Prior to the removal of xfs_ialloc_next_ag, we would increment the agi
rotor and return the *old* value.  atomic_inc_return returns the new
value, which causes mkfs to allocate the root directory in AG 1.  Put
back the old behavior (at least for mkfs) by subtracting 1 here.

Fixes: 20a5eab4 ("xfs: convert xfs_ialloc_next_ag() to an atomic")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

If the end position of a GETFSMAP query overlaps an allocated space and
we're using the free space info to generate fsmap info, the akeys
information gets fed into the fsmap formatter with bad results.
Zero-init the space.

Reported-by: syzbot+090ae72d552e6bd93cfe@syzkaller.appspotmail.com
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

Merge tag 'xfs-alloc-perag-conversion' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs into xfs-6.3-merge-A

xfs: per-ag centric allocation alogrithms

This series continues the work towards making shrinking a filesystem
possible.  We need to be able to stop operations from taking place
on AGs that need to be removed by a shrink, so before shrink can be
implemented we need to have the infrastructure in place to prevent
incursion into AGs that are going to be, or are in the process, of
being removed from active duty.

The focus of this is making operations that depend on access to AGs
use the perag to access and pin the AG in active use, thereby
creating a barrier we can use to delay shrink until all active uses
of an AG have been drained and new uses are prevented.

This series starts by fixing some existing issues that are exposed
by changes later in the series. They stand alone, so can be picked
up independently of the rest of this patchset.

The most complex of these fixes is cleaning up the mess that is the
AGF deadlock avoidance algorithm. This algorithm stores the first
block that is allocated in a transaction in tp->t_firstblock, then
uses this to try to limit future allocations within the transaction
to AGs at or higher than the filesystem block stored in
tp->t_firstblock. This depends on one of the initial bug fixes in
the series to move the deadlock avoidance checks to
xfs_alloc_vextent(), and then builds on it to relax the constraints
of the avoidance algorithm to only be active when a deadlock is
possible.

We also update the algorithm to record allocations from higher AGs
that are allocated from, because we when we need to lock more than
two AGs we still have to ensure lock order is correct. Therefore we
can't lock AGs in the order 1, 3, 2, even though tp->t_firstblock
indicates that we've allocated from AG 1 and so AG is valid to lock.
It's not valid, because we already hold AG 3 locked, and so
tp->t-first_block should actually point at AG 3, not AG 1 in this
situation.

It should now be obvious that the deadlock avoidance algorithm
should record AGs, not filesystem blocks. So the series then changes
the transaction to store the highest AG we've allocated in rather
than a filesystem block we allocated.  This makes it obvious what
the constraints are, and trivial to update as we lock and allocate
from various AGs.

With all the bug fixes out of the way, the series then starts
converting the code to use active references. Active reference
counts are used by high level code that needs to prevent the AG from
being taken out from under it by a shrink operation. The high level
code needs to be able to handle not getting an active reference
gracefully, and the shrink code will need to wait for active
references to drain before continuing.

Active references are implemented just as reference counts right now
- an active reference is taken at perag init during mount, and all
other active references are dependent on the active reference count
being greater than zero. This gives us an initial method of stopping
new active references without needing other infrastructure; just
drop the reference taken at filesystem mount time and when the
refcount then falls to zero no new references can be taken.

In future, this will need to take into account AG control state
(e.g. offline, no alloc, etc) as well as the reference count, but
right now we can implement a basic barrier for shrink with just
reference count manipulations. As such, patches to convert the perag
state to atomic opstate fields similar to the xfs_mount and xlog
opstate fields follow the initial active perag reference counting
patches.

The first target for active reference conversion is the
for_each_perag*() iterators. This captures a lot of high level code
that should skip offline AGs, and introduces the ability to
differentiate between a lookup that didn't have an online AG and the
end of the AG iteration range.

From there, the inode allocation AG selection is converted to active
references, and the perag is driven deeper into the inode allocation
and btree code to replace the xfs_mount. Most of the inode
allocation code operates on a single AG once it is selected, hence
it should pass the perag as the primary referenced object around for
allocation, not the xfs_mount. There is a bit of churn here, but it
emphasises that inode allocation is inherently an allocation group
based operation.

