Re: [RFC V2 3/3] xfs: Add support to shrink multiple empty AGs

["Nirjhar Roy (IBM)" <nirjhar.roy.lists@gmail.com>]

On 10/15/25 04:43, Darrick J. Wong wrote:
> On Tue, Sep 16, 2025 at 08:34:09PM +0530, Nirjhar Roy (IBM) wrote:
>> This patch is based on a previous RFC[1] by Gao Xiang and various
>> ideas proposed by Dave Chinner in the RFC[1].
>>
>> This patch adds the functionality to shrink the filesystem beyond
>> 1 AG. We can remove only empty AGs in order to prevent loss
>> of data. Before I summarize the overall steps of the shrink
>> process, I would like to introduce some of the terminologies:
>>
>> 1. Empty AG - An AG that is completely un-used, and no block
>>     is being used/allocated for data or metadata and no
>>     log blocks are allocated here. This ensures that the
>>     removal of this AG doesn't result in any loss of data.
> This isn't quite accurate -- the AG can have blocks in use, but only for
> the root blocks of the per-AG metadata btrees.  But that's fairly minor.

Okay, yeah maybe I will re-define it to

Empty AG - An AG that has no user data and log data. This will ensure that
    removal of this AG doesn't result in any data loss.

Does the above look fine?

>
>> 2. Active/Online AG - Online AG and active AG will be used
>>     interchangebly. An AG is active or online when all the regular
>>     operations can be done on it. When we mount a filesystem, all
>>     the AGs are by default online/active. In terms of implementation,
>>     an online AG will have number of active references greater than 0
>>     (default is 1 i.e, an AG by default is online/active).
>>
>> 3. AG offlining/deactivation - AG offlining and AG deactivation will
>>     be used interchangebly. An AG is said to be offlined/deactivated
>>     when no new high level operation can be started on the AG. This is
>>     implemented with the help of active references. When the active
>>     reference count of an AG is 0, the AG is said to be deactivated.
>>     No new active reference can be taken if the present active reference
>>     count is 0. This way a barrier is formed from preventing new high
>>     level operations to get started on an already offlined AG.
>>
>> 4. Reactivating an AG - If we try to remove an offlined AG but for some
>>     reason, we can't, then we reactivate the AG i.e, the AG will once
>>     more be in an usable state i.e, the active reference count will be
>>     set to 1. All the high level operations can now be performed on this
>>     AG. In terms of implementation, in order to activate an AG, we
>>     atomically set the active reference count to 1.
>>
>> 5. AG removal - This means that an AG no longer exists in the filesystem.
>>     It will be reflected in the usable/total size of the device too
>>     (using tools like df).
> An offline AG can still have positive passive refcount if it's in the
> process of being removed from the filesystem, right?
Yes, that is correct.
>
>> 6. New tail AG - This refers to the last AG that will be formed after
>>     the removal of 1 or more AGs. For example, if there are 4 AGs, each
>>     with 32 blocks, then there are total of 4 * 32 = 128 blocks. Now,
>>     if we remove 40 blocks, AG 3(indexed at 0) will be completely
>>     removed (32 blocks) and from AG 2, we will remove 8 blocks.
>>     So AG 2 will be the new tail AG.
>>
>> 7. Old tail AG - This is the last AG before the start of the shrink
>>     process. If the number of blocks removed is less than the AG
>>     size, then the old tail AG will be the same as the new tail
>>     AG.
>>
>> 8. AG stabilization - This simply means that the in-memory contents
>>     are synched to the disk.
>>
>> The overall steps for the removal of AG(s) are as follows:
>> PHASE 1: Preparing the AGs for removal
>> 1. Deactivate the AGs to be removed completely - This is done
>>     by the function xfs_shrinkfs_deactivate_ags(). The steps to deactivate
>>     an AG are as follows(function is xfs_perag_deactivate()):
>>       1.a Manually reserve/reduce from the global fdblock free counters
>>           the perag pagf_freeblks + pagf_flcount. This is done in order
>>           to prevent a race where, some AGs have been offlined but
>>           the delayed  allocator has already promised some bytes
>>           and the real extent/block allocation is failing due to the
>>           AG(s) being offline.
>>           If the overall shrink succeeds, we will again manually
>>           restore these counters just before the shrink transaction
>>           commits and let these global counters get adjusted
>>           automatically later.
> Wouldn't it be more correct to say that the shrink operation reserves to
> the shrink transaction the space to be removed from the incore fdblocks
> and either commits that change to the ondisk fdblocks (shrink succeeds)
> or gives it back (shrink fails)?
Well, during AG deactivation, we reduce the free fdblock in-core counter 
and then manually again restore/add these numbers before the shrink 
transaction commits so that the transaction commit can do the final 
adjustment to the counters. The manual subtraction and 
addition/restoration is done irrespective of whether the shrink succeeds 
or fails. The reason why I am doing this manual subtraction is to 
prevent the race (mentioned in point 1.a) and then manually doing the 
addition again - so that the subtraction of the fdblocks isn't done 
twice. The manual addition/restoration is done in the function 
"xfs_growfs_data_private()". Does that make sense?
>
>>       1.b Wait for the active reference to come to 0.
>>           This is done so that no other entity is racing while the removal
>>           is in progress i.e, no new high level operation can start on that
>>           AG while we are trying to remove the AG.
>>           AG deactivation will fail if the AG is non-empty at the time of
>>           deactivation.
>> 2. Once we have waited for the active references to come down to 0,
>>     we make sure that all the pending operations on that AG are completed
>>     and the in-core and on-disk structures are in synch i.e, the AG is
>>     stabilized on to the disk.
> Pending operations, as in whatever has passive refcounts (file ops,
> defer intent chains, etc)?
Yes.
>
>>     The steps to stablize the AG onto the disk are as follows:
>>     2.a We need to flush and empty the logs and wait for all the pending
>>         I/Os to complete - for this, perform a log force+ail push by
>>         calling xfs_ail_push_all_sync(). This also ensures that
>>         none of the future logged transactions will refer to these
>>         AGs during log recovery in case if sudden shutdown/crash
>>         happens while we are trying to remove these AGs. We also sync
>>         the superblock with the disk.
>>     2.b Wait for all the pending I/O to complete.
> (Redundant with 2a, yes?)
Yeah, right. I can remove this.
>
>>     2.c Wait for all the busy extents for the target AGs to be resolved
>>        (done by the function xfs_extent_busy_wait_ags())
>>     2.d Flush the xfs_discard_wq workqueue
>> 3. Once the AG is deactivated and stabilized on to the disk, we check if
>>     all the target AGs are empty, and if not, we fail the shrink process.
>>     We are not supporting partial shrink i.e, the shrink will
>>     either completely fail or completely succeed.
>>
>> PHASE 2: Shrink new tail group, punch out totally empty groups
>> 4. Once the preparation phase is over, we start the actual removal
>>     process. This is done in the function xfs_shrinkfs_remove_ags().
