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Re: [PATCH 3/10] xfs: consolidate xfs_inumbers

To: Jeff Liu <jeff.liu@xxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Subject: Re: [PATCH 3/10] xfs: consolidate xfs_inumbers
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Mon, 06 Jan 2014 10:05:06 -0500
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <52CA4BDB.7010804@xxxxxxxxxx>
References: <52BEB3FA.5080402@xxxxxxxxxx> <52C72362.2000005@xxxxxxxxxx> <52CA4BDB.7010804@xxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0
On 01/06/2014 01:23 AM, Jeff Liu wrote:
> On 01/04 2014 04:53 AM, Brian Foster wrote:
>> On 12/28/2013 06:20 AM, Jeff Liu wrote:
>>> From: Jie Liu <jeff.liu@xxxxxxxxxx>
>>>
>>> To fetch the file system number tables, we currently just ignore the
>>> errors and proceed to loop over the next AG or bump agino to the next
>>> chunk in case of btree operations failed, that is not properly because
>>> those errors might hint us potential file system problems.
>>>
>>> This patch rework xfs_inumbers() to handle the btree operation errors
>>> as well as the loop conditions.  Also, add pre-checkups for the given
>>> inode, we can save alloc/free the format buffer once against an invalid
>>> inode number.
>>>
>>> Signed-off-by: Jie Liu <jeff.liu@xxxxxxxxxx>
>>> ---
>>>  fs/xfs/xfs_itable.c | 163 
>>> +++++++++++++++++++++++-----------------------------
>>>  1 file changed, 72 insertions(+), 91 deletions(-)
>>>
>>> diff --git a/fs/xfs/xfs_itable.c b/fs/xfs/xfs_itable.c
>>> index 692671c..4d262f6 100644
>>> --- a/fs/xfs/xfs_itable.c
>>> +++ b/fs/xfs/xfs_itable.c
...
>>> -           error = xfs_inobt_get_rec(cur, &r, &i);
>>> -           if (error || i == 0) {
>>> -                   xfs_buf_relse(agbp);
>>> -                   agbp = NULL;
>>> -                   xfs_btree_del_cursor(cur, XFS_BTREE_NOERROR);
>>> -                   cur = NULL;
>>> -                   agno++;
>>> -                   agino = 0;
>>> -                   continue;
>>> +           error = xfs_inobt_lookup(cur, agino, XFS_LOOKUP_GE, &stat);
>>> +           if (error)
>>> +                   break;
>>
>> Isn't this lookup only needed after cursor initialization? i.e., we
>> lookup once and increment through the records via xfs_btree_increment()
>> below.
> 
> Yes, but please see my comments below.
>>
...
>>> +           if (!--left)
>>> +                   break;
>>> +
>>> +           error = xfs_btree_increment(cur, 0, &stat);
>>> +           if (error)
>>> +                   break;
>>> +           if (stat) {
>>> +                   /*
>>> +                    * The agino value has already been bumped, just try
>>> +                    * to skip up to it.
>>> +                    */
>>> +                   agino += XFS_INODES_PER_CHUNK;
>>> +                   continue;
>>>             }
>>
>> Maybe it's just me, but this reads a little funny to me. In particular
>> because we only get here if stat == 1. I wonder if this would look a bit
>> cleaner if we pulled the next_ag labeled block below up into the goto,
>> since that appears to be the only reference. Then just let the loop fall
>> through.
> 
> Actually, we would never hint xfs_btree_increment() at all no matter the 
> default
> or with my current change, because the left variable value is 1 as per the 
> user
> space imap implementation, thus, it should be decreased to 0 in this point.
> 
> The problem is, how we define the user call interface originally, why it does 
> not
> support a 2nd argument which can be used to specified a desired icount to 
> return a
> limited number of inode mapping tables? i.e,
> 
> imap_f() {
> 
>         if (argc != 2)
>                 nent = 1;
>         else
>                 nent = atoi(argv[1]);
> 
>       ....
> }
> 
> The imap command does not support another options but initialize 
> bulkreq.icount = nent = 1;
> In kernel, the bcount is evaluated to 1 and if (bufidx == bcount) should be 
> true anyway as
> below code logic:
>  
> bcount = MIN(left, (int)(PAGE_SIZE / sizeof(*buffer)));
> 
> if (bufidx == bcount) {
>       ....
>       formatter();
>       ....
> }

