Re: [PATCH] xfs: account extra freespace btree splits for multiple allocations

Gao Xiang
2022-11-20T22:33:06Z

Hi Dave,

On Wed, Nov 16, 2022 at 01:51:06PM +1100, Dave Chinner wrote:
> On Tue, Nov 15, 2022 at 03:57:54PM +0800, Gao Xiang wrote:
> > On Sun, Nov 13, 2022 at 08:45:45AM +1100, Dave Chinner wrote:
> > > On Sat, Nov 12, 2022 at 07:46:33AM +0800, Gao Xiang wrote:
> > > > On Sat, Nov 12, 2022 at 07:39:05AM +1100, Dave Chinner wrote:
> > > > > On Wed, Nov 09, 2022 at 11:48:02AM +0800, Gao Xiang wrote:
> > > > > > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > > > > > index 6261599bb389..684c67310175 100644
> > > > > > --- a/fs/xfs/libxfs/xfs_alloc.c
> > > > > > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > > > > > @@ -2630,7 +2630,12 @@ xfs_alloc_fix_freelist(
> > > > > > goto out_agbp_relse;
> > > > > > }
> > > > > >
> > > > > > - need = xfs_alloc_min_freelist(mp, pag);
> > > > > > + /
> > > > > > + * Also need to fulfill freespace btree splits by reservaing more
> > > > > > + * blocks to perform multiple allocations from a single AG and
> > > > > > + * transaction if needed.
> > > > > > + */
> > > > > > + need = xfs_alloc_min_freelist(mp, pag) * (1 + args->postallocs);
> > > > > > if (!xfs_alloc_space_available(args, need, flags |
> > > > > > XFS_ALLOC_FLAG_CHECK))
> > > > > > goto out_agbp_relse;
> > > > > > @@ -2654,7 +2659,7 @@ xfs_alloc_fix_freelist(
> > > > > > xfs_agfl_reset(tp, agbp, pag);
> > > > > >
> > > > > > /* If there isn't enough total space or single-extent, reject it. */
> > > > > > - need = xfs_alloc_min_freelist(mp, pag);
> > > > > > + need = xfs_alloc_min_freelist(mp, pag) * (1 + args->postallocs);
> > > > > > if (!xfs_alloc_space_available(args, need, flags))
> > > > > > goto out_agbp_relse;
> > > > > >
> > > > > > diff --git a/fs/xfs/libxfs/xfs_alloc.h b/fs/xfs/libxfs/xfs_alloc.h
> > > > > > index 2c3f762dfb58..be7f15d6a40d 100644
> > > > > > --- a/fs/xfs/libxfs/xfs_alloc.h
> > > > > > +++ b/fs/xfs/libxfs/xfs_alloc.h
> > > > > > @@ -73,6 +73,7 @@ typedef struct xfs_alloc_arg {
> > > > > > int datatype; /* mask defining data type treatment */
> > > > > > char wasdel; /* set if allocation was prev delayed */
> > > > > > char wasfromfl; /* set if allocation is from freelist */
> > > > > > + bool postallocs; /* number of post-allocations */
> > > > > > struct xfs_owner_info oinfo; /* owner of blocks being allocated */
> > > > > > enum xfs_ag_resv_type resv; /* block reservation to use */
> > > > > > #ifdef DEBUG
> > > > > > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > > > > > index 49d0d4ea63fc..ed92c6a314b6 100644
> > > > > > --- a/fs/xfs/libxfs/xfs_bmap.c
> > > > > > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > > > > > @@ -3497,6 +3497,7 @@ xfs_bmap_exact_minlen_extent_alloc(
> > > > > > args.alignment = 1;
> > > > > > args.minalignslop = 0;
> > > > > >
> > > > > > + args.postallocs = 1;
> > > > > > args.minleft = ap->minleft;
> > > > > > args.wasdel = ap->wasdel;
> > > > > > args.resv = XFS_AG_RESV_NONE;
> > > > > > @@ -3658,6 +3659,7 @@ xfs_bmap_btalloc(
> > > > > > args.alignment = 1;
> > > > > > args.minalignslop = 0;
> > > > > > }
> > > > > > + args.postallocs = 1;
> > > > > > args.minleft = ap->minleft;
> > > > > > args.wasdel = ap->wasdel;
> > > > > > args.resv = XFS_AG_RESV_NONE;
> > > > >
> > > > > That's not going to work. What happens when we do a full bno
> > > > > split? Or we do both a bno and a cnt split in the same allocation?
> > > >
> > > > I'm not sure if I got your point or not. I think it reserves another
> > > > full splits in the first allocation by doing:
> > > >
> > > > need = xfs_alloc_min_freelist(mp, pag) * (1 + args->postallocs);
> > > >
> > > > as I wrote above.
> > >
> > > You're changing the BMBT reservation code. If the first "post-extent
> > > BMBT block allocation" does a full split of both the bno/cnt trees,
> > > then this uses all the AGFL reservations made.
> >
> > Emmm... I have to align my understanding of this first, I think one
> > example of what you meant is
> > 1. allocate an extent for an inode with minleft = 1;
> > 2. then do extents-to-btree allocation with one block, even minleft
> > was reserved as 1 in the previous allocation but such one-block
> > allocation from non-AGFL can cause full bno/cnt splits, which
> > could takes xfs_alloc_min_freelist() blocks from AGFL and could
> > take up all AGFL blocks?
> >
> > If my understanding above is like what you said, I think the current
> > codebase may also have a chance to eat up all AGFL blocks in the first
> > allocation since more agfl blocks are only filled in
> > xfs_alloc_fix_freelist(), but later xfs_alloc_ag_vextent() could
> > cause full bno/cnt splits as well?
>