>>     Here we first remove the blocks, then update the metadata of
>>     new last tail AG and then remove the  AGs (and their associated
>>     data structures) one by one (in function xfs_shrinkfs_remove_ag()).
>> 5. In the end we log the changes and commit the transaction.
> What do we commit?  I think the shrink transaction has:
>
> 1. bnobt/cntbt changes to remove the post-tail space
> 2. AG header length updates
> 3. Superblock update to change sb_dblocks
Yeah, we also commit free data blocks(XFS_TRANS_SB_FDBLOCKS) and agcount 
(XFS_TRANS_SB_AGCOUNT).
>
>> Removal of each AG is done by the function xfs_shrinkfs_remove_ag().
> Removal of each incore AG structure?
I will update the description.
>
>> The steps can be outlined as follows:
>> 1. Free the per AG reservation - this will result in correct free
>>     space/used space information.
>> 2. Freeing the intents drain queue.
>> 3. Freeing busy extents list.
>> 4. Remove the perag cached buffers and then the buffer cache.
>> 5. Freeing the struct xfs_group pointer - Before this is done, we
>>     assert that all the active and passive references are down to 0.
>>     We remove all the cached buffers associated with the offlined AGs
>>     to be removed - this releases the passive references of the AGs
>>     consumed by the cached buffers.
>>
>> [1] https://lore.kernel.org/all/20210414195240.1802221-1-hsiangkao@redhat.com/
>>
>> Signed-off-by: Nirjhar Roy (IBM) <nirjhar.roy.lists@gmail.com>
>> Inspired-by: Gao Xiang <hsiangkao@linux.alibaba.com>
>> Suggested-by: Dave Chinner <david@fromorbit.com>
>> ---
>>   fs/xfs/libxfs/xfs_ag.c        | 165 +++++++++++++++-
>>   fs/xfs/libxfs/xfs_ag.h        |  14 ++
>>   fs/xfs/libxfs/xfs_alloc.c     |   9 +-
>>   fs/xfs/xfs_buf.c              |  78 ++++++++
>>   fs/xfs/xfs_buf.h              |   1 +
>>   fs/xfs/xfs_buf_item_recover.c |  37 ++--
>>   fs/xfs/xfs_extent_busy.c      |  30 +++
>>   fs/xfs/xfs_extent_busy.h      |   2 +
>>   fs/xfs/xfs_fsops.c            | 343 ++++++++++++++++++++++++++++++++--
>>   fs/xfs/xfs_trans.c            |   1 -
>>   10 files changed, 641 insertions(+), 39 deletions(-)
>>
>> diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
>> index f2b35d59d51e..1bdcd4c6d264 100644
>> --- a/fs/xfs/libxfs/xfs_ag.c
>> +++ b/fs/xfs/libxfs/xfs_ag.c
>> @@ -193,20 +193,32 @@ xfs_agino_range(
>>   }
>>   
>>   /*
>> - * Update the perag of the previous tail AG if it has been changed during
>> - * recovery (i.e. recovery of a growfs).
>> + * This function does the following:
>> + * - Updates the previous perag tail if prev_agcount < current agcount i.e, the
>> + *   filesystem has grown OR
>> + * - Updates the current tail AG when prev_agcount > current agcount i.e, the
>> + *   filesystem has shrunk beyond 1 AG OR
>> + * - Updates the current tail AG when only the last AG was shrunk or grown i.e,
>> + *   prev_agcount == mp->m_sb.sb_agcount.
>>    */
>>   int
>>   xfs_update_last_ag_size(
>>   	struct xfs_mount	*mp,
>>   	xfs_agnumber_t		prev_agcount)
>>   {
>> -	struct xfs_perag	*pag = xfs_perag_grab(mp, prev_agcount - 1);
>> +	xfs_agnumber_t		agno;
>> +	struct xfs_perag	*pag;
>>   
>> +	if (prev_agcount >= mp->m_sb.sb_agcount)
>> +		agno = mp->m_sb.sb_agcount - 1;
>> +	else
>> +		agno = prev_agcount - 1;
>> +
>> +	pag = xfs_perag_grab(mp, agno);
>>   	if (!pag)
>>   		return -EFSCORRUPTED;
>> -	pag_group(pag)->xg_block_count = __xfs_ag_block_count(mp,
>> -			prev_agcount - 1, mp->m_sb.sb_agcount,
>> +	pag_group(pag)->xg_block_count = __xfs_ag_block_count(mp, agno,
>> +			mp->m_sb.sb_agcount,
>>   			mp->m_sb.sb_dblocks);
>>   	__xfs_agino_range(mp, pag_group(pag)->xg_block_count, &pag->agino_min,
>>   			&pag->agino_max);
>> @@ -290,6 +302,48 @@ xfs_initialize_perag(
>>   	return error;
>>   }
>>   
>> +void
>> +xfs_perag_activate(struct xfs_perag	*pag)
>> +{
>> +	ASSERT(!xfs_ag_is_active(pag));
>> +	init_waitqueue_head(&pag_group(pag)->xg_active_wq);
>> +	atomic_set(&pag_group(pag)->xg_active_ref, 1);
>> +	xfs_add_fdblocks(pag_mount(pag), pag->pagf_freeblks +
>> +			pag->pagf_flcount);
>> +}
>> +
>> +bool
>> +xfs_perag_deactivate(struct xfs_perag	*pag)
>> +{
>> +	int	error = 0;
>> +
>> +	ASSERT(xfs_ag_is_active(pag));
>> +	if (!xfs_ag_is_empty(pag))
>> +		return false;
>> +	/*
>> +	 * Manually reduce/reserve (pagf_freeblks + pagf_flcount) worth of
>> +	 * free datablocks from the global counters. This is necessary
>> +	 * in order to prevent a race where, some AGs have been temporarily
>> +	 * offlined but the delayed allocator has already promised some bytes
>> +	 * and later the real extent/block allocation is failing due to
>> +	 * the AG(s) being offline.
>> +	 * If the overall shrink succeeds, we will again
>> +	 * manually restore these counters just before the shrink transaction
>> +	 * commits and let these global counters get adjusted automatically
>> +	 * later.
>> +	 */
>> +	error = xfs_dec_fdblocks(pag_mount(pag),
>> +			pag->pagf_freeblks + pag->pagf_flcount, false);
>> +	if (error)
>> +		return false;
>> +	xfs_perag_rele(pag);
>> +	do {
>> +		wait_event(pag_group(pag)->xg_active_wq,
>> +			!xfs_ag_is_active(pag));
>> +	} while (xfs_ag_is_active(pag));
> wait_event_killable, so that a fatal signal can interrupt the
> deactivation process?
Oh ,okay. I thought wait_event() is killable or a spurious wake-up can 
take place - that is why I put it in a loop. I will remove the loop.
