linux-ext4 - Re: [PATCH 4/4] e2fsck: Do not discard itable if discard doen't zero data

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Message-ID: <4F55182A.20805@redhat.com>
Date:	Mon, 05 Mar 2012 13:46:50 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH 4/4] e2fsck: Do not discard itable if discard doen't zero
 data

On 03/05/2012 01:49 AM, Lukas Czerner wrote:
> We do not want to discard inode table if the underlying device does not
> return zeros when reading non-provisioned blocks. The reason is that if
> the inode table is not zeroed yet, then discard would not help us since
> we would have to zero it anyway. In the case that inode table was
> already zeroed, then the discard would cause subsequent reads to contain
> non-deterministic data so we would not be able to assume that the inode
> table was zeroed and we would need to zero it again, which does not
> really make sense.
> 
> This commit adds check to prevent inode table from being discarded if
> the discard does not zero data.
> 
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>

seems fine

Reviewed-by: Eric Sandeen <sandeen@...hat.com>

> ---
>  e2fsck/pass5.c |   13 ++++++++++++-
>  1 files changed, 12 insertions(+), 1 deletions(-)
> 
> diff --git a/e2fsck/pass5.c b/e2fsck/pass5.c
> index 741e6dd..9e63037 100644
> --- a/e2fsck/pass5.c
> +++ b/e2fsck/pass5.c
> @@ -116,7 +116,18 @@ static void e2fsck_discard_inodes(e2fsck_t ctx, int group,
>  		ctx->options &= ~E2F_OPT_DISCARD;
>  	}
>  
> -	if (!(ctx->options & E2F_OPT_DISCARD))
> +	/*
> +	 * Do not attempt to discard if E2F_OPT_DISCARD is not set. And also
> +	 * skip the discard on this group if discard does not zero data.
> +	 * The reason is that if the inode table is not zeroed discard would
> +	 * no help us since we need to zero it anyway, or if the inode table
> +	 * is zeroed then the read after discard would not be deterministic
> +	 * anyway and we would not be able to assume that this inode table
> +	 * was zeroed anymore so we would have to zero it again, which does
> +	 * not really make sense.
> +	 */
> +	if (!(ctx->options & E2F_OPT_DISCARD) ||
> +	    !io_channel_discard_zeroes_data(fs->io))
>  		return;
>  
>  	/*

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