linux-kernel - Re: XFS stack overhead

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Date:	Wed, 21 Oct 2009 21:16:48 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, esandeen@...hat.com,
	cebbert@...hat.com, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: XFS stack overhead


* Eric Sandeen <sandeen@...hat.com> wrote:

> Arjan van de Ven wrote:
> > On Wed, 21 Oct 2009 10:50:02 -0500
> > Eric Sandeen <sandeen@...hat.com> wrote:
> > 
> >> Ingo Molnar wrote:
> >>> (Cc:-ed Arjan too.)
> >>>
> >>> * Dave Jones <davej@...hat.com> wrote:
> >>>
> >>>> 113c5413cf9051cc50b88befdc42e3402bb92115 introduced a change that
> >>>> made CC_STACKPROTECTOR_ALL not-selectable if someone enables 
> >>>> CC_STACKPROTECTOR.
> >>>>
> >>>> We've noticed in Fedora that this has introduced noticable
> >>>> overhead on some functions, including those which don't even have
> >>>> any on-stack variables.
> >>>>
> >>>> According to the gcc manpage, -fstack-protector will protect
> >>>> functions with as little as 8 bytes of stack usage. So we're
> >>>> introducing a huge amount of overhead, to close a small amount of
> >>>> vulnerability (the >0 && <8 case).
> >>>>
> >>>> The overhead as it stands right now means this whole option is 
> >>>> unusable for a distro kernel without reverting the above commit.
> >>> Exactly what workload showed overhead, and how much?
> >>>
> >>> 	Ingo
> >> I had xfs blowing up pretty nicely; granted, xfs is not svelte but it
> >> was never this bad before.
> >>
> > 
> > do you have any indication that SP actually increases the stack
> > footprint by that much? it's only a few bytes....
> > 
> > 
> 
> Here's a sample of some of the largest xfs stack users,
> and the effect stack-protector had on them.  This was just
> done with objdump -d xfs.ko | scripts/checkstack.pl; I don't
> know if there's extra runtime stack overhead w/ stackprotector?
> 
> -Eric
> 
> function                  nostack stackprot delta delta %
> xfs_bmapi                      376      408    32  9%
> xfs_bulkstat                   328      344    16  5%
> _xfs_trans_commit              296      312    16  5%
> xfs_iomap_write_delay          264      280    16  6%
> xfs_file_ioctl                 248      312    64 26%
> xfs_symlink                    248      264    16  6%
> xfs_bunmapi                    232      280    48 21%
> xlog_do_recovery_pass          232      248    16  7%
> xfs_trans_unreserve_and_mod_sb 224      240    16  7%
> xfs_bmap_del_extent            216      248    32 15%
> xfs_cluster_write              216      232    16  7%
> xfs_file_compat_ioctl          216      296    80 37%
> xfs_attr_set_int               200      216    16  8%
> xfs_bmap_add_extent_delay_real 200      248    48 24%

Note that those are very large stack frames to begin with.

3496 bytes - that's a _lot_ - can anyone even run XFS with 4K stacks on? 

With stackprotector it's 3928 - a 12% increase - which certainly does 
not help - but the basic problem is the large stack footprint to begin 
with.

Also, the posting apparently mixes 'stack overhead' with 'runtime 
overhead'.

	Ingo
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