>
>> +	return true;
>> +}
>> +
>>   static int
>>   xfs_get_aghdr_buf(
>>   	struct xfs_mount	*mp,
>> @@ -758,7 +812,6 @@ xfs_ag_shrink_space(
>>   	xfs_agblock_t		aglen;
>>   	int			error, err2;
>>   
>> -	ASSERT(pag_agno(pag) == mp->m_sb.sb_agcount - 1);
>>   	error = xfs_ialloc_read_agi(pag, *tpp, 0, &agibp);
>>   	if (error)
>>   		return error;
>> @@ -872,6 +925,106 @@ xfs_ag_shrink_space(
>>   	return err2;
>>   }
>>   
>> +/*
>> + * This function checks whether an AG is empty. An AG is eligible to be
>> + * removed if it is empty.
>> + */
>> +bool
>> +xfs_ag_is_empty(struct xfs_perag	*pag)
> xfs_perag_is_empty?
Noted.
>
>> +{
>> +	struct xfs_buf		*agfbp = NULL;
>> +	struct xfs_mount	*mp = pag_mount(pag);
>> +	bool			is_empty = false;
>> +	int			error = 0;
>> +	struct xfs_agf		*agf = NULL;
>> +
>> +	/*
>> +	 * Read the on-disk data structures to get the correct length of the AG.
>> +	 * All the AGs have the same length except the last AG.
>> +	 */
>> +	error = xfs_alloc_read_agf(pag, NULL, 0, &agfbp);
>> +	if (!error) {
>> +		agf = agfbp->b_addr;
>> +		/*
>> +		 * We don't need to check if the log blocks belong here since
>> +		 * the log blocks are taken from the number of free blocks, and
>> +		 * if the given AG has log blocks, then those many number of
>> +		 * blocks will be consumed from the number of free blocks and
>> +		 * the AG empty condition will not hold true.
>> +		 */
>> +		if (pag->pagf_freeblks + pag->pagf_flcount +
>> +			mp->m_ag_prealloc_blocks ==
>> +			be32_to_cpu(agf->agf_length)) {
>> +			is_empty = true;
> Indenting problems...
Noted.
>
>> +		}
>> +		xfs_buf_relse(agfbp);
>> +	}
>> +	return is_empty;
>> +}
>> +
>> +/*
>> + * This function removes an entire empty AG. Before removing the struct
>> + * xfs_perag reference, it removes the associated data structures. Before
>> + * removing an AG, the caller must ensure that the AG has been deactivated with
>> + * no active references and it has been fully stabilized on the disk.
>> + */
>> +void
>> +xfs_shrinkfs_remove_ag(
>> +	struct xfs_mount	*mp,
>> +	xfs_agnumber_t		agno)
>> +{
>> +	struct xfs_group	*xg = NULL;
>> +	struct xfs_perag	*cur_pag = NULL;
>> +
>> +	/*
>> +	 * Number of AGs can't be less than 2
>> +	 */
>> +	ASSERT(agno >= 2);
>> +	xg = xa_erase(&mp->m_groups[XG_TYPE_AG].xa, agno);
>> +	cur_pag = to_perag(xg);
>> +
>> +	ASSERT(!xfs_ag_is_active(cur_pag));
>> +	/*
>> +	 * Since we are freeing the AG, we should clear the perag reservations
>> +	 * for the corresponding AGs.
>> +	 */
>> +	xfs_ag_resv_free(cur_pag);
>> +	/*
>> +	 * We have already ensured in the AG preparation phase that all intents
>> +	 * for the offlined AGs have been resolved. So it safe to free it here.
>> +	 */
>> +	xfs_defer_drain_free(&xg->xg_intents_drain);
>> +	/*
>> +	 * We have already ensured in the AG preparation phase that all busy
>> +	 * extents for the offlined AGs have been resolved. So it safe to free
>> +	 * it here.
>> +	 */
>> +	kfree(xg->xg_busy_extents);
>> +	cancel_delayed_work_sync(&cur_pag->pag_blockgc_work);
>> +
>> +	/*
>> +	 * Remove all the cached buffers for the given AG.
>> +	 */
>> +	xfs_buf_cache_invalidate(cur_pag);
>> +	/*
>> +	 * Now that the cached buffers have been released, remove the
>> +	 * cache/hashtable itself. We should not change the order of the buffer
>> +	 * removal and cache removal.
>> +	 */
>> +	xfs_buf_cache_destroy(&cur_pag->pag_bcache);
>> +	/*
>> +	 * One final assert, before we remove the xg. Since the cached buffers
>> +	 * for the offlined AGs are already removed, their passive references
>> +	 * should be 0. Also, the active references are 0 too, so no new
>> +	 * operation can start and race and get new references.
>> +	 */
>> +	XFS_IS_CORRUPT(mp, atomic_read(&pag_group(cur_pag)->xg_ref) != 0);
>> +	/*
>> +	 * Finally free the struct xfs_perag of the AG.
>> +	 */
>> +	kfree_rcu_mightsleep(xg);
>> +}
>> +
>>   void
>>   xfs_growfs_compute_deltas(
>>   	struct xfs_mount	*mp,
>> diff --git a/fs/xfs/libxfs/xfs_ag.h b/fs/xfs/libxfs/xfs_ag.h
>> index f7b56d486468..bd30421eded5 100644
>> --- a/fs/xfs/libxfs/xfs_ag.h
>> +++ b/fs/xfs/libxfs/xfs_ag.h
>> @@ -112,6 +112,11 @@ static inline xfs_agnumber_t pag_agno(const struct xfs_perag *pag)
>>   	return pag->pag_group.xg_gno;
>>   }
>>   
>> +static inline bool xfs_ag_is_active(struct xfs_perag	*pag)
> xfs_perag_is_active
Noted.
>
>> +{
>> +	return atomic_read(&pag_group(pag)->xg_active_ref) > 0;
>> +}
>> +
>>   /*
>>    * Per-AG operational state. These are atomic flag bits.
>>    */
>> @@ -140,6 +145,7 @@ void xfs_free_perag_range(struct xfs_mount *mp, xfs_agnumber_t first_agno,
>>   		xfs_agnumber_t end_agno);
>>   int xfs_initialize_perag_data(struct xfs_mount *mp, xfs_agnumber_t agno);
>>   int xfs_update_last_ag_size(struct xfs_mount *mp, xfs_agnumber_t prev_agcount);
>> +bool xfs_ag_is_empty(struct xfs_perag *pag);
>>   
>>   /* Passive AG references */
>>   static inline struct xfs_perag *
>> @@ -263,6 +269,9 @@ xfs_ag_contains_log(struct xfs_mount *mp, xfs_agnumber_t agno)
>>   	       agno == XFS_FSB_TO_AGNO(mp, mp->m_sb.sb_logstart);
>>   }
>>   
>> +void xfs_perag_activate(struct xfs_perag *pag);
>> +bool xfs_perag_deactivate(struct xfs_perag *pag);
>> +
>>   static inline struct xfs_perag *
>>   xfs_perag_next_wrap(
>>   	struct xfs_perag	*pag,
>> @@ -290,6 +299,10 @@ xfs_perag_next_wrap(
>>   	return NULL;
>>   }
>>   
>> +#define for_each_perag_range_reverse(agno, oagcount, nagcount) \
>> +	for ((agno) = ((oagcount) - 1); (typeof(oagcount))(agno) >= \
>> +		((typeof(oagcount))(nagcount) - 1); (agno)--)
>> +
>>   /*
>>    * Iterate all AGs from start_agno through wrap_agno, then restart_agno through
>>    * (start_agno - 1).