Next the bmap/alloc interface undergoes a major untangling,
reworking xfs_bmap_btalloc() into separate allocation operations for
different contexts and failure handling behaviours. This then allows
us to completely remove the xfs_alloc_vextent() layer via
restructuring the xfs_alloc_vextent/xfs_alloc_ag_vextent() into a
set of realtively simple helper function that describe the
allocation that they are doing. e.g.  xfs_alloc_vextent_exact_bno().

This allows the requirements for accessing AGs to be allocation
context dependent. The allocations that require operation on a
single AG generally can't tolerate failure after the allocation
method and AG has been decided on, and hence the caller needs to
manage the active references to ensure the allocation does not race
with shrink removing the selected AG for the duration of the
operation that requires access to that allocation group.

Other allocations iterate AGs and so the first AG is just a hint -
these do not need to pin a perag first as they can tolerate not
being able to access an AG by simply skipping over it. These require
new perag iteration functions that can start at arbitrary AGs and
wrap around at arbitrary AGs, hence a new set for
for_each_perag_wrap*() helpers to do this.

Next is the rework of the filestreams allocator. This doesn't change
any functionality, but gets rid of the unnecessary multi-pass
selection algorithm when the selected AG is not available. It
currently does a lookup pass which might iterate all AGs to select
an AG, then checks if the AG is acceptible and if not does a "new
AG" pass that is essentially identical to the lookup pass. Both of
these scans also do the same "longest extent in AG" check before
selecting an AG as is done after the AG is selected.

IOWs, the filestreams algorithm can be greatly simplified into a
single new AG selection pass if the there is no current association
or the currently associated AG doesn't have enough contiguous free
space for the allocation to proceed.  With this simplification of
the filestreams allocator, it's then trivial to convert it to use
for_each_perag_wrap() for the AG scan algorithm.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

* tag 'xfs-alloc-perag-conversion' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs: (42 commits)
  xfs: refactor the filestreams allocator pick functions
  xfs: return a referenced perag from filestreams allocator
  xfs: pass perag to filestreams tracing
  xfs: use for_each_perag_wrap in xfs_filestream_pick_ag
  xfs: track an active perag reference in filestreams
  xfs: factor out MRU hit case in xfs_filestream_select_ag
  xfs: remove xfs_filestream_select_ag() longest extent check
  xfs: merge new filestream AG selection into xfs_filestream_select_ag()
  xfs: merge filestream AG lookup into xfs_filestream_select_ag()
  xfs: move xfs_bmap_btalloc_filestreams() to xfs_filestreams.c
  xfs: use xfs_bmap_longest_free_extent() in filestreams
  xfs: get rid of notinit from xfs_bmap_longest_free_extent
  xfs: factor out filestreams from xfs_bmap_btalloc_nullfb
  xfs: convert trim to use for_each_perag_range
  xfs: convert xfs_alloc_vextent_iterate_ags() to use perag walker
  xfs: move the minimum agno checks into xfs_alloc_vextent_check_args
  xfs: fold xfs_alloc_ag_vextent() into callers
  xfs: move allocation accounting to xfs_alloc_vextent_set_fsbno()
  xfs: introduce xfs_alloc_vextent_prepare()
  xfs: introduce xfs_alloc_vextent_exact_bno()
  ...

Now that the filestreams allocator is largely rewritten,
restructure the main entry point and pick function to seperate out
the different operations cleanly. The MRU lookup function should not
handle the start AG selection on MRU lookup failure, and nor should
the pick function handle building the association that is inserted
into the MRU.

This leaves the filestreams allocator fairly clean and easy to
understand, returning to the caller with an active perag reference
and a target block to allocate at.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Now that the filestreams AG selection tracks active perags, we need
to return an active perag to the core allocator code. This is
because the file allocation the filestreams code will run are AG
specific allocations and so need to pin the AG until the allocations
complete.

We cannot rely on the filestreams item reference to do this - the
filestreams association can be torn down at any time, hence we
need to have a separate reference for the allocation process to pin
the AG after it has been selected.

