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Re: [PATCH] xfs: reduce stack usage in xfs_bmap_btalloc()

To: Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: reduce stack usage in xfs_bmap_btalloc()
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Sat, 26 Apr 2008 13:54:54 -0500
Cc: David Chinner <dgc@xxxxxxx>, xfs@xxxxxxxxxxx, Adrian Bunk <bunk@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx
In-reply-to: <200804261651.02078.vda.linux@xxxxxxxxxxxxxx>
References: <200804261651.02078.vda.linux@xxxxxxxxxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Thunderbird 2.0.0.12 (Macintosh/20080213)
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@xxxxxxxxxxxxxx>
> --
> 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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