>> @@ -331,6 +344,7 @@ struct aghdr_init_data {
>>   int xfs_ag_init_headers(struct xfs_mount *mp, struct aghdr_init_data *id);
>>   int xfs_ag_shrink_space(struct xfs_perag *pag, struct xfs_trans **tpp,
>>   			xfs_extlen_t delta);
>> +void xfs_shrinkfs_remove_ag(struct xfs_mount *mp, xfs_agnumber_t agno);
>>   void
>>   xfs_growfs_compute_deltas(struct xfs_mount *mp, xfs_rfsblock_t nb,
>>   	int64_t *deltap, xfs_agnumber_t *nagcountp);
>> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
>> index 000cc7f4a3ce..e16803214223 100644
>> --- a/fs/xfs/libxfs/xfs_alloc.c
>> +++ b/fs/xfs/libxfs/xfs_alloc.c
>> @@ -3209,11 +3209,12 @@ xfs_validate_ag_length(
>>   	if (length != mp->m_sb.sb_agblocks) {
>>   		/*
>>   		 * During growfs, the new last AG can get here before we
>> -		 * have updated the superblock. Give it a pass on the seqno
>> -		 * check.
>> +		 * have updated the superblock. During shrink, the new last AG
>> +		 * will be updated and the AGs from newag to old AG will be
>> +		 * removed. So seqno here maybe not be equal to
>> +		 * mp->m_sb.sb_agcount - 1 since the super block is not yet
>> +		 * updated globally.
>>   		 */
>> -		if (bp->b_pag && seqno != mp->m_sb.sb_agcount - 1)
>> -			return __this_address;
> Shrinking should be rare, maybe we should have a SHRINKING state flag
> that turns this off?
I couldn't follow the above suggestion entirely. Can you please shed 
some more details as to what you meant by the above comment?
>
>>   		if (length < XFS_MIN_AG_BLOCKS)
>>   			return __this_address;
>>   		if (length > mp->m_sb.sb_agblocks)
>> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
>> index f9ef3b2a332a..56be9a0afb00 100644
>> --- a/fs/xfs/xfs_buf.c
>> +++ b/fs/xfs/xfs_buf.c
>> @@ -951,6 +951,84 @@ xfs_buf_rele(
>>   		xfs_buf_rele_cached(bp);
>>   }
>>   
>> +/*
>> + * This function populates a list of all the cached buffers of the given AG
>> + * in the to_be_free list head.
>> + */
>> +static void
>> +xfs_buf_cache_grab_all(
>> +	struct xfs_perag	*pag,
>> +	struct list_head	*to_be_freed)
>> +{
>> +	struct xfs_buf		*bp;
>> +	struct rhashtable_iter	iter;
>> +
>> +	rhashtable_walk_enter(&pag->pag_bcache.bc_hash, &iter);
>> +	do {
>> +		rhashtable_walk_start(&iter);
>> +		while ((bp = rhashtable_walk_next(&iter)) && !IS_ERR(bp)) {
>> +			ASSERT(list_empty(&bp->b_list));
>> +			ASSERT(list_empty(&bp->b_li_list));
>> +			list_add_tail(&bp->b_list, to_be_freed);
>> +		}
>> +		rhashtable_walk_stop(&iter);
>> +	} while (cond_resched(), bp == ERR_PTR(-EAGAIN));
>> +	rhashtable_walk_exit(&iter);
>> +}
>> +
>> +/*
>> + * This function frees all the cached buffers (struct xfs_buf) associated with
>> + * the given offline AG. The caller must ensure that the AG which is passed
>> + * is offline and completely stabilized on the disk. Also, the caller should
>> + * ensure that all the cached buffers are not queued for any pending i/o
>> + * i.e, the b_list for all the cached buffers are empty - since we will be using
>> + * b_list to get list of all the bufs that need to be freed.
>> + */
>> +void
>> +xfs_buf_cache_invalidate(struct xfs_perag	*pag)
>> +{
>> +	/*
>> +	 * First get the list of buffers we want to free.
>> +	 * We need to populate to_be_freed list and cannot directly free
>> +	 * the buffers during the hashtable walk. rhashtable_walk_start() takes
>> +	 * an RCU and xfs_buf_rele eventually calls xfs_buf_free (for
>> +	 * cached buffers). xfs_buf_free() might sleep (depending on the
>> +	 * whether the buffer was allocated using vmalloc or kmalloc) and
>> +	 * cannot be called within an RCU context. Hence we first populate
>> +	 * the buffers within an RCU context and free them outside it.
>> +	 */
>> +	struct list_head	to_be_freed;
>> +	struct xfs_buf		*bp, *tmp;
>> +
>> +	ASSERT(!xfs_ag_is_active(pag));
>> +
>> +	INIT_LIST_HEAD(&to_be_freed);
>> +
>> +	xfs_buf_cache_grab_all(pag, &to_be_freed);
>> +	list_for_each_entry_safe(bp, tmp, &to_be_freed, b_list) {
>> +		list_del(&bp->b_list);
>> +		spin_lock(&bp->b_lock);
>> +		ASSERT(bp->b_pag == pag);
>> +		ASSERT(!xfs_buf_is_uncached(bp));
>> +		/*
>> +		 * Since we have made sure that this is being called on an
>> +		 * AG with active refcount = 0, the b_hold value of any cached
>> +		 * buffer should not exceed 1 (i.e, the default value) and hence
>> +		 * can be safely removed. Hence, it should also be in an
>> +		 * unlocked state.
>> +		 */
>> +		ASSERT(bp->b_hold == 1);
>> +		ASSERT(!xfs_buf_islocked(bp));
>> +		/*
>> +		 * We should set b_lru_ref to 0 so that it gets deleted from
>> +		 * the lru during the call to xfs_buf_rele.
>> +		 */
>> +		atomic_set(&bp->b_lru_ref, 0);
>> +		spin_unlock(&bp->b_lock);
>> +		xfs_buf_rele(bp);
>> +	}
>> +}
>> +
>>   /*
>>    *	Lock a buffer object, if it is not already locked.