This means there is some perag juggling in allocation failure
fallback paths as they will do all AG scans in the case the AG
specific allocation fails. Hence we need to track the perag
reference that the filestream allocator returned to make sure we
don't leak it on repeated allocation failure.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Pass perags instead of raw ag numbers, avoiding the need for the
special peek function for the tracing code.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

xfs_filestream_pick_ag() is now ready to rework to use
for_each_perag_wrap() for iterating the perags during the AG
selection scan.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Rather than just track the agno of the reference, track a referenced
perag pointer instead. This will allow active filestreams to prevent
AGs from going away until the filestreams have been torn down.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Because it now stands out like a sore thumb. Factoring out this case
starts the process of simplifying xfs_filestream_select_ag() again.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Picking a new AG checks the longest free extent in the AG is valid,
so there's no need to repeat the check in
xfs_filestream_select_ag(). Remove it.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

This is largely a wrapper around xfs_filestream_pick_ag() that
repeats a lot of the lookups that we just merged back into
xfs_filestream_select_ag() from the lookup code. Merge the
xfs_filestream_new_ag() code back into _select_ag() to get rid
of all the unnecessary logic.

Indeed, this makes it obvious that if we have no parent inode,
the filestreams allocator always selects AG 0 regardless of whether
it is fit for purpose or not.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

The lookup currently either returns the cached filestream AG or it
calls xfs_filestreams_select_lengths() to looks up a new AG. This
has verify the AG that is selected, so we end up doing "select a new
AG loop in a couple of places when only one really is needed.  Merge
the initial lookup functionality with the length selection so that
we only need to do a single pick loop on lookup or verification
failure.

This undoes a lot of the factoring that enabled the selection to be
moved over to the filestreams code. It makes
xfs_filestream_select_ag() an awful messier, but it has to be made
worse before it can get better in future patches...
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

xfs_bmap_btalloc_filestreams() calls two filestreams functions to
select the AG to allocate from. Both those functions end up in
the same selection function that iterates all AGs multiple times.
Worst case, xfs_bmap_btalloc_filestreams() can iterate all AGs 4
times just to select the initial AG to allocate in.

Move the AG selection to fs/xfs/xfs_filestreams.c as a single
interface so that the inefficient AG interation is contained
entirely within the filestreams code. This will allow the
implementation to be simplified and made more efficient in future
patches.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

The code in xfs_bmap_longest_free_extent() is open coded in
xfs_filestream_pick_ag(). Export xfs_bmap_longest_free_extent and
call it from the filestreams code instead.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

It is only set if reading the AGF gets a EAGAIN error. Just return
the EAGAIN error and handle that error in the callers.

This means we can remove the not_init parameter from
xfs_bmap_select_minlen(), too, because the use of not_init there is
pessimistic. If we can't read the agf, it won't increase blen.

The only time we actually care whether we checked all the AGFs for
contiguous free space is when the best length is less than the
minimum allocation length. If not_init is set, then we ignore blen
and set the minimum alloc length to the absolute minimum, not the
best length we know already is present.

However, if blen is less than the minimum we're going to ignore it
anyway, regardless of whether we scanned all the AGFs or not.  Hence
not_init can go away, because we only use if blen is good from
the scanned AGs otherwise we ignore it altogether and use minlen.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

There's many if (filestreams) {} else {} branches in this function.
Split it out into a filestreams specific function so that we can
then work directly on cleaning up the filestreams code without
impacting the rest of the allocation algorithms.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

To convert it to using active perag references and hence make it
shrink safe.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Now that the AG iteration code in the core allocation code has been
cleaned up, we can easily convert it to use a for_each_perag..()
variant to use active references and skip AGs that it can't get
active references on.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

All of the allocation functions now extract the minimum allowed AG
from the transaction and then use it in some way. The allocation
functions that are restricted to a single AG all check if the
AG requested can be allocated from and return an error if so. These
all set args->agno appropriately.

All the allocation functions that iterate AGs use it to calculate
the scan start AG. args->agno is not set until the iterator starts
walking AGs.

Hence we can easily set up a conditional check against the minimum
AG allowed in xfs_alloc_vextent_check_args() based on whether
args->agno contains NULLAGNUMBER or not and move all the repeated
setup code to xfs_alloc_vextent_check_args(), further simplifying
the allocation functions.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

We don't need the multiplexing xfs_alloc_ag_vextent() provided
anymore - we can just call the exact/near/size variants directly.
This allows us to remove args->type completely and stop using
args->fsbno as an input to the allocator algorithms.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Move it from xfs_alloc_ag_vextent() so we can get rid of that layer.
Rename xfs_alloc_vextent_set_fsbno() to xfs_alloc_vextent_finish()
to indicate that it's function is finishing off the allocation that
we've run now that it contains much more functionality.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Now that we have wrapper functions for each type of allocation we
can ask for, we can start unravelling xfs_alloc_ag_vextent(). That
is essentially just a prepare stage, the allocation multiplexer
and a post-allocation accounting step is the allocation proceeded.

