Re: [PATCH] Prevent extent btree block allocation failures

[Dave Chinner <david@fromorbit.com>]

On Mon, Jun 23, 2008 at 04:40:26PM +1000, Lachlan McIlroy wrote:
> Dave Chinner wrote:
>> On Mon, Jun 23, 2008 at 03:57:22PM +1000, Lachlan McIlroy wrote:
>>> Dave Chinner wrote:
>>>> On Fri, Jun 20, 2008 at 03:21:20PM +1000, Dave Chinner wrote:
>>>>> On Thu, Jun 19, 2008 at 05:28:50PM +1000, Lachlan McIlroy wrote:
>>>>>> There's something else that looks suspicious to me - this code in
>>>>>> xfs_bmap_btalloc() is setting minleft to 0.  Doesn't this go against
>>>>>> what you were saying about setting minleft to be the space we might
>>>>>> need for the btree operations?
>>>>>>
>>>>>> 	if (args.fsbno == NULLFSBLOCK && nullfb) {
>>>>>> 		args.fsbno = 0;
>>>>>> 		args.type = XFS_ALLOCTYPE_FIRST_AG;
>>>>>> 		args.total = ap->minlen;
>>>>>> 		args.minleft = 0;
>>>>>> 		if ((error = xfs_alloc_vextent(&args)))
>>>>>> 			return error;
>>>>>> 		ap->low = 1;
>>>>>> 	}
>>>>> Hmmm - that looks suspicious. In xfs_bmapi(), when we are doing a
>>>>> write and *firstblock == NULLFSBLOCK (which leads to nullfb being
>>>>> set in the above code), we do:
>>>>>
>>>>>         if (wr && *firstblock == NULLFSBLOCK) {
>>>>>                 if (XFS_IFORK_FORMAT(ip, whichfork) == XFS_DINODE_FMT_BTREE)
>>>>>                         minleft = be16_to_cpu(ifp->if_broot->bb_level) + 1;
>>>>>                 else
>>>>>                         minleft = 1;
>>>>>         } else
>>>>>                 minleft = 0;
>>>>>
>>>>> If we are in btree format we set the minleft to the number of blocks needed
>>>>> for a split. If we are in extent or local format, change to extent of btree
>>>>> format requires one extra block.
>>>>>
>>>>> The above code you point out definitely breaks this - we haven't done a
>>>>> previous allocation so we can start from the first AG, but we sure as
>>>>> hell still need minleft set to the number of blocks needed for a
>>>>> format change or btree split.
>>>> Just to point out yet another problem in this code (one that's just
>>>> been tripped over @ agami) is unwritten extent conversion.
>>>>
>>>> Basically, we don't do an allocation here, so when we end up in
>>>> xfs_bmap_add_extent_unwritten_real() with a null firstblock. Hence
>>>> the cases where conversion can cause a split - case
>>>> MASK(LEFT_FILLING), MASK(RIGHT_FILLING) and 0 (convert the middle of
>>>> an extent) - we can select an AG that doesn't have enough space for
>>>> the entire split as we've ignored the number of blocks we might
>>>> need to allocate in the split (the minleft parameter) entirely.
>>>>
>>>> I suspect that xfs_bmbt_split() needs to handle the null first block
>>>> case slightly differently - the minleft parameter passed to the
>>>> allocation should not be zero - it should be the number of levels
>>>> above the current level left in the tree. i.e:
>>>>
>>>> 	minleft = be16_to_cpu(ifp->if_broot->bb_level) + 1;
>>>>
>>>> If we've already got a firstblock set, then this should have already
>>>> been taken into account (i.e. we still need to fix the low space
>>>> case where it got ignored as we were discussing).
>>> Funny.  I tested the exact same change last week to try to fix the same
>>> problem.  Seemed to work okay.
>>
>> Cool. Got a patch for review?
>
> I couldn't find the original patch that calculated minleft as above - instead
> here's a variant that addresses the double insert problem by retrieving the
> reservation amount from the transaction.  It could very well be overkill though.

No, that seems valid; all allocations need to pass in a reservation
for a number of blocks needed for the transaction to proceed. I did
a quick check and everything appears to be reserving only what
is necessary for a bmbt split (or two)...

> --- fs/xfs/xfs_bmap_btree.c_1.169	2008-06-16 17:25:10.000000000 +1000
> +++ fs/xfs/xfs_bmap_btree.c	2008-06-16 18:32:45.000000000 +1000
> @@ -1496,9 +1496,12 @@ xfs_bmbt_split(
> 	if (args.fsbno == NULLFSBLOCK) {
> 		args.fsbno = lbno;
> 		args.type = XFS_ALLOCTYPE_START_BNO;
> -	} else
> +		args.minleft = xfs_trans_get_block_res(args.tp);
> +	} else {

Might be worth a comment ;)

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com