>>    *
>> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
>> index b269e115d9ac..9b054bc8a96f 100644
>> --- a/fs/xfs/xfs_buf.h
>> +++ b/fs/xfs/xfs_buf.h
>> @@ -281,6 +281,7 @@ void xfs_buf_hold(struct xfs_buf *bp);
>>   
>>   /* Releasing Buffers */
>>   extern void xfs_buf_rele(struct xfs_buf *);
>> +void xfs_buf_cache_invalidate(struct xfs_perag	*pag);
>>   
>>   /* Locking and Unlocking Buffers */
>>   extern int xfs_buf_trylock(struct xfs_buf *);
>> diff --git a/fs/xfs/xfs_buf_item_recover.c b/fs/xfs/xfs_buf_item_recover.c
>> index 5d58e2ae4972..5fe7fd1931f5 100644
>> --- a/fs/xfs/xfs_buf_item_recover.c
>> +++ b/fs/xfs/xfs_buf_item_recover.c
>> @@ -737,8 +737,7 @@ xlog_recover_do_primary_sb_buffer(
>>   	xfs_sb_from_disk(&mp->m_sb, dsb);
>>   
>>   	if (mp->m_sb.sb_agcount < orig_agcount) {
>> -		xfs_alert(mp, "Shrinking AG count in log recovery not supported");
>> -		return -EFSCORRUPTED;
>> +		xfs_warn_experimental(mp, XFS_EXPERIMENTAL_SHRINK);
>>   	}
>>   	if (mp->m_sb.sb_rgcount < orig_rgcount) {
>>   		xfs_warn(mp,
>> @@ -764,18 +763,28 @@ xlog_recover_do_primary_sb_buffer(
>>   		if (error)
>>   			return error;
>>   	}
>> -
>> -	/*
>> -	 * Initialize the new perags, and also update various block and inode
>> -	 * allocator setting based off the number of AGs or total blocks.
>> -	 * Because of the latter this also needs to happen if the agcount did
>> -	 * not change.
>> -	 */
>> -	error = xfs_initialize_perag(mp, orig_agcount, mp->m_sb.sb_agcount,
>> -			mp->m_sb.sb_dblocks, &mp->m_maxagi);
>> -	if (error) {
>> -		xfs_warn(mp, "Failed recovery per-ag init: %d", error);
>> -		return error;
>> +	if (orig_agcount > mp->m_sb.sb_agcount) {
>> +		/*
>> +		 * Remove the old AGs that were removed previously by a growfs
>> +		 */
>> +		xfs_free_perag_range(mp, mp->m_sb.sb_agcount, orig_agcount);
>> +		mp->m_maxagi = xfs_set_inode_alloc(mp, mp->m_sb.sb_agcount);
>> +		mp->m_ag_prealloc_blocks = xfs_prealloc_blocks(mp);
>> +	} else {
>> +		/*
>> +		 * Initialize the new perags, and also the update various block
>> +		 * and inode allocator setting based off the number of AGs or
>> +		 * total blocks.
>> +		 * Because of the latter, this also needs to happen if the
>> +		 * agcount did not change.
>> +		 */
>> +		error = xfs_initialize_perag(mp, orig_agcount,
>> +				mp->m_sb.sb_agcount,
>> +				mp->m_sb.sb_dblocks, &mp->m_maxagi);
>> +		if (error) {
>> +			xfs_warn(mp, "Failed recovery per-ag init: %d", error);
>> +			return error;
>> +		}
>>   	}
>>   	mp->m_alloc_set_aside = xfs_alloc_set_aside(mp);
>>   
>> diff --git a/fs/xfs/xfs_extent_busy.c b/fs/xfs/xfs_extent_busy.c
>> index da3161572735..1dba9da27a31 100644
>> --- a/fs/xfs/xfs_extent_busy.c
>> +++ b/fs/xfs/xfs_extent_busy.c
>> @@ -676,6 +676,36 @@ xfs_extent_busy_wait_all(
>>   			xfs_extent_busy_wait_group(rtg_group(rtg));
>>   }
>>   
>> +/*
>> + * Similar to xfs_extent_busy_wait_all() - It waits for all the busy extents to
>> + * get resolved for the range of AGs provided. For now, this function is
>> + * introduced to be used in online shrink process. Unlike
>> + * xfs_extent_busy_wait_all(), this takes a passive reference, because this
>> + * function is expected to be called for the AGs whose active reference has
>> + * been reduced to 0 i.e, offline AGs.
>> + *
>> + * @mp - The xfs mount point
>> + * @first_agno - The 0 based AG index of the range of AGs from which we will
>> + *     start.
>> + * @end_agno - The 0 based AG index of the range of AGs from till which we will
>> + *     traverse.
>> + */
>> +void
>> +xfs_extent_busy_wait_ags(
>> +	struct xfs_mount	*mp,
>> +	xfs_agnumber_t		first_agno,
>> +	xfs_agnumber_t		end_agno)
>> +{
>> +	xfs_agnumber_t		agno;
>> +	struct xfs_perag	*pag = NULL;
>> +
>> +	for_each_perag_range_reverse(agno, end_agno + 1, first_agno + 1) {
>> +		pag = xfs_perag_get(mp, agno);
>> +		xfs_extent_busy_wait_group(pag_group(pag));
>> +		xfs_perag_put(pag);
>> +	}
>> +}
>> +
>>   /*
>>    * Callback for list_sort to sort busy extents by the group they reside in.
>>    */
>> diff --git a/fs/xfs/xfs_extent_busy.h b/fs/xfs/xfs_extent_busy.h
>> index 3e6e019b6146..6fcab714be07 100644
>> --- a/fs/xfs/xfs_extent_busy.h
>> +++ b/fs/xfs/xfs_extent_busy.h
>> @@ -57,6 +57,8 @@ bool xfs_extent_busy_trim(struct xfs_group *xg, xfs_extlen_t minlen,
>>   		unsigned *busy_gen);
>>   int xfs_extent_busy_flush(struct xfs_trans *tp, struct xfs_group *xg,
>>   		unsigned busy_gen, uint32_t alloc_flags);
>> +void xfs_extent_busy_wait_ags(struct xfs_mount *mp, xfs_agnumber_t first_agno,
>> +		xfs_agnumber_t end_agno);
>>   void xfs_extent_busy_wait_all(struct xfs_mount *mp);
>>   bool xfs_extent_busy_list_empty(struct xfs_group *xg, unsigned int *busy_gen);
>>   struct xfs_extent_busy_tree *xfs_extent_busy_alloc(void);
>> diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
>> index 8353e2f186f6..199d48403514 100644
>> --- a/fs/xfs/xfs_fsops.c
>> +++ b/fs/xfs/xfs_fsops.c
>> @@ -25,6 +25,7 @@
>>   #include "xfs_rtrmap_btree.h"
>>   #include "xfs_rtrefcount_btree.h"
>>   #include "xfs_metafile.h"
>> +#include "xfs_trans_priv.h"
>>   
>>   /*
>>    * Write new AG headers to disk. Non-transactional, but need to be
>> @@ -83,6 +84,291 @@ xfs_resizefs_init_new_ags(
>>   	return error;
>>   }
>>   
>> +static int
>> +xfs_shrinkfs_stablize_ags(
> s/stablize/stabilize/g
>
> or maybe "quiesce" to fit with the xfs language?
