Re: [PATCH 07/42] xfs: active perag reference counting

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Jan 19, 2023 at 09:44:30AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> We need to be able to dynamically remove instantiated AGs from
> memory safely, either for shrinking the filesystem or paging AG
> state in and out of memory (e.g. supporting millions of AGs). This
> means we need to be able to safely exclude operations from accessing
> perags while dynamic removal is in progress.
> 
> To do this, introduce the concept of active and passive references.
> Active references are required for high level operations that make
> use of an AG for a given operation (e.g. allocation) and pin the
> perag in memory for the duration of the operation that is operating
> on the perag (e.g. transaction scope). This means we can fail to get
> an active reference to an AG, hence callers of the new active
> reference API must be able to handle lookup failure gracefully.
> 
> Passive references are used in low level code, where we might need
> to access the perag structure for the purposes of completing high
> level operations. For example, buffers need to use passive
> references because:
> - we need to be able to do metadata IO during operations like grow
>   and shrink transactions where high level active references to the
>   AG have already been blocked
> - buffers need to pin the perag until they are reclaimed from
>   memory, something that high level code has no direct control over.
> - unused cached buffers should not prevent a shrink from being
>   started.
> 
> Hence we have active references that will form exclusion barriers
> for operations to be performed on an AG, and passive references that
> will prevent reclaim of the perag until all objects with passive
> references have been reclaimed themselves.

This is going to be fun to rebase the online fsck series on top of. :)

If I'm understanding correctly, active perag refs are for high level
code that wants to call down into an AG to do some operation
(allocating, freeing, scanning, whatever)?  So I think online fsck
uniformly wants xfs_perag_grab/rele, right?

Passive refs are (I think) for lower level code that's wants to call up
into an AG to finish off something that was already started?  And
probably by upper level code?  So the amount of code that actually wants
a passive reference is pretty small?

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

Is there any code that iterates perag structures via passive references?
I think the answer to this is 'no'?

