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

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Date:	Mon, 12 Dec 2011 16:01:56 -0800
From:	Eric Gouriou <egouriou@...gle.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
Cc:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"achender@...ux.vnet.ibm.com" <achender@...ux.vnet.ibm.com>,
	"hughd@...gle.com" <hughd@...gle.com>,
	"tytso@....edu" <tytso@....edu>
Subject: Re: [PATCH 1/2] ext4: remove a wrong BUG_ON in ext4_ext_convert_to_initialized

[Yet another resend, without HTML and _with_ the list CC'ed. Apologies
for the spam]

On Fri, Dec 9, 2011 at 19:36, Yongqiang Yang <xiaoqiangnk@...il.com> wrote:
>
>
> On Saturday, December 10, 2011, Eric Gouriou <egouriou@...gle.com> wrote:
>> 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.
> It's easy to reproduce, I think.   Just need to write beyond fallocated
> blocks, write back would submit a request to ext4_map_blocks with blocks
> part of which are fallocated while other part of which are not, then the
> bug-on would happen.

Thanks, this makes sense.

>
> I met the bug_on during fsx and after the patch applied it works normal.
>
> I think Allison also tested with this patch.
>
> Yongqiang..
>
>>
>>> Signed-off-by: Yongqiang Yang <xiaoqiangnk@...il.com>

Reviewed-by: Eric Gouriou <egouriou@...gle.com>

 Thanks - Eric

>>> ---
>>>  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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>>
>
> --
> Best Wishes
> Yongqiang Yang
>
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