Yeah, quiesce sounds better.
>
>> +	struct xfs_mount	*mp,
>> +	xfs_agnumber_t		oagcount,
>> +	xfs_agnumber_t		nagcount)
>> +{
>> +	int	error = 0;
>> +	int	count = 0;
>> +
>> +	/*
>> +	 * We should wait for the log to be empty and all the pending I/Os to
>> +	 * be completed so that the AGs are completely stabilized before we
>> +	 * start tearing them down. Flushing the AIL and synching the superblock
>> +	 * here ensures that none of the future logged transactions will refer
>> +	 * to these AGs during log recovery in case if sudden shutdown/crash
>> +	 * happens while we are trying to remove these AGs.
>> +	 * The following code is similar to xfs_log_quiesce() and xfs_log_cover.
>> +	 *
>> +	 * We are doing a xfs_sync_sb_buf + AIL flush twice. The first
>> +	 * xfs_sync_sb_buf writes a checkpoint, then the first AIL flush makes
>> +	 * the first checkpoint stable. The second set of xfs_sync_sb_buf + AIL
>> +	 * flush synchs the on-disk LSN with the in-core LSN.
>> +	 * Unlike xfs_log_cover(), we don't necessarily want the background
>> +	 * filesytem activity/log activity to stop (like in case of unmount
>> +	 * or freeze).
>> +	 */
>> +	cancel_delayed_work_sync(&mp->m_log->l_work);
>> +	error = xfs_log_force(mp, XFS_LOG_SYNC);
>> +	if (error)
>> +		goto out;
>> +
>> +	error = xfs_sync_sb_buf(mp, false);
>> +	if (error)
>> +		goto out;
>> +
>> +	xfs_ail_push_all_sync(mp->m_ail);
>> +	xfs_buftarg_wait(mp->m_ddev_targp);
>> +	xfs_buf_lock(mp->m_sb_bp);
>> +	xfs_buf_unlock(mp->m_sb_bp);
>> +
>> +	/*
>> +	 * The first xfs_sync_sb serves as a reference for the in-core tail
>> +	 * pointer and the second one updates the on-disk tail with the in-core
>> +	 * lsn. This is similar to what is being done in xfs_log_cover, however
>> +	 * here we are explicitly doing this twice in order to ensure forward
>> +	 * progress as, during shrink the filesystem is active.
>> +	 */
>> +	for (count = 0; count < 2; count++) {
>> +		error = xfs_sync_sb(mp, true);
>> +		if (error)
>> +			goto out;
>> +		xfs_ail_push_all_sync(mp->m_ail);
>> +	}
>> +
>> +	/*
>> +	 * Wait for all the busy extents to get resolved along with pending trim
>> +	 * ops for all the offlined AGs.
>> +	 */
>> +	xfs_extent_busy_wait_ags(mp, nagcount, oagcount - 1);
>> +	flush_workqueue(xfs_discard_wq);
>> +out:
>> +	xfs_log_work_queue(mp);
>> +	return error;
>> +}
>> +
>> +/*
>> + * Get new active references for all the AGs. This might be called when
>> + * shrinkage process encounters a failure at an intermediate stage after the
>> + * active references of all/some of the target AGs have become 0.
>> + */
>> +static void
>> +xfs_shrinkfs_reactivate_ags(
>> +	struct xfs_mount	*mp,
>> +	xfs_agnumber_t		oagcount,
>> +	xfs_agnumber_t		nagcount)
>> +{
>> +	struct xfs_perag	*pag = NULL;
>> +	xfs_agnumber_t		agno;
>> +
>> +	ASSERT(nagcount < oagcount);
>> +
>> +	for_each_perag_range_reverse(agno, oagcount, nagcount + 1) {
>> +		pag = xfs_perag_get(mp, agno);
>> +		xfs_perag_activate(pag);
>> +		xfs_perag_put(pag);
>> +	}
>> +}
>> +
>> +/*
>> + * The function deactivates or puts the AGs to an offline mode. AG deactivation
>> + * or AG offlining means that no new operation can be started on that AG. The AG
>> + * still exists, however no new high level operation (like extent allocation)
>> + * can be started. In terms of implementation, an AG is taken offline or is
>> + * deactivated when xg_active_ref of the struct xfs_perag is 0 i.e, the number
>> + * of active references becomes 0.
>> + * Since active references act as a form of barrier, so once the active
>> + * reference of an AG is 0, no new entity can get an active reference and in
>> + * this way we ensure that once an AG is offline (i.e, active reference count is
>> + * 0), no one will be able to start a new operation in it unless the active
>> + * reference count is explicitly set to 1 i.e, the AG is made online/activated.
>> + */
>> +static int
>> +xfs_shrinkfs_deactivate_ags(
>> +	struct xfs_mount	*mp,
>> +	xfs_agnumber_t		oagcount,
>> +	xfs_agnumber_t		nagcount)
>> +{
>> +	int			error = 0;
>> +	struct xfs_perag	*pag = NULL;
>> +	xfs_agnumber_t		agno;
>> +
>> +	ASSERT(nagcount < oagcount);
>> +
>> +	/*
>> +	 * If we are removing 1 or more entire AGs, we only need to take those
>> +	 * AGs offline which we are planning to remove completely. The new tail
>> +	 * AG which will be partially shrunk should not be taken offline - since
>> +	 * we will be doing an online operation on them, just like any other
>> +	 * high level operation. For complete AG removal, we need to take them
>> +	 * offline since we cannot start any new operation on them as they will
>> +	 * be removed eventually.
>> +	 *
>> +	 * However, if the number of blocks that we are trying to remove is
>> +	 * an exact multiple of the AG size (in blocks), then the new tail AG
>> +	 * will not be shrunk at all.
>> +	 */
>> +	for_each_perag_range_reverse(agno, oagcount, nagcount + 1) {
>> +		pag = xfs_perag_get(mp, agno);
> I keep seeing this for_each_perag() -> xfs_perag_get code.  The regular
> for_each_perag macros take a pag pointer and set it to an actively
> referenced perag.
>
> I wonder if this new macro ought to behave like that too, but then I
> guess you'd need to indicate that it coughs up passive references, not
> active ones, and ... yeah.
Oh okay, so the macro should itself take the passive reference, and we 
don't need to explicitly take it inside the loop. Yeah, I can make the 
change.
>
>> +		if (!xfs_perag_deactivate(pag)) {
>> +			xfs_perag_put(pag);
>> +			if (agno < oagcount - 1)
>> +				xfs_shrinkfs_reactivate_ags(mp, oagcount,
>> +					agno + 1);
>> +			return -ENOTEMPTY;
>> +		}
>> +		xfs_perag_put(pag);
>> +	}
>> +	/*
>> +	 * Now that we have deactivated/offlined the AGs, we need to make sure
>> +	 * that all the pending operations are completed and the in-core and
>> +	 * the on disk contents are completely in synch i.e, AGs are stablized
>> +	 * on to the disk.
