linux-ext4 - Re: [PATCH] ext4: limit group search loop for non-extent files

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Date:	Sun, 5 May 2013 20:26:35 -0400 (EDT)
From:	Lachlan McIlroy <lmcilroy@...hat.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: limit group search loop for non-extent files

----- Original Message -----
> From: Lachlan McIlroy <lmcilroy@...hat.com>
> 
> In the case where we are allocating for a non-extent file,
> we must limit the groups we allocate from to those below
> 2^32 blocks, and ext4_mb_regular_allocator() attempts to
> do this initially by putting a cap on ngroups for the
> subsequent search loop.
> 
> However, the initial target group comes in from the
> allocation context (ac), and it may already be beyond
> the artificially limited ngroups.  In this case,
> the limit
> 
> 	if (group == ngroups)
> 		group = 0;
> 
> at the top of the loop is never true, and the loop will
> run away.
> 
> Catch this case inside the loop and reset the search to
> start at group 0.
> 
> [sandeen@...hat.com: add commit msg & comments]
> 
> Signed-off-by: Eric Sandeen <sandeen@...hat.com>
> ---
> 
> p.s. I suppose this limit could go after the assignment
> from ac->ac_g_ex.fe_group, depending on your sense
> of style...
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index ee6614b..f54baee 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1994,7 +1994,11 @@ repeat:
>  		group = ac->ac_g_ex.fe_group;
>  
>  		for (i = 0; i < ngroups; group++, i++) {
> -			if (group == ngroups)
> +			/*
> +			 * Artificially restricted ngroups for non-extent
> +			 * files makes group > ngroups possible on first loop.
> +			 */
> +			if (group >= ngroups)
>  				group = 0;
>  
>  			/* This now checks without needing the buddy page */
> 
> --
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> 

Thanks for following through with this Eric.  Looks good to me.
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