linux-ext4 - Re: [PATCH 1/2] ext4: remove a wrong BUG_ON in ext4_ext_convert_to

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Date:	Fri, 9 Dec 2011 14:41:44 -0800
From:	Eric Gouriou <egouriou@...gle.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
Cc:	linux-ext4@...r.kernel.org, achender@...ux.vnet.ibm.com,
	hughd@...gle.com, tytso@....edu
Subject: Re: [PATCH 1/2] ext4: remove a wrong BUG_ON in ext4_ext_convert_to_initialized

On Tue, Dec 6, 2011 at 21:21, Yongqiang Yang <xiaoqiangnk@...il.com> wrote:
> If a file is fallocated on a hole, map->m_lblk + map->m_len may be greater
> than ee_block + ee_len.

Could you please detail a scenario that leads to this check being invalid?
As I'm to blame for the faulty BUG_ON I'd like to use this as an opportunity
to get properly edified.

> Signed-off-by: Yongqiang Yang <xiaoqiangnk@...il.com>
> ---
>  fs/ext4/extents.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 6f0300e..29bb629 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2943,7 +2943,6 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
>        /* Pre-conditions */
>        BUG_ON(!ext4_ext_is_uninitialized(ex));
>        BUG_ON(!in_range(map->m_lblk, ee_block, ee_len));
> -       BUG_ON(map->m_lblk + map->m_len > ee_block + ee_len);

For a bit I thought this would break the fast path logic, however it gets
protected by the checks marked /*L1*/ and /*L2*/ since m_lblk == ee_block
(L1) and m_len < ee_len (L2).

 Regards - Eric

>
>        /*
>         * Attempt to transfer newly initialized blocks from the currently
> --
> 1.7.5.1
>
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