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Re: [PATCH 1/2] set minleft in xfs_bmbt_split()

To: Lachlan McIlroy <lachlan@xxxxxxx>, xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Subject: Re: [PATCH 1/2] set minleft in xfs_bmbt_split()
From: Lachlan McIlroy <lachlan@xxxxxxx>
Date: Tue, 24 Jun 2008 16:16:50 +1000
In-reply-to: <20080624052556.GR16257@xxxxxxxxxxxxxxxxxxxxx>
References: <48605744.8070400@xxxxxxx> <20080624045841.GN16257@xxxxxxxxxxxxxxxxxxxxx> <4860847B.7070703@xxxxxxx> <20080624052556.GR16257@xxxxxxxxxxxxxxxxxxxxx>
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Dave Chinner wrote:
On Tue, Jun 24, 2008 at 03:22:03PM +1000, Lachlan McIlroy wrote:
Dave Chinner wrote:
On Tue, Jun 24, 2008 at 12:09:08PM +1000, Lachlan McIlroy wrote:
The bmap btree split code relies on a previous data extent allocation
(from xfs_bmap_btalloc()) to find an AG that has sufficient space
to perform a full btree split, when inserting the extent.  When
converting unwritten extents we don't allocate a data extent so a
btree split will be the first allocation.  In this case we need to
set minleft so the allocator will pick an AG that has space to
complete the split(s).
looks good, execpt...


--- 2.6.x-agno2.orig/fs/xfs/xfs_bmap_btree.c
+++ 2.6.x-agno2/fs/xfs/xfs_bmap_btree.c
@@ -1493,12 +1493,20 @@ xfs_bmbt_split(
        left = XFS_BUF_TO_BMBT_BLOCK(lbp);
        args.fsbno = cur->bc_private.b.firstblock;
        args.firstblock = args.fsbno;
+       args.minleft = 0;
        if (args.fsbno == NULLFSBLOCK) {
                args.fsbno = lbno;
                args.type = XFS_ALLOCTYPE_START_BNO;
+               /*
+                * Make sure there is sufficient room left in the AG to
+                * complete a full tree split for an extent insert.  If
+                * we are converting the middle part of an extent then
+                * we may need space for two tree splits.
+                */
+               args.minleft = xfs_trans_get_block_res(args.tp);
I'd mention in this comment that transaction reservation needs to
take this specifically into account.
How would transaction reservation take this into account?  Are you
implying they could do something different if they knew about this

No, I'm implying that the transaction reservation better be correct.
i.e. anywhere that unwritten extent conversion can take place needs
to supply a reservation of enough blocks to allow a double split to
occur.  i.e. we are relying on the caller to get this right, so we
need to ensure that a comment explaining the potential landmine is

Okay.  With the change above we can still have xfs_bmbt_split() fail
to allocate btree blocks if it cannot find a single AG with enough
free space.  Although in my testing I haven't seen it fail yet.

We really don't want to fail here because we have already partly
modified the extent btree (ie for the case where we convert the middle
part of an unwritten extent we do an xfs_bmbt_update before the insert)
and we dont undo the damage.  Also a failure to allocate in
xfs_bmbt_split() will be caught by the XFS_WANT_CORRUPTED_GOTO() in
xfs_bmbt_insert() and the error set to EFSCORRUPTED which is only going
to create confusion.  I have another patch that allows xfs_bmbt_split()
and xfs_bmbt_newroot() to fall back to the lowspace algorithm - I'm
just testing it now.

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