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Re: [PATCH] Prevent extent btree block allocation failures

To: Lachlan McIlroy <lachlan@xxxxxxx>, xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Subject: Re: [PATCH] Prevent extent btree block allocation failures
From: Lachlan McIlroy <lachlan@xxxxxxx>
Date: Mon, 23 Jun 2008 15:24:45 +1000
In-reply-to: <20080620052120.GA3700@disturbed>
References: <485223E4.6030404@xxxxxxx> <20080613155708.GG3700@disturbed> <485603FD.2080204@xxxxxxx> <200806161010.22476.dchinner@xxxxxxxxx> <48571A57.5090901@xxxxxxx> <20080617073949.GP3700@disturbed> <485A0AB2.4060009@xxxxxxx> <20080620052120.GA3700@disturbed>
Reply-to: lachlan@xxxxxxx
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Dave Chinner wrote:
On Thu, Jun 19, 2008 at 05:28:50PM +1000, Lachlan McIlroy wrote:
Dave Chinner wrote:
On Tue, Jun 17, 2008 at 11:58:47AM +1000, Lachlan McIlroy wrote:
Dave Chinner wrote:
On Sunday 15 June 2008 11:11 pm, Lachlan McIlroy wrote:
I'm well aware of that particular deadlock involving the freelist - I
hit it while testing.  If you look closely at the code that deadlock
can occur with or without the AG locking avoidance logic.  This is
because the rest of the transaction is unaware that an AG has been
locked due to a freelist operation.
Yes, which is why you need to prevent freelist modifications occurring
when you can't allocate anything out of the AG.
That sounds reasonable but it isn't consistent with the deadlock I saw.
One of the threads that was deadlocked had tried to allocate a data extent
in AG3 but didn't find the space.  It had modified, and hence locked, AG3
due to modifying the freelist but since it didn't get the space it needed
it had to go on to another AG.
That sounds like an exact allocation failure - there is enough
space, a large enough extent but no free space at the exact block
required. This is exactly the case that occurred with the inode
allocation - and then allocation in the same AG failed because of
alignment that wasn't taken into account by the first exact
allocation attempt. Perhaps the minalignslop calculation in
xfs_bmap_btalloc() is incorrect...
Okay I'll look into that.

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.

I see it sets a lowspace indicator which filters back up and into
some btree operations.  It appears the purpose of this feature is to
allow allocations to search for space in other AGs as in this example
from xfs_bmap_extents_to_btree():

        if (*firstblock == NULLFSBLOCK) {
                args.type = XFS_ALLOCTYPE_START_BNO;
                args.fsbno = XFS_INO_TO_FSB(mp, ip->i_ino);
        } else if (flist->xbf_low) {
                args.type = XFS_ALLOCTYPE_START_BNO;
                args.fsbno = *firstblock;
        } else {
                args.type = XFS_ALLOCTYPE_NEAR_BNO;
                args.fsbno = *firstblock;
        }

Hmmm - the only place xbf_low is used in the extent-to-btree conversion. I
don't have access to the revision history anymore, so i can't find out what
bug the xbf_low condition was added for. It certainly looks like it is
allowing the btree block to be allocated in a different AG to data block.

The lowspace algorithm was added way back in the early '90s and has been
'tweaked' many times since.


This is sort of what I was trying to do with my patch but without the
special lowspace condition.  This lowspace feature is probably broken
because there was a similar special case in xfs_bmbt_split() that got
removed with the changes that fixed the AG out-of-order locking problem.

@@ -1569,12 +1569,11 @@
        lbno = XFS_DADDR_TO_FSB(args.mp, XFS_BUF_ADDR(lbp));
        left = XFS_BUF_TO_BMBT_BLOCK(lbp);
        args.fsbno = cur->bc_private.b.firstblock;
+       args.firstblock = args.fsbno;
        if (args.fsbno == NULLFSBLOCK) {
                args.fsbno = lbno;
                args.type = XFS_ALLOCTYPE_START_BNO;
-       } else if (cur->bc_private.b.flist->xbf_low)
-               args.type = XFS_ALLOCTYPE_FIRST_AG;
-       else
+       } else
                args.type = XFS_ALLOCTYPE_NEAR_BNO;
        args.mod = args.minleft = args.alignment = args.total = args.isfl =
                args.userdata = args.minalignslop = 0;

This could be why we have allocations failing now.

