linux-kernel - Re: [PATCH] xfs: reduce stack usage in xfs_bmap

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Date:	Sat, 26 Apr 2008 13:54:54 -0500
From:	Eric Sandeen <sandeen@...deen.net>
To:	Denys Vlasenko <vda.linux@...glemail.com>
CC:	David Chinner <dgc@....com>, xfs@....sgi.com,
	Adrian Bunk <bunk@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xfs: reduce stack usage in xfs_bmap_btalloc()

Denys Vlasenko wrote:
> Hi David,
> 
> This patch reduces xfs_bmap_btalloc() stack usage by 50 bytes
> by moving part of its body into a helper function.
> 
> This results in some variables not taking stack space in
> xfs_bmap_btalloc() anymore.
> 
> The helper itself does not call anything stack-deep.
> Stack-deep call to xfs_alloc_vextent() happen
> in xfs_bmap_btalloc(), as before.
> 
> Compile tested only.
> 
> Signed-off-by: Denys Vlasenko <vda.linux@...glemail.com>
> --
> vda
> 

Looks like a very good approach, it pushes a lot of large local vars off
into the helper.

There is one build-time problem if DEBUG is turned on:

        if (args.fsbno != NULLFSBLOCK) {
                ap->firstblock = ap->rval = args.fsbno;
                ASSERT(nullfb || fb_agno == args.agno ||
                       (ap->low && fb_agno < args.agno));

in xfs_bmap_btalloc, which no longer has an fb_agno variable which the
ASSERT macro uses.

Thanks,

-Eric
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