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

Darrick J. Wong
2023-02-11T04:06:25Z

On Fri, Feb 10, 2023 at 09:17:50AM +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 patch introduce xfs_perag_grab()/xfs_perag_rele() as the API
> for active AG reference functionality. We also need to convert the
> for_each_perag*() iterators to use active references, which will
> start the process of converting high level code over to using active
> references. Conversion of non-iterator based code to active
> references will be done in followup patches.
>
> Note that the implementation using reference counting is really just
> a development vehicle for the API to ensure we don't have any leaks
> in the callers. Once we need to remove perag structures from memory
> dyanmically, we will need a much more robust per-ag state transition
> mechanism for preventing new references from being taken while we
> wait for existing references to drain before removal from memory can
> occur....
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Allison Henderson <allison.henderson@oracle.com>

It'll be interesting to see how the perag reference changes play out
over the next year...

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> 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 2ac98d8ddbfd..eb5e49d44f13 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
>