Hmmm - yes, could be. Well found, Lachlan. Was there an equivalent change
to the allocation of a new root block?

Yeah but it got dropped 14 years ago in a code cleanup change!  Looks like
it was by mistake too.  There used to be another special case for converting
delayed allocations that had the same semantics as this low space trick - it
used XFS_ALLOCTYPE_FIRST_AG instead - maybe to try a little harder to find
space for cases where it is too late to return an error to the user.


Maybe it should
have been left in and XFS_ALLOCTYPE_FIRST_AG changed to
XFS_ALLOCTYPE_START_BNO.  But even then it could still fail because the
AG deadlock avoidance code may prevent us from searching the AG that has
the space we need.

Right. But it would definitely be more likely to find space than the current
code without re-introducing the deadlock.

Should we ditch this lowspace special condition (and the code in
xfs_bmap_btalloc()) and insist that all the space we need (using minleft)
should come from one AG?

Well, we could, but I suspect that one condition that it is used it
is safe to do so. That is, the logic goes like this:

        - allocate the last extent in an AG. By definition, that has not
          caused a AGF btree split as the trees are now empty.
        - because we haven't split any AGF btrees, we still have an unused
          transaction reservation for full AGF btree splits.
        - seeing as we have a full reservation, we can safely allocate in a
          different AG without overrunning a transaction reservation.

However, we still need to obey the AGF locking order.

Hmmmm - perhaps before allocating with minleft = 0 we need to
check if we can allocate the rest of the blocks from another AG and
lock both AGs in the correct order first, recheck we can allocate
from both of them after they are locked but before modifying anything
and only then proceed. If we can't find two AGs to allocate from then
we can safely ENOSPC without any problems. In this special case we'd then
be able to search the entire FS for space and hence only get an ENOSPC
if we are really at ENOSPC. Can you pick holes in this?

Sounds like it should work.  We may need to lock more than two AGs at once to
find all the space we need.  Since we can't lock AGs out of order then if we get
to the last AG and we still don't have enough space then we will need to try
again but start at an earlier AG (say AG 0 which should work).

So the code in xfs_alloc_vextent() that uses XFS_ALLOCTYPE_FIRST_AG and sets
minleft to 0 should work.  If we start at AG 0 and we've done the proper 
reservations
then we should eventually find all the space we need - as long as everything 
that
needs to allocate space obeys the lowspace algorithm and we always kick off each
search for space from the AG we last allocated from.


I'm worried with this approach that we could have delayed allocations and
unwritten extents that need to be converted but we can't do it because we
don't have the space we might need (but probably don't).
Delayed allocation is the issue - unwritten extent conversion failure will
simply return an error and leave the extent unwritten.
That's still a problem though - if we can't convert unwritten extents then
we can't clean dirty pages and we wont be able to unmount the filesystem.
There will be errors logged and the extents will remain unwritten.
The filesystem should still be able to be unmounted.
So returning an error from unwritten extent conversion will not re-dirty the
pages?  So we effectively throw the user's data away?

Yes, I think that can happen async writes. For anything that is sync the
error will be propagated back to application. Often there is nothing to
report errors back to on async writes - I'm not sure if the errors get
logged in that case, though...

I suspect that this is a holdover from before we had ENOSPC checking on
mmap writes - that could result in mmap oversubscribing space and the
data could not ever be written. In low memory conditions this could
deadlock the machine if we did not throw the pages away. We probably
need to reevisit this now that ->page_mkwrite() prevents
oversubscription from occurring....

Cheers,

Dave.


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