Hi Jeff,

I see, but this is just a characteristic of the imap command in xfs_io.
I can use an argument just by changing argmax from 0 to 1. Perhaps this
is just a bug, since 'help imap' provides syntax that allows a parameter:

xfs_io> help imap
imap [nentries] -- inode map for filesystem of current file

> 
> But, if the left value could be specified from the user progs, maybe we 
> cannot simply
> assuming "stat == 1" is always be true, in particular, in next path for 
> implementing
> per AG inumber mechanism, xfs_btree_increment() would probably succeed but 
> "stat == 0"
> if there is no right neighbors, and if the user could specified the 2nd imap 
> option.
> 

Hmm, that looked odd to me as well once you pointed that out. A quick
printk check shows that xfs_btree_increment() does not fail in this
scenario, but the subsequent xfs_inobt_get_rec() in the following
iteration sets i == 0 and skips to the next ag appropriately. I agree
that a tmp == 0 check in this scenario would be slightly more intuitive
though.

But either way, we should probably maintain the general algorithm of
walking the btree explicitly rather than incrementing agino and issuing
the lookup each iteration of the loop.

> Hence, in this patch, I only intended to fix the btree error handling 
> mechanism as a
> preparation step to implement per AG inumber.
> 

Right... I see that the following patches clean this up further (though
I haven't looked in detail yet).

>>  
>> Also, I think the agino addition here becomes unnecessary when the
>> lookup issue above is addressed.
>>
>>> -   }
>>> +
>>> +next_ag:
>>> +           xfs_btree_del_cursor(cur, XFS_BTREE_NOERROR);
>>> +           cur = NULL;
>>> +           xfs_buf_relse(agbp);
>>> +           agbp = NULL;
>>> +           agino = 0;
>>> +   } while (++agno < mp->m_sb.sb_agcount);
>>> +
>>
>> ... and just thinking about the logic that way highlights the bug here,
>> where we bump agno due to the continue above (where IIUC, we intend to
>> only move forward within the ag). Perhaps the ++agno should be part of
>> the broken off 'next_ag' logic as well.
> 
> Can you please point out a bit more about the bug?  We bump agno due to
> current run out of the current AG but the user want to show more imaps.
> 

agno is bumped in the while condition at the end of the loop. This will
also be executed in the following block if xfs_btree_increment()
succeeds and finds a record to the right:

                error = xfs_btree_increment(cur, 0, &stat);
                if (error)
                        break;
                if (stat) {
                        ...
                        agino += XFS_INODES_PER_CHUNK;
------>                 continue;
                }

The continue statement will execute the loop condition to determine
whether it should iterate the loop again or break. In this case, you're
still moving along the original AG, but agno has been bumped
erroneously. This error looks to be hidden by the non-looping scenario
where nentries is 1, but make the xfsprogs fix I described above and
you'll probably see what I'm describing. I only see a subset of the
original imap output using a record count of 10 on an fs with a couple
thousand inodes spread across a couple AGs.

FWIW, it does look like the following patch fixes this particular
problem via the introduction of xfs_perag_inumbers(), but we shouldn't
introduce transient errors into the patch stream if we can help it. It
looks like the comments about the repeated inobt lookups still apply
though (and the implementation as of patch 4 is still skipping inodes
with nentries > 1, perhaps due to advancing agino multiple times per
iteration..?).

Brian

> 
> Thanks,
> -Jeff
> 

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