Thanks for your detailed reply.

Sorry for late reply. I pinged you on IRC last Friday but seems you are
busy, so I try to reply here as well.

> Yes, the second allocation here might only require 1 block, which is
> what args->minleft says. But the problem is that nothing is
> reserving AGFL blocks for those nested extent allocations...

Yeah, that is also what I meant.

>
> ..... because the assumption is that AGFL blocks come from free
> space and so when we are at ENOSPC bno/cnt btrees *do no require
> splits* so will not consume extra space. Hence allocation at ENOSPC
> doesn't need to take into account AGFL block usage because the AGFL
> will not be consumed.

I noticed another thing. I think the reason why the first allocation
in this case caused a cntbt split is that Zorro's workload set
sunit/swidth. Therefore, due to align requirement, I assume it
called xfs_alloc_fix_len() to fix up agbno and len.

Actually I found our workload has the similar sunit/swidth setup and
I am thinking about this these days. One thing is that why we need
freespace btree splits when consuming free blocks.

Another thing is that considering we're near ENOSPC, and bno/cnt
btrees has only a few records. If we allocates without alignment,
I also think bno/cnt btrees do no require splits so it will not
consume extra space since the overall extents only decrease.

Yet how about allocating with alignment? It seems that it can add
another free extent in order to fulfill the alignment. I'm not sure
if it can cause some corner cases here.

>
> Similarly, if we have enough free space records to split a free
> space btree block, we have enough free space to refill the AGFL
> multiple times and we don't have to reserve space for them.
>
> IOWs, the allocation code has, historically, never had to care about
> AGFL refilling when the AG is near ENOSPC as nothing will consume
> AGFL blocks when the AG is near empty.
>
> This is the design assumption that AG reservations broke. This is
> why I'm asking you to look into taking blocks that are supposedly
> reserved for the AGFL, because as reserved space is used, the
> bno/cnt btrees will shrink and return those blocks to free space and
> hence they are still available for reserved allocations to use as
> the real physical ENOSPC condition approaches.

