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

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

On 10/16/25 21:23, Darrick J. Wong wrote:
> On Thu, Oct 16, 2025 at 02:46:08PM +0530, Nirjhar Roy (IBM) wrote:
>> On 10/16/25 00:56, Darrick J. Wong wrote:
>>> On Wed, Oct 15, 2025 at 04:32:59PM +0530, Nirjhar Roy (IBM) wrote:
>>>> 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?
>>> Still no -- it can't have bmbt blocks either, which are not user data
>>> per se.  How about:
>>>
>>> "Empty AG - An AG with no allocated space other than AG headers, empty
>>> AG btree root blocks, and AGFL reserved blocks.  Removal of this AG will
>>> not result in any data loss."
>> Yeah, this looks 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?
>>> I know what you're describing now, but let's say the shrink process
>>> does:
>>>
>>> 0. Fill the filesystem until there are only 400 blocks left at the end.
>>> 1. Take (say) 400 blocks from fdblocks
>>> 2. Deactivate/shrink AGs
>>> 3. Add 400 back to fdblocks
>>> 4. xfs_trans_mod_sb(tp, XFS_TRANS_SB_FDBLOCKS, -400)
>>> 5. Commit
>>>
>>> What prevents another process from taking the 400 blocks in between
>>> steps 3 and 4 and causing the superblock to be written out with fdblocks
>>> set to -400?
>> Yeah, right. There is a short window between step 3 and 4 where the race can
>> still occur.
>>> Does removing step 3 and changing step 4 to be
>>>
>>> 4. xfs_trans_mod_sb(tp, XFS_TRANS_SB_RES_FDBLOCKS, -400)
>>>
>>> fix this race?  We've already subtracted 400 from the incore fdblocks,
>>> so now all we need to do is subtract 400 from the ondisk fdblocks.
>> Yes, XFS_TRANS_SB_RES_FDBLOCKS does fix the issue. Thank you so much for
>> pointing our XFS_TRANS_SB_RES_FDBLOCKS. This is really helpful. I have
>> verified that this works. I have added fstests[1] that can reproduce such
>> races.
>>
>> Btw, what does the substring "RES" signify in the constant
>> XFS_TRANS_SB_RES_FDBLOCKS?
> "already reserved", i.e. you already subtracted the quantity out of the
> incore fdblocks.  Most callers do that by passing dblocks > 0 in
> xfs_trans_alloc (transaction block reservation) but writeback also does
> this when converting delalloc reservations into unwritten space.
Okay, makes sense. Thank you.
>
>> [1]
>> https://lore.kernel.org/all/cover.1758035262.git.nirjhar.roy.lists@gmail.com/
>>
>>>>>>         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).
>>> Oh, right.
>>>
>>>>>> 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.
>>> The loop is fine, but doesn't wait_event put the process in
>>> UNINTERRUPTIBLE state?
>> Yes, I have done that intentionally. The reason behind this is that, let's
>> say we have offlined ag3, ag2 and while the shrink process is waiting for
>> ag1 to go offline, the process is interrupted. This will put the filesystem
>> in an unusable state, since we have interrupted the shrink process and now
>> ag{3,2} are offline. Do you agree with this?
> Well, if you can reactivate ag[23] then I'd say that the user should be
> able to ^C the shrinkfs and have the fs go back to the way it was.  If
> not, then wait_event/uninterruptible is ok.

Well, yeah, I can actually reactivate ag23. So I can do something like

do {
         ret = wait_event(pag_group(pag)->xg_active_wq,
                                     !xfs_ag_is_active(pag));
         if (ret ==  -ERESTARTSYS) {
             /* reactivate AGs from (pag_agno(pag) + 1) to 
((pag_mount(pag))->m_sb.sb_agcount - 1) */
             /* clear shrinking bit */
         }

     } while (xfs_ag_is_active(pag));

How does the above look?

--NR

>
>>>>>> +	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?
>>> Define an XFS_OPSTATE_SHRINKING state flag, set it before starting a
>>> shrink, and clear it before finishing/aborting the shrink.  Then this
>>> check becomes:
>>>
>>> /* filesystem is shrinking */
>>> #define XFS_OPSTATE_SHRINKING	21
>>> __XFS_IS_OPSTATE(shrinking, SHRINKING)
>>>
>>> 	if (!xfs_is_shrinking(mp) &&
>>> 	    bp->b_pag && seqno != mp->m_sb.sb_agcount - 1)
>>> 		return __this_address;
>> Okay, it makes sense. I can make the change suggested above.
> Thanks!
>
> --D
>
>>>>>>     		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.
>>> I dunno if it really is a good idea to hide a passive walk behind a
>>> macro though.  Maybe just change the name to
>>> for_each_agno_range_reverse() and leave the explicit xfs_perag_get/put
>>> calls?
>> Okay.
>>
>> --NR
>>
>>>>>> +		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
>>>>
>>>>
>> -- 
>> Nirjhar Roy
>> Linux Kernel Developer
>> IBM, Bangalore
>>
>>
-- 
Nirjhar Roy
Linux Kernel Developer
IBM, Bangalore