The current xfs_alloc_vextent*() wrappers all have a prepare stage,
the allocation operation and a post-allocation accounting step.

We can consolidate this by moving the AG alloc prep code into the
wrapper functions, the accounting code in the wrapper accounting
functions, and cut out the multiplexer layer entirely.

This patch consolidates the AG preparation stage.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Two of the callers to xfs_alloc_vextent_this_ag() actually want
exact block number allocation, not anywhere-in-ag allocation. Split
this out from _this_ag() as a first class citizen so no external
extent allocation code needs to care about args->type anymore.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

The remaining callers of xfs_alloc_vextent() are all doing NEAR_BNO
allocations. We can replace that function with a new
xfs_alloc_vextent_near_bno() function that does this explicitly.

We also multiplex NEAR_BNO allocations through
xfs_alloc_vextent_this_ag via args->type. Replace all of these with
direct calls to xfs_alloc_vextent_near_bno(), too.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Change obvious callers of single AG allocation to use
xfs_alloc_vextent_start_bno(). Callers no long need to specify
XFS_ALLOCTYPE_START_BNO, and so the type can be driven inward and
removed.

While doing this, also pass the allocation target fsb as a parameter
rather than encoding it in args->fsbno.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Change obvious callers of single AG allocation to use
xfs_alloc_vextent_first_ag(). This gets rid of
XFS_ALLOCTYPE_FIRST_AG as the type used within
xfs_alloc_vextent_first_ag() during iteration is _THIS_AG. Hence we
can remove the setting of args->type from all the callers of
_first_ag() and remove the alloctype.

While doing this, pass the allocation target fsb as a parameter
rather than encoding it in args->fsbno. This starts the process
of making args->fsbno an output only variable rather than
input/output.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

There are several different contexts xfs_bmap_btalloc() handles, and
large chunks of the code execute independent allocation contexts.
Try to untangle this mess a bit.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Change obvious callers of single AG allocation to use
xfs_alloc_vextent_this_ag(). Drive the per-ag grabbing out to the
callers, too, so that callers with active references don't need
to do new lookups just for an allocation in a context that already
has a perag reference.

The only remaining caller that does single AG allocation through
xfs_alloc_vextent() is xfs_bmap_btalloc() with
XFS_ALLOCTYPE_NEAR_BNO. That is going to need more untangling before
it can be converted cleanly.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

There's a bit of a recursive conundrum around
xfs_alloc_ag_vextent(). We can't first call xfs_alloc_ag_vextent()
without preparing the AGFL for the allocation, and preparing the
AGFL calls xfs_alloc_ag_vextent() to prepare the AGFL for the
allocation. This "double allocation" requirement is not really clear
from the current xfs_alloc_fix_freelist() calls that are sprinkled
through the allocation code.

It's not helped that xfs_alloc_ag_vextent() can actually allocate
from the AGFL itself, but there's special code to prevent AGFL prep
allocations from allocating from the free list it's trying to prep.
The naming is also not consistent: args->wasfromfl is true when we
allocated _from_ the free list, but the indication that we are
allocating _for_ the free list is via checking that (args->resv ==
XFS_AG_RESV_AGFL).

So, lets make this "allocation required for allocation" situation
clear by moving it all inside xfs_alloc_ag_vextent(). The freelist
allocation is a specific XFS_ALLOCTYPE_THIS_AG allocation, which
translated directly to xfs_alloc_ag_vextent_size() allocation.

This enables us to replace __xfs_alloc_vextent_this_ag() with a call
to xfs_alloc_ag_vextent(), and we drive the freelist fixing further
into the per-ag allocation algorithm.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

The core of the per-ag iteration is effectively doing a "this ag"
allocation on one AG at a time. Use the same code to implement the
core "this ag" allocation in both xfs_alloc_vextent_this_ag()
and xfs_alloc_vextent_iterate_ags().

This means we only call xfs_alloc_ag_vextent() from one place so we
can easily collapse the call stack in future patches.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

It's a multiplexing mess that can be greatly simplified, and really
needs to be simplified to allow active per-ag references to
propagate from initial AG selection code the the bmapi code.