The code changes look all right.  If the answers to the above questions
are "yes", "yes", "yes", and "no", then:
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> Note that the implementation using reference counting is really just
> a development vehicle for the API to ensure we don't have any leaks
> in the callers. Once we need to remove perag structures from memory
> dyanmically, we will need a much more robust per-ag state transition
> mechanism for preventing new references from being taken while we
> wait for existing references to drain before removal from memory can
> occur....
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/libxfs/xfs_ag.c    | 70 +++++++++++++++++++++++++++++++++++++++
>  fs/xfs/libxfs/xfs_ag.h    | 31 ++++++++++++-----
>  fs/xfs/scrub/bmap.c       |  2 +-
>  fs/xfs/scrub/fscounters.c |  4 +--
>  fs/xfs/xfs_fsmap.c        |  4 +--
>  fs/xfs/xfs_icache.c       |  2 +-
>  fs/xfs/xfs_iwalk.c        |  6 ++--
>  fs/xfs/xfs_reflink.c      |  2 +-
>  fs/xfs/xfs_trace.h        |  3 ++
>  9 files changed, 105 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
> index bb0c700afe3c..46e25c682bf4 100644
> --- a/fs/xfs/libxfs/xfs_ag.c
> +++ b/fs/xfs/libxfs/xfs_ag.c
> @@ -94,6 +94,68 @@ xfs_perag_put(
>  	trace_xfs_perag_put(pag->pag_mount, pag->pag_agno, ref, _RET_IP_);
>  }
>  
> +/*
> + * Active references for perag structures. This is for short term access to the
> + * per ag structures for walking trees or accessing state. If an AG is being
> + * shrunk or is offline, then this will fail to find that AG and return NULL
> + * instead.
> + */
> +struct xfs_perag *
> +xfs_perag_grab(
> +	struct xfs_mount	*mp,
> +	xfs_agnumber_t		agno)
> +{
> +	struct xfs_perag	*pag;
> +
> +	rcu_read_lock();
> +	pag = radix_tree_lookup(&mp->m_perag_tree, agno);
> +	if (pag) {
> +		trace_xfs_perag_grab(mp, pag->pag_agno,
> +				atomic_read(&pag->pag_active_ref), _RET_IP_);
> +		if (!atomic_inc_not_zero(&pag->pag_active_ref))
> +			pag = NULL;
> +	}
> +	rcu_read_unlock();
> +	return pag;
> +}
> +
> +/*
> + * search from @first to find the next perag with the given tag set.
> + */
> +struct xfs_perag *
> +xfs_perag_grab_tag(
> +	struct xfs_mount	*mp,
> +	xfs_agnumber_t		first,
> +	int			tag)
> +{
> +	struct xfs_perag	*pag;
> +	int			found;
> +
> +	rcu_read_lock();
> +	found = radix_tree_gang_lookup_tag(&mp->m_perag_tree,
> +					(void **)&pag, first, 1, tag);
> +	if (found <= 0) {
> +		rcu_read_unlock();
> +		return NULL;
> +	}
> +	trace_xfs_perag_grab_tag(mp, pag->pag_agno,
> +			atomic_read(&pag->pag_active_ref), _RET_IP_);
> +	if (!atomic_inc_not_zero(&pag->pag_active_ref))
> +		pag = NULL;
> +	rcu_read_unlock();
> +	return pag;
> +}
> +
> +void
> +xfs_perag_rele(
> +	struct xfs_perag	*pag)
> +{
> +	trace_xfs_perag_rele(pag->pag_mount, pag->pag_agno,
> +			atomic_read(&pag->pag_active_ref), _RET_IP_);
> +	if (atomic_dec_and_test(&pag->pag_active_ref))
> +		wake_up(&pag->pag_active_wq);
> +}
> +
>  /*
>   * xfs_initialize_perag_data
>   *
> @@ -196,6 +258,10 @@ xfs_free_perag(
>  		cancel_delayed_work_sync(&pag->pag_blockgc_work);
>  		xfs_buf_hash_destroy(pag);
>  
> +		/* drop the mount's active reference */
> +		xfs_perag_rele(pag);
> +		XFS_IS_CORRUPT(pag->pag_mount,
> +				atomic_read(&pag->pag_active_ref) != 0);
>  		call_rcu(&pag->rcu_head, __xfs_free_perag);
>  	}
>  }
> @@ -314,6 +380,7 @@ xfs_initialize_perag(
