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Re: [PATCH] Prevent extent btree block allocation failures

To: Lachlan McIlroy <lachlan@xxxxxxx>
Subject: Re: [PATCH] Prevent extent btree block allocation failures
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 23 Jun 2008 16:14:21 +1000
Cc: xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
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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?

> In the case where we convert the middle of an existing unwritten extent
> we need to insert two new extents.  I might be paranoid here but I'll
> assume the worst case scenario and that we'll need space for two complete
> tree splits.

Yes, I think so. Certainly, if you look at the block reservation in
xfs_iomap_write_unwritten():

892         resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;

#define XFS_DIOSTRAT_SPACE_RES(mp, v)   \
        (XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK) + (v))

#define XFS_EXTENTADD_SPACE_RES(mp,w)   (XFS_BM_MAXLEVELS(mp,w) - 1)

It reserves enough blocks for 2 bmbt splits so I think this is
definitely a possibility we need to handle.

> The first allocation for the first insert will set minleft
> correctly but what about the allocations for splits during the second
> insert?  We could run out of space in the chosen AG because minleft wasn't
> enough.

Yeah, so we probably need pass a flag in the cursor to indicate
it's a double split case when doing the first allocation in
xfs_bmbt_split....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx


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