This splits the code out into separate a parameter checking
function, an iterator function, and allocation completion functions
and then implements the individual policies using these functions.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

In several places we iterate every AG from a specific start agno and
wrap back to the first AG when we reach the end of the filesystem to
continue searching. We don't have a primitive for this iteration
yet, so add one for conversion of these algorithms to per-ag based
iteration.

The filestream AG select code is a mess, and this initially makes it
worse. The per-ag selection needs to be driven completely into the
filestream code to clean this up and it will be done in a future
patch that makes the filestream allocator use active per-ag
references correctly.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

We currently don't have any flags or operational state in the
xfs_perag except for the pagf_init and pagi_init flags. And the
agflreset flag. Oh, there's also the pagf_metadata and pagi_inodeok
flags, too.

For controlling per-ag operations, we are going to need some atomic
state flags. Hence add an opstate field similar to what we already
have in the mount and log, and convert all these state flags across
to atomic bit operations.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

This is currently a spinlock lock protected rotor which can be
implemented with a single atomic operation. Change it to be more
efficient and get rid of the m_agirotor_lock. Noticed while
converting the inode allocation AG selection loop to active perag
references.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Lots of code in the inobt infrastructure is passed both xfs_mount
and perags. We only need perags for the per-ag inode allocation
code, so reduce the duplication by passing only the perags as the
primary object.

This ends up reducing the code size by a bit:

	   text    data     bss     dec     hex filename
orig	1138878  323979     548 1463405  16546d (TOTALS)
patched	1138709  323979     548 1463236  1653c4 (TOTALS)
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Convert the inode allocation routines to use active perag references
or references held by callers rather than grab their own. Also drive
the perag further inwards to replace xfs_mounts when doing
operations on a specific AG.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Callers have referenced perags but they don't pass it into
xfs_imap() so it takes it's own reference. Fix that so we can change
inode allocation over to using active references.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

So that they all output the same information in the traces to make
debugging refcount issues easier.

This means that all the lookup/drop functions no longer need to use
the full memory barrier atomic operations (atomic*_return()) so
will have less overhead when tracing is off. The set/clear tag
tracepoints no longer abuse the reference count to pass the tag -
the tag being cleared is obvious from the _RET_IP_ that is recorded
in the trace point.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

We need to be able to dynamically remove instantiated AGs from
memory safely, either for shrinking the filesystem or paging AG
state in and out of memory (e.g. supporting millions of AGs). This
means we need to be able to safely exclude operations from accessing
perags while dynamic removal is in progress.

To do this, introduce the concept of active and passive references.
Active references are required for high level operations that make
use of an AG for a given operation (e.g. allocation) and pin the
perag in memory for the duration of the operation that is operating
on the perag (e.g. transaction scope). This means we can fail to get
an active reference to an AG, hence callers of the new active
reference API must be able to handle lookup failure gracefully.

Passive references are used in low level code, where we might need
to access the perag structure for the purposes of completing high
level operations. For example, buffers need to use passive
references because:
- we need to be able to do metadata IO during operations like grow
  and shrink transactions where high level active references to the
  AG have already been blocked
- buffers need to pin the perag until they are reclaimed from
  memory, something that high level code has no direct control over.
- unused cached buffers should not prevent a shrink from being
  started.

Hence we have active references that will form exclusion barriers
for operations to be performed on an AG, and passive references that
will prevent reclaim of the perag until all objects with passive
references have been reclaimed themselves.

This patch introduce xfs_perag_grab()/xfs_perag_rele() as the API
for active AG reference functionality. We also need to convert the
for_each_perag*() iterators to use active references, which will
start the process of converting high level code over to using active
references. Conversion of non-iterator based code to active
references will be done in followup patches.

Note that the implementation using reference counting is really just
a development vehicle for the API to ensure we don't have any leaks
in the callers. Once we need to remove perag structures from memory
dyanmically, we will need a much more robust per-ag state transition
mechanism for preventing new references from being taken while we
wait for existing references to drain before removal from memory can
occur....
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

We can error out of an allocation transaction when updating BMBT
blocks when things go wrong. This can be a btree corruption, and
unexpected ENOSPC, etc. In these cases, we already have deferred ops
queued for the first allocation that has been done, and we just want
to cancel out the transaction and shut down the filesystem on error.

In fact, we do just that for production systems - the assert that we
can't have a transaction with defer ops attached unless we are
already shut down is bogus and gets in the way of debugging
whatever issue is actually causing the transaction to be cancelled.

Remove the assert because it is causing spurious test failures to
hang test machines.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>