>> +	 */
>> +	error = xfs_shrinkfs_stablize_ags(mp, oagcount, nagcount);
>> +	if (error) {
>> +		xfs_shrinkfs_reactivate_ags(mp, oagcount, nagcount);
>> +		return error;
>> +	}
>> +
>> +	return error;
> Nit: this could be return 0.
Noted.
>
>> +}
>> +
>> +/*
>> + * This function does 3 things:
>> + * 1. Deactivate the AGs i.e, wait for all the active references to come to 0.
>> + * 2. Checks whether all the AGs that shrink process needs to remove are empty.
>> + *    If at least one of the target AGs is non-empty, shrink fails and
>> + *    xfs_shrinkfs_reactivate_ags() is called.
>> + * 3. Calculates the total number of fdblocks (free data blocks) that will be
>> + *    removed and stores in id->nfree.
>> + * Please look into the individual functions for more details and the definition
>> + * of the terminologies.
>> + */
>> +static int
>> +xfs_shrinkfs_prepare_ags(
>> +	struct xfs_mount	*mp,
>> +	xfs_agnumber_t		oagcount,
>> +	xfs_agnumber_t		nagcount,
>> +	struct aghdr_init_data	*id)
>> +{
>> +
>> +	struct xfs_perag	*pag = NULL;
>> +	xfs_agnumber_t		agno;
>> +	int			error = 0;
>> +
>> +	ASSERT(nagcount < oagcount);
>> +
>> +	/*
>> +	 * Deactivating/offlining the AGs i.e waiting for the active references
>> +	 * to come down to 0.
>> +	 */
>> +	error = xfs_shrinkfs_deactivate_ags(mp, oagcount, nagcount);
>> +	if (error)
>> +		return error;
>> +	/*
>> +	 * At this point the AGs have been deactivated/offlined and the in-core
>> +	 * and the on-disk are synch. So now we need to check whether all the
>> +	 * AGs that we are trying to remove/delete are empty. Since we are not
>> +	 * supporting partial shrink success (i.e, the entire requested size
>> +	 * will be removed or none), we will bail out with a failure code even
>> +	 * if 1 AG is non-empty.
>> +	 */
>> +	for_each_perag_range_reverse(agno, oagcount, nagcount + 1) {
>> +		pag = xfs_perag_get(mp, agno);
>> +		if (!xfs_ag_is_empty(pag)) {
>> +			/* Error out even if one AG is non-empty */
>> +			error = -ENOTEMPTY;
>> +			xfs_perag_put(pag);
>> +			xfs_shrinkfs_reactivate_ags(mp, oagcount, nagcount);
>> +			return error;
> return -ENOTEMPTY ?
Yeah, I can directly return -ENOTEMPTY.
>
> FWIW, I think this code looks mostly sane.  Pending answers to my
> questions, it might be close to ready for testing.  Apologies for the
> very long delay in getting to this.  $problems :/

Thank you so much for taking out time and reviewing the code. I have 
tried to answer the questions that you have asked. Please let me know if 
you have additional questions. I, too, have asked some questions about a 
couple of your comments. Once I have clarity on those, I will send the 
next revision.

--NR

>
> --D
>
>> +		}
>> +		/*
>> +		 * Since these are removed, these free blocks should also be
>> +		 * subtracted from the total list of free blocks.
>> +		 */
>> +		id->nfree += (pag->pagf_freeblks + pag->pagf_flcount);
>> +		xfs_perag_put(pag);
>> +	}
>> +	return 0;
>> +}
>> +
>> +/*
>> + * This function does the job of fully removing the blocks and empty AGs (
>> + * depending of the values of oagcount and nagcount). By removal it means,
>> + * removal of all the perag data structures, other data structures associated
>> + * with it and all the perag cached buffers (when AGs are removed). Once this
>> + * function succeeds, the AGs/blocks will no longer exist.
>> + * The overall steps are as follows (details are in the function):
>> + * - calculate the number of blocks that will be removed from the new tail AG
>> + *   i.e, the AG that will be shrunk partially.
>> + * - call xfs_shrinkfs_remove_ag() that removes the perag cached buffers,
>> + *   then frees the perag reservation, other associated datastructures and
>> + *   finally the in-memory perag group instance.
>> + */
>> +static int
>> +xfs_shrinkfs_remove_ags(
>> +	struct xfs_mount	*mp,
>> +	struct xfs_trans	**tp,
>> +	xfs_agnumber_t		oagcount,
>> +	xfs_agnumber_t		nagcount,
>> +	int64_t			delta_rem,
>> +	xfs_agnumber_t		*nagmax)
>> +{
>> +	xfs_agnumber_t		agno;
>> +	int			error = 0;
>> +	struct xfs_perag	*cur_pag = NULL;
>> +
>> +	/*
>> +	 * This loop is calculating the number of blocks that needs to be
>> +	 * removed from the new tail AG. If delta_rem is 0 after the loop exits,
>> +	 * then it means that the number of blocks we want to remove is a
>> +	 * multiple of AG size (in blocks).
>> +	 */
>> +	for_each_perag_range_reverse(agno, oagcount, nagcount + 1) {
>> +		cur_pag = xfs_perag_get(mp, agno);
>> +		delta_rem -= xfs_ag_block_count(mp, agno);
>> +		xfs_perag_put(cur_pag);
>> +	}
>> +	/*
>> +	 * We are first removing blocks from the AG that will form the new tail
>> +	 * AG. The reason is that, if we encounter an error here, we can simply
>> +	 * reactivate the AGs (by calling xfs_shrinkfs_reactivate_ags()).
>> +	 * Removal of complete empty AGs always succeed anyway. However if we
>> +	 * remove the empty AGs first (which will succeed) and then the new
>> +	 * last AG shrink fails, then we will again have to re-initialize the
>> +	 * removed AGs. Hence the former approach seems more efficient to me.
>> +	 */
>> +	if (delta_rem) {
>> +		/*
>> +		 * Remove delta_rem blocks from the AG that will form the new
>> +		 * tail AG after the AGs are removed. If the number of blocks to
>> +		 * be removed is a multiple of AG size, then nothing is done
>> +		 * here.
>> +		 */
>> +		cur_pag = xfs_perag_get(mp, nagcount - 1);
>> +		error = xfs_ag_shrink_space(cur_pag, tp, delta_rem);
>> +		xfs_perag_put(cur_pag);
>> +		if (error) {
>> +			if (nagcount < oagcount)
>> +				xfs_shrinkfs_reactivate_ags(mp, oagcount,
>> +					nagcount);
>> +			return error;
>> +		}
>> +	}
>> +	/*
>> +	 * Now, in this final step we remove the perag instance and the
>> +	 * associated datastructures and cached buffers. This fully removes the
>> +	 * AG.