>  		INIT_DELAYED_WORK(&pag->pag_blockgc_work, xfs_blockgc_worker);
>  		INIT_RADIX_TREE(&pag->pag_ici_root, GFP_ATOMIC);
>  		init_waitqueue_head(&pag->pagb_wait);
> +		init_waitqueue_head(&pag->pag_active_wq);
>  		pag->pagb_count = 0;
>  		pag->pagb_tree = RB_ROOT;
>  #endif /* __KERNEL__ */
> @@ -322,6 +389,9 @@ xfs_initialize_perag(
>  		if (error)
>  			goto out_remove_pag;
>  
> +		/* Active ref owned by mount indicates AG is online. */
> +		atomic_set(&pag->pag_active_ref, 1);
> +
>  		/* first new pag is fully initialized */
>  		if (first_initialised == NULLAGNUMBER)
>  			first_initialised = index;
> diff --git a/fs/xfs/libxfs/xfs_ag.h b/fs/xfs/libxfs/xfs_ag.h
> index 191b22b9a35b..aeb21c8df201 100644
> --- a/fs/xfs/libxfs/xfs_ag.h
> +++ b/fs/xfs/libxfs/xfs_ag.h
> @@ -32,7 +32,9 @@ struct xfs_ag_resv {
>  struct xfs_perag {
>  	struct xfs_mount *pag_mount;	/* owner filesystem */
>  	xfs_agnumber_t	pag_agno;	/* AG this structure belongs to */
> -	atomic_t	pag_ref;	/* perag reference count */
> +	atomic_t	pag_ref;	/* passive reference count */
> +	atomic_t	pag_active_ref;	/* active reference count */
> +	wait_queue_head_t pag_active_wq;/* woken active_ref falls to zero */
>  	char		pagf_init;	/* this agf's entry is initialized */
>  	char		pagi_init;	/* this agi's entry is initialized */
>  	char		pagf_metadata;	/* the agf is preferred to be metadata */
> @@ -111,11 +113,18 @@ int xfs_initialize_perag(struct xfs_mount *mp, xfs_agnumber_t agcount,
>  int xfs_initialize_perag_data(struct xfs_mount *mp, xfs_agnumber_t agno);
>  void xfs_free_perag(struct xfs_mount *mp);
>  
> +/* Passive AG references */
>  struct xfs_perag *xfs_perag_get(struct xfs_mount *mp, xfs_agnumber_t agno);
>  struct xfs_perag *xfs_perag_get_tag(struct xfs_mount *mp, xfs_agnumber_t agno,
>  		unsigned int tag);
>  void xfs_perag_put(struct xfs_perag *pag);
>  
> +/* Active AG references */
> +struct xfs_perag *xfs_perag_grab(struct xfs_mount *, xfs_agnumber_t);
> +struct xfs_perag *xfs_perag_grab_tag(struct xfs_mount *, xfs_agnumber_t,
> +				   int tag);
> +void xfs_perag_rele(struct xfs_perag *pag);
> +
>  /*
>   * Per-ag geometry infomation and validation
>   */
> @@ -193,14 +202,18 @@ xfs_perag_next(
>  	struct xfs_mount	*mp = pag->pag_mount;
>  
>  	*agno = pag->pag_agno + 1;
> -	xfs_perag_put(pag);
> -	if (*agno > end_agno)
> -		return NULL;
> -	return xfs_perag_get(mp, *agno);
> +	xfs_perag_rele(pag);
> +	while (*agno <= end_agno) {
> +		pag = xfs_perag_grab(mp, *agno);
> +		if (pag)
> +			return pag;
> +		(*agno)++;
> +	}
> +	return NULL;
>  }
>  
>  #define for_each_perag_range(mp, agno, end_agno, pag) \
> -	for ((pag) = xfs_perag_get((mp), (agno)); \
> +	for ((pag) = xfs_perag_grab((mp), (agno)); \
>  		(pag) != NULL; \
>  		(pag) = xfs_perag_next((pag), &(agno), (end_agno)))
>  
> @@ -213,11 +226,11 @@ xfs_perag_next(
>  	for_each_perag_from((mp), (agno), (pag))
>  
>  #define for_each_perag_tag(mp, agno, pag, tag) \
> -	for ((agno) = 0, (pag) = xfs_perag_get_tag((mp), 0, (tag)); \
> +	for ((agno) = 0, (pag) = xfs_perag_grab_tag((mp), 0, (tag)); \
>  		(pag) != NULL; \
>  		(agno) = (pag)->pag_agno + 1, \
> -		xfs_perag_put(pag), \
> -		(pag) = xfs_perag_get_tag((mp), (agno), (tag)))
> +		xfs_perag_rele(pag), \
> +		(pag) = xfs_perag_grab_tag((mp), (agno), (tag)))
>  
>  struct aghdr_init_data {
>  	/* per ag data */
> diff --git a/fs/xfs/scrub/bmap.c b/fs/xfs/scrub/bmap.c