Yeah, intuitively I also imagine like what you said. However, does it
have strictly monotonicity, especially with stripe alignment setup?

>
> The more I look at this, the more I think overall answer to this
> problem is to allow AGFL refilling to ignore AG reserves rather than
> causing ENOSPC....

Could you give more details how to fit this case? Also we have a
short talk last Wednesday (sorry that I had an urgent thing at that
time). You mentioned "the simple solution is something like
min(ag reservation blocks, needed AGFL blocks) instead of accounting
them separately", could you give an example for this case as well?

Nevertheless, I wonder if `need` now can be fixed up as
need = xfs_alloc_min_freelist(mp, pag) * (1 + args->minleft);

>
> ----
>
> Regardless of the above, answers to the rest of you questions follow.
>
> > Please help correct me if my understanding about your ask is wrong.
> >
> > >
> > > How many blocks does a BMBT split need to allocate?
> >
> > IMO, a full bmbt split can allocate btree level blocks at maximum,
> > but if these block allocation cause bno/cnt btree splits, that
> > needs more than such blocks.
>
> And how many individual allocations does that require?

ok, you're right, so args->minleft may be enough.

>
> > So I think that's why AGFL is needed
> > for XFS. IOWs, that is to prepare enough blocks for bno/cnt splits
> > to avoid cyclic dependency.
>
> The AGFL is there to ensure any *one* space allocation succeeds.
>
> > But I'm not sure if the current AGFL reservation works properly
> > if multiple allocations must be succeeded in the same AG, see below..)
>
> Right, it does not provide any guarantees across mutliple successive
> allocations like an extent + BMBT split chain. That's what
> args->minleft is supposed to provide.
>
> However, it does provide the guarantee that when near ENOSPC,
> bno/cnt splits and hence AGFL consumption will not occur, thereby
> ensuring that if args->minleft is reserved correctly, operation
> right up to ENOSPC will work correctly without AGFL reservations
> because the AGFL will not be consumed.
>
> Hence my comments above about the problem being the way AG
> reservations moved ENOSPC from "AG physically empty" to "AG still
> has thousands of free extents but remaining space unavailable to
> user data allocation".

Partially because of that, the main reason I think may be due to
stripe alignment.

>
> > > > > Regardless, I don't see anything wrong with the allocation setup -
> > > > > it's telling the allocation code exactly what it needs for
> > > > > subsequent BMBT block allocations to succeed (i.e. args->minleft).
> > > >
> > > > In the long term, I think the main point is that args->minleft doesn't
> > > > have the exact meaning. I don't know how many blocks should be counted
> > > > by args->minleft or other ways.
> > >
> > > args->minleft has an *exact* meaning - that the AG must have that
> > > many blocks left available for potential btree record insertion
> > > allocations after the initial extent is allocated. For inode fork
> > > allocations, the BMBT blocks required is defined by
> > > xfs_bmapi_minleft(). For inode chunk extent allocation and inobt
> > > record insertion, it is defined by the pre-calculated
> > > igeo->inobt_maxlevels variable.
> > >
> > > IOWs, this "postalloc" concept is redundant - minleft already
> > > provides the maximum number of single block allocations that need to
> > > have space reserved in the AG for the initial extent allocation to
> > > succeed. i.e. the allocation setup is already taking into account
> > > blocks needed for extra allocations within the AG, but that's not
> > > being handled correctly by the AG allocation code.
> >
> > I don't think it's the case as I described in the patch commit message,
> > if we go over the words at the top, the main point is
> >
> > In the first allocation, minleft = 1, the current allocator assumes
> > it can allocate an extent with 27 blocks (the remaining blocks are
> > 18276 per-AG reservation, 6 for AGFL reservation, 1 for inode extents
> > -to-btree for the following allocation).
> >
> > But here in order to finish this allocation with 27 blocks, it splits
> > cntbt so that it takes another unexpected block from AGFL, and
> > that wasn't accounted in minleft (or with any other fields) before.
> >
> > I don't think it can be directly described by minleft because
> > such extra bno/cntbt reservation needs more knowledge of bno/cntbt
> > internals (such as current bno/cnt btree levels), so I don't think
> > it should belong to BMBT allocation code at least.
> >
> > So here I introduced another variable to describe the total number
> > of post-allocations, I think it's just enough to resolve the inode
> > extents-to-btree bno/cntbt reservation issue.
>
> extents-to-btree is the degenerate case of a btree split. It's
> moving the in-inode extent block to a single btree root block - it's
> the same case as having a multi-level BMBT and splitting a single
> leaf block during an xfs_btree_insert() call. Both require a second
> discrete allocation to be made in the same transaction from the same
> AG.
>
> But if that xfs_btree_insert() call triggers a multi-level btree
> split, we've now got more than 1 "post allocation" allocation being
> done - there's one allocation for every level that needs to have a
> block split. To handle this, we'd need to set up this args.postalloc
> variable with the number of allocations a btree split might require.
>
> What I'm trying to tell you is that args->minleft is already
> configured with exactly this number of blocks/post-allocations that
> the btree split might require, and hence allow the allocation code
> to select an AG with the right amount of space needed before it
> starts.