>> +	 */
>> +	for_each_perag_range_reverse(agno, oagcount, nagcount + 1)
>> +		xfs_shrinkfs_remove_ag(mp, agno);
>> +	*nagmax = xfs_set_inode_alloc(mp, nagcount);
>> +	return error;
>> +}
>> +
>>   /*
>>    * growfs operations
>>    */
>> @@ -98,10 +384,11 @@ xfs_growfs_data_private(
>>   	xfs_agnumber_t		nagcount;
>>   	xfs_agnumber_t		nagimax = 0;
>>   	int64_t			delta;
>> +	xfs_rfsblock_t		nb_div, nb_mod;
>>   	bool			lastag_extended = false;
>>   	struct xfs_trans	*tp;
>>   	struct aghdr_init_data	id = {};
>> -	struct xfs_perag	*last_pag;
>> +	struct xfs_perag	*last_pag = NULL;
>>   
>>   	error = xfs_sb_validate_fsb_count(&mp->m_sb, nb);
>>   	if (error)
>> @@ -122,6 +409,13 @@ xfs_growfs_data_private(
>>   	if (error)
>>   		return error;
>>   	xfs_growfs_compute_deltas(mp, nb, &delta, &nagcount);
>> +	/*
>> +	 * Fail if the new tail AG length is < XFS_MIN_AG_BLOCKS during shrink
>> +	 */
>> +	nb_div = nb;
>> +	nb_mod = do_div(nb_div, mp->m_sb.sb_agblocks);
>> +	if (delta < 0 && nb_mod && nb_mod < XFS_MIN_AG_BLOCKS)
>> +		return -EINVAL;
>>   
>>   	/*
>>   	 * Reject filesystems with a single AG because they are not
>> @@ -134,15 +428,19 @@ xfs_growfs_data_private(
>>   	/* No work to do */
>>   	if (delta == 0)
>>   		return 0;
>> -
>> -	/* TODO: shrinking the entire AGs hasn't yet completed */
>> -	if (nagcount < oagcount)
>> -		return -EINVAL;
>> +	if (nagcount < oagcount) {
>> +		error = xfs_shrinkfs_prepare_ags(mp, oagcount, nagcount, &id);
>> +		if (error)
>> +			return error;
>> +	}
>>   
>>   	/* allocate the new per-ag structures */
>>   	error = xfs_initialize_perag(mp, oagcount, nagcount, nb, &nagimax);
>> -	if (error)
>> +	if (error) {
>> +		if (nagcount < oagcount)
>> +			xfs_shrinkfs_reactivate_ags(mp, oagcount, nagcount);
>>   		return error;
>> +	}
>>   
>>   	if (delta > 0)
>>   		error = xfs_trans_alloc(mp, &M_RES(mp)->tr_growdata,
>> @@ -151,32 +449,44 @@ xfs_growfs_data_private(
>>   	else
>>   		error = xfs_trans_alloc(mp, &M_RES(mp)->tr_growdata, -delta, 0,
>>   				0, &tp);
>> -	if (error)
>> +	if (error) {
>> +		if (nagcount < oagcount)
>> +			xfs_shrinkfs_reactivate_ags(mp, oagcount, nagcount);
>>   		goto out_free_unused_perag;
>> +	}
>>   
>> -	last_pag = xfs_perag_get(mp, oagcount - 1);
>>   	if (delta > 0) {
>> +		last_pag = xfs_perag_get(mp, oagcount - 1);
>>   		error = xfs_resizefs_init_new_ags(tp, &id, oagcount, nagcount,
>>   				delta, last_pag, &lastag_extended);
>> +		xfs_perag_put(last_pag);
>>   	} else {
>>   		xfs_warn_experimental(mp, XFS_EXPERIMENTAL_SHRINK);
>> -		error = xfs_ag_shrink_space(last_pag, &tp, -delta);
>> +		error = xfs_shrinkfs_remove_ags(mp, &tp, oagcount, nagcount,
>> +				-delta, &nagimax);
>>   	}
>> -	xfs_perag_put(last_pag);
>>   	if (error)
>>   		goto out_trans_cancel;
>> +	/*
>> +	 * Adjust the free data blocks back which we manually reduced during
>> +	 * AG deactivation.
>> +	 */
>> +	if (nagcount < oagcount)
>> +		xfs_add_fdblocks(mp, id.nfree);
>>   
>>   	/*
>>   	 * Update changed superblock fields transactionally. These are not
>>   	 * seen by the rest of the world until the transaction commit applies
>>   	 * them atomically to the superblock.
>>   	 */
>> -	if (nagcount > oagcount)
>> -		xfs_trans_mod_sb(tp, XFS_TRANS_SB_AGCOUNT, nagcount - oagcount);
>> +	if (nagcount != oagcount)
>> +		xfs_trans_mod_sb(tp, XFS_TRANS_SB_AGCOUNT,
>> +			(int64_t)nagcount - (int64_t)oagcount);
>>   	if (delta)
>>   		xfs_trans_mod_sb(tp, XFS_TRANS_SB_DBLOCKS, delta);
>>   	if (id.nfree)
>> -		xfs_trans_mod_sb(tp, XFS_TRANS_SB_FDBLOCKS, id.nfree);
>> +		xfs_trans_mod_sb(tp, XFS_TRANS_SB_FDBLOCKS,
>> +			delta > 0 ? id.nfree : (int64_t)-id.nfree);
>>   
>>   	/*
>>   	 * Sync sb counters now to reflect the updated values. This is
>> @@ -188,12 +498,17 @@ xfs_growfs_data_private(
>>   
>>   	xfs_trans_set_sync(tp);
>>   	error = xfs_trans_commit(tp);
>> -	if (error)
>> +	if (error) {
>> +		if (nagcount < oagcount)
>> +			xfs_shrinkfs_reactivate_ags(mp, oagcount, nagcount);
>>   		return error;
>> +	}
>>   
>>   	/* New allocation groups fully initialized, so update mount struct */
>>   	if (nagimax)
>>   		mp->m_maxagi = nagimax;
>> +	if (nagcount < oagcount)
>> +		mp->m_ag_prealloc_blocks = xfs_prealloc_blocks(mp);
>>   	xfs_set_low_space_thresholds(mp);
>>   	mp->m_alloc_set_aside = xfs_alloc_set_aside(mp);
>>   
>> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
>> index 575e7028f423..c5467f52356f 100644
>> --- a/fs/xfs/xfs_trans.c
>> +++ b/fs/xfs/xfs_trans.c
>> @@ -409,7 +409,6 @@ xfs_trans_mod_sb(
>>   		tp->t_dblocks_delta += delta;
>>   		break;
>>   	case XFS_TRANS_SB_AGCOUNT:
>> -		ASSERT(delta > 0);
>>   		tp->t_agcount_delta += delta;
>>   		break;
>>   	case XFS_TRANS_SB_IMAXPCT:
>> -- 
>> 2.43.5
>>
>>
-- 
Nirjhar Roy
Linux Kernel Developer
IBM, Bangalore