> index d50d0eab196a..dbbc7037074c 100644
> --- a/fs/xfs/scrub/bmap.c
> +++ b/fs/xfs/scrub/bmap.c
> @@ -662,7 +662,7 @@ xchk_bmap_check_rmaps(
>  		error = xchk_bmap_check_ag_rmaps(sc, whichfork, pag);
>  		if (error ||
>  		    (sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT)) {
> -			xfs_perag_put(pag);
> +			xfs_perag_rele(pag);
>  			return error;
>  		}
>  	}
> diff --git a/fs/xfs/scrub/fscounters.c b/fs/xfs/scrub/fscounters.c
> index 4777e7b89fdc..ef97670970c3 100644
> --- a/fs/xfs/scrub/fscounters.c
> +++ b/fs/xfs/scrub/fscounters.c
> @@ -117,7 +117,7 @@ xchk_fscount_warmup(
>  	if (agi_bp)
>  		xfs_buf_relse(agi_bp);
>  	if (pag)
> -		xfs_perag_put(pag);
> +		xfs_perag_rele(pag);
>  	return error;
>  }
>  
> @@ -249,7 +249,7 @@ xchk_fscount_aggregate_agcounts(
>  
>  	}
>  	if (pag)
> -		xfs_perag_put(pag);
> +		xfs_perag_rele(pag);
>  	if (error) {
>  		xchk_set_incomplete(sc);
>  		return error;
> diff --git a/fs/xfs/xfs_fsmap.c b/fs/xfs/xfs_fsmap.c
> index 88a88506ffff..120d284a03fe 100644
> --- a/fs/xfs/xfs_fsmap.c
> +++ b/fs/xfs/xfs_fsmap.c
> @@ -688,11 +688,11 @@ __xfs_getfsmap_datadev(
>  		info->agf_bp = NULL;
>  	}
>  	if (info->pag) {
> -		xfs_perag_put(info->pag);
> +		xfs_perag_rele(info->pag);
>  		info->pag = NULL;
>  	} else if (pag) {
>  		/* loop termination case */
> -		xfs_perag_put(pag);
> +		xfs_perag_rele(pag);
>  	}
>  
>  	return error;
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index ddeaccc04aec..0f4a014dded3 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -1767,7 +1767,7 @@ xfs_icwalk(
>  		if (error) {
>  			last_error = error;
>  			if (error == -EFSCORRUPTED) {
> -				xfs_perag_put(pag);
> +				xfs_perag_rele(pag);
>  				break;
>  			}
>  		}
> diff --git a/fs/xfs/xfs_iwalk.c b/fs/xfs/xfs_iwalk.c
> index 7558486f4937..c31857d903a4 100644
> --- a/fs/xfs/xfs_iwalk.c
> +++ b/fs/xfs/xfs_iwalk.c
> @@ -591,7 +591,7 @@ xfs_iwalk(
>  	}
>  
>  	if (iwag.pag)
> -		xfs_perag_put(pag);
> +		xfs_perag_rele(pag);
>  	xfs_iwalk_free(&iwag);
>  	return error;
>  }
> @@ -683,7 +683,7 @@ xfs_iwalk_threaded(
>  			break;
>  	}
>  	if (pag)
> -		xfs_perag_put(pag);
> +		xfs_perag_rele(pag);
>  	if (polled)
>  		xfs_pwork_poll(&pctl);
>  	return xfs_pwork_destroy(&pctl);
> @@ -776,7 +776,7 @@ xfs_inobt_walk(
>  	}
>  
>  	if (iwag.pag)
> -		xfs_perag_put(pag);
> +		xfs_perag_rele(pag);
>  	xfs_iwalk_free(&iwag);
>  	return error;
>  }
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index 57bf59ff4854..f5dc46ce9803 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -927,7 +927,7 @@ xfs_reflink_recover_cow(
>  	for_each_perag(mp, agno, pag) {
>  		error = xfs_refcount_recover_cow_leftovers(mp, pag);
>  		if (error) {
> -			xfs_perag_put(pag);
> +			xfs_perag_rele(pag);
>  			break;
>  		}
>  	}
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index 7dc57db6aa42..f0b62054ea68 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -189,6 +189,9 @@ DEFINE_EVENT(xfs_perag_class, name,	\
>  DEFINE_PERAG_REF_EVENT(xfs_perag_get);
>  DEFINE_PERAG_REF_EVENT(xfs_perag_get_tag);
>  DEFINE_PERAG_REF_EVENT(xfs_perag_put);
> +DEFINE_PERAG_REF_EVENT(xfs_perag_grab);
> +DEFINE_PERAG_REF_EVENT(xfs_perag_grab_tag);
> +DEFINE_PERAG_REF_EVENT(xfs_perag_rele);
>  DEFINE_PERAG_REF_EVENT(xfs_perag_set_inode_tag);
>  DEFINE_PERAG_REF_EVENT(xfs_perag_clear_inode_tag);
>  
> -- 
> 2.39.0
>