Yeah, you are right.

>
>
> > > On review, it is quite possible that args->minleft is not being
> > > handled by the BMBT and inobt block allocation code correctly.
> > > Shouldn't btree block allocation drop args->minleft by 1 for
> > > each block that is allocated?
> >
> > At least, in order to convert from inode extents-to-btree, we need
> > another block for the following allocation, so minleft = 1 here.
> >
> > if (ifp->if_format != XFS_DINODE_FMT_BTREE)
> > return 1;
>
> Yes, as I said above, that's the degenerate case where we only need
> to allocate a root block and set the btree level to 1.
>
> > So I guess what you meant is
> > return be16_to_cpu(ifp->if_broot->bb_level) + 1; ?
> >
> > I don't know why it has another 1 here,
>
> It's a btree. What does a full height btree split do?
>
> It adds a block to each existing level, and splits the root block
> into two. Which means we need to increase the tree height by 1 and
> allocate a new root block. IOWs, the number of allocations/blocks
> needed by a full split is (current height + 1).

Agreed, sorry for missing that point.

>
> > yet even if we account an
> > extra block here, I think it doesn't have some critical result
> > since the worst case is that it just returns -ENOSPC in advance.
> >
> > But in principle, most users use terabytes XFS, so I think such
> > one extra block doesn't matter too much. I will update this if
> > such 1 is meaningless, but it doesn't actually contribute to the
> > real shutdown issue.
>
> I think you misunderstood what I was asking. Let's unroll the
> extent allocation/BMBT record insert loop:
>
> extent allocation
> args.minleft = bb_level + 1
> xfs_alloc_vextent(args)
> bmbt record insert
> xfs_btree_insert()
> leaf split
> xfs_bmbt_alloc_block()
> args.minleft = ???
> xfs_alloc_vextent(args)
> level 1 node split
> xfs_bmbt_alloc_block()
> args.minleft = ???
> xfs_alloc_vextent(args)
> level 2 node split
> xfs_bmbt_alloc_block()
> args.minleft = ???
> xfs_alloc_vextent(args)
> ....
> root split
> xfs_bmbt_alloc_block()
> args.minleft = ???
> xfs_alloc_vextent(args)
>
> A BMBT split results in a chain of individual allocations. What
> should args.minleft be set to on each of these allocations, and
> what context do we have to ensure it is set correctly?
> So the question I was asking was whether what we are doing with
> args->minleft for each allocation in the chain is correct, and
> whether they need modification if we have to take into account the
> AGFL block refilling that may need to occur after each BMBT block
> allocation?
>
> Indeed, if we get the initial extent allocation reservation correct,
> does minleft even matter for the rest of the allocations in the
> chain?
>
> Looking at xfs_bmbt_alloc_block(), it sets args.minleft = 0 if there
> was a previous allocation in the transaction (i.e. args.fsbno !=
> NULLFSBLOCK). It assumes that the original extent reservation set
> args.minleft appropriately to reserve enough space for all
> subsequent calls to xfs_bmbt_alloc_block() in this transaction to
> succeed.
>
> Hence, given the way it is implemented right now, all we need to do
> is ensure that the initial allocation has all the space reservation
> the entire operation may need and the rest is good, yes?
>
> xfs_bmap_extents_to_btree() also sets args->minleft = 0, so as long
> as the first allocation in the transaction has reserved enough
> blocks in args->minleft it doesn't need any special help, either.
>
> So, yes, you are right that avoiding ENOSPC when running multiple
> allocations in a single transaction is all based on the initial
> allocation ensuring there is enough space in the AG for all
> subsequent allocations to succeed. But there's a lot more to it than
> that....

