_link in RCU-walk. - V2

On Mon, Mar 16, 2015 at 03:43:19PM +1100, NeilBrown wrote:
> Hi Al,
>  I believe this series addresses all your concerns about
>  my first attempt.
>  The first patch results in nameidata being almost completely
>  localized to namei.c :-)  It also highlights out-of-date
>  documentation in automount-support.txt :-(
> 
>  It also exposes (and removes) some ... interesting code in lustre.
>  I'm not sure how safe it is to remove that.... I didn't think
>  recursive symlinks used extra stack.

Recursive nested symlinks *do* use extra stack; it's not in fs code, though.
make fs/namei.s and check link_path_walk; AFAICS, on amd64 it's 192 bytes per
level, on sparc64 - 256, sparc32 and ppc32 - 144, ppc64 - obscenely fat 336...

It's more that lustre is an extreme stack hog; call its methods on slightly
deeper stack and you are screwed.  I don't _know_ if that's pure paranoia -
might very well be.  OTOH, it might be not paranoid enough.  OTTH, if it
manages to survive 5 levels on 4K stack, it ought to survive 8 levels on 8K
one; if 3 times the footprint of link_path_walk pushes the total by more 4K,
there's no way in hell to fit 5 times that footprint into 4K stack, nevermind
the rest of call chain...
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