Yeah..

>
> > > > > The problem here is that the internal allocation code is failing to
> > > > > handle the corner case where space is just about gone correctly.
> > > > >
> > > > > As I pointed out previously - we have a huge amount of reserve space
> > > > > available in the AG here, so why not use some of the reserve space
> > > > > to get out of this temporary deficit corner case? We can argue that
> > > > > it's not really a deficit, either, because moving free blocks to the
> > > > > free list still accounts them as unused and free, so could still
> > > > > make up part of the unused reservation....
> > > > >
> > > > > i.e. is the problem here simply that we don't allow AGFL blocks to
> > > > > be considered part of the reserved free space?
> > > >
> > > > I don't know how to simply reuse per-AG reservation blocks for this,
> > >
> > > I don't know either, which is *why I asked the question*. i.e. I'm
> > > asking for you to investigate a potential alternative solution that
> > > challenges a design assumption this code makes. i.e. AGBNO and AGCNT
> > > btree blocks are considered free space because when we are at ENOSPC
> > > they are empty.
> > >
> > > However, with this ag reservation code, we can be at ENOSPC when
> > > there are still tens of thousands of free extents, and hence the
> > > AGBNO and AGCNT btree blocks are used space, not free space. The
> > > AGFL accounting is based on AGFL blocks being considered free space,
> > > which matches the AG btree blocks being considered free space, and
> > > so maybe the root of the problem here is the assumption that AG
> > > btree blocks and AGFL blocks are accounted as free space rather than
> > > part of this new "reserved space"....
> >
> > I have strong feeling that the current per-AG reservation code (or
> > AGFL reservation as in xfs_alloc_min_freelist() ) doesn't work
> > properly for multiple allocations in the same AG in order to make
> > sure such multiple allocations all succeed.
> >
> > Also, a wilder question is that I'm not sure why such multiple
> > allocations in oneshot _cannot_ be handled with a defer ops as
> > some new log intent, so that we don't need to care about minleft
> > messy anymore.
>
> We do use intents and deferred ops for BMBT freeing and reflink
> based insertion, but those only log changes to individual records in
> the btree. They do not record internal btree shape changes at all.
> Yes, we could convert normal extent allocation to use these intents
> as well, but that doesn't solve the problem of chained allocations
> within a single AG.
>
> IOWs, the chain of allocations for a BMBT split I mention above
> still exists for record level intents. To handle the btree split
> case as a chain of intents involves a whole new level of complexity
> and overhead in the btree code, and likely introduces more problems
> at ENOSPC than it solves...

My question was that if we convert some allocations into dfops, does
inode extents-to-btree needs to be strictly allocated in this AG
anymore? ... Does that tend to be a long-term plan?

Sorry I'm not good at replying a long email, hopefully reflect what I'm
thinking.

Thanks,
Gao Xiang

>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com