Re: [PATCH 25/43] xfs: add support for zoned space reservations

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

On Wed, Dec 11, 2024 at 09:54:50AM +0100, Christoph Hellwig wrote:
> For zoned file systems garbage collection (GC) has to take the iolock
> and mmaplock after moving data to a new place to synchronize with
> readers.  This means waiting for garbage collection with the iolock can
> deadlock.
> 
> To avoid this, the worst case required blocks have to be reserved before
> taking the iolock, which is done using a new RTAVAILABLE counter that
> tracks blocks that are free to write into and don't require garbage
> collection.  The new helpers try to take these available blocks, and
> if there aren't enough available it wakes and waits for GC.  This is
> done using a list of on-stack reservations to ensure fairness.
> 
> Co-developed-by: Hans Holmberg <hans.holmberg@wdc.com>
> Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/Makefile              |   3 +-
>  fs/xfs/libxfs/xfs_bmap.c     |  15 ++-
>  fs/xfs/xfs_zone_alloc.h      |  15 +++
>  fs/xfs/xfs_zone_priv.h       |   2 +
>  fs/xfs/xfs_zone_space_resv.c | 244 +++++++++++++++++++++++++++++++++++
>  5 files changed, 274 insertions(+), 5 deletions(-)
>  create mode 100644 fs/xfs/xfs_zone_space_resv.c
> 
> diff --git a/fs/xfs/Makefile b/fs/xfs/Makefile
> index 28bd2627e9ef..bdedf4bdb1db 100644
> --- a/fs/xfs/Makefile
> +++ b/fs/xfs/Makefile
> @@ -138,7 +138,8 @@ xfs-$(CONFIG_XFS_QUOTA)		+= xfs_dquot.o \
>  
>  # xfs_rtbitmap is shared with libxfs
>  xfs-$(CONFIG_XFS_RT)		+= xfs_rtalloc.o \
> -				   xfs_zone_alloc.o
> +				   xfs_zone_alloc.o \
> +				   xfs_zone_space_resv.o
>  
>  xfs-$(CONFIG_XFS_POSIX_ACL)	+= xfs_acl.o
>  xfs-$(CONFIG_SYSCTL)		+= xfs_sysctl.o
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 512f1ceca47f..625d853f248b 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -40,6 +40,7 @@
>  #include "xfs_symlink_remote.h"
>  #include "xfs_inode_util.h"
>  #include "xfs_rtgroup.h"
> +#include "xfs_zone_alloc.h"
>  
>  struct kmem_cache		*xfs_bmap_intent_cache;
>  
> @@ -4787,12 +4788,18 @@ xfs_bmap_del_extent_delay(
>  	da_diff = da_old - da_new;
>  	fdblocks = da_diff;
>  
> -	if (bflags & XFS_BMAPI_REMAP)
> +	if (bflags & XFS_BMAPI_REMAP) {
>  		;
> -	else if (isrt)
> -		xfs_add_frextents(mp, xfs_blen_to_rtbxlen(mp, del->br_blockcount));
> -	else
> +	} else if (isrt) {
> +		xfs_rtxlen_t	rtxlen;
> +
> +		rtxlen = xfs_blen_to_rtbxlen(mp, del->br_blockcount);
> +		if (xfs_is_zoned_inode(ip))
> +			xfs_zoned_add_available(mp, rtxlen);
> +		xfs_add_frextents(mp, rtxlen);
> +	} else {
>  		fdblocks += del->br_blockcount;
> +	}
>  
>  	xfs_add_fdblocks(mp, fdblocks);
>  	xfs_mod_delalloc(ip, -(int64_t)del->br_blockcount, -da_diff);
> diff --git a/fs/xfs/xfs_zone_alloc.h b/fs/xfs/xfs_zone_alloc.h
> index 37a49f4ce40c..6d0404c2c46c 100644
> --- a/fs/xfs/xfs_zone_alloc.h
> +++ b/fs/xfs/xfs_zone_alloc.h
> @@ -5,6 +5,21 @@
>  struct iomap_ioend;
>  struct xfs_open_zone;
>  
> +struct xfs_zone_alloc_ctx {
> +	struct xfs_open_zone	*open_zone;
> +	xfs_filblks_t		reserved_blocks;
> +};
> +
> +#define XFS_ZR_GREEDY		(1U << 0)
> +#define XFS_ZR_NOWAIT		(1U << 1)
> +#define XFS_ZR_RESERVED		(1U << 2)

What do these flag values mean?  Can we put that into comments?

> +int xfs_zoned_space_reserve(struct xfs_inode *ip, xfs_filblks_t count_fsb,
> +		unsigned int flags, struct xfs_zone_alloc_ctx *ac);
> +void xfs_zoned_space_unreserve(struct xfs_inode *ip,
> +		struct xfs_zone_alloc_ctx *ac);
> +void xfs_zoned_add_available(struct xfs_mount *mp, xfs_filblks_t count_fsb);
> +
>  void xfs_zone_alloc_and_submit(struct iomap_ioend *ioend,
>  		struct xfs_open_zone **oz);
>  int xfs_zone_free_blocks(struct xfs_trans *tp, struct xfs_rtgroup *rtg,
> diff --git a/fs/xfs/xfs_zone_priv.h b/fs/xfs/xfs_zone_priv.h
> index ae1556871596..f56f3ca8ea00 100644
> --- a/fs/xfs/xfs_zone_priv.h
> +++ b/fs/xfs/xfs_zone_priv.h
> @@ -82,4 +82,6 @@ struct xfs_zone_info {
>  
>  struct xfs_open_zone *xfs_open_zone(struct xfs_mount *mp, bool is_gc);
>  
> +void xfs_zoned_resv_wake_all(struct xfs_mount *mp);
> +
>  #endif /* _XFS_ZONE_PRIV_H */
> diff --git a/fs/xfs/xfs_zone_space_resv.c b/fs/xfs/xfs_zone_space_resv.c
> new file mode 100644
> index 000000000000..5ee525e18759
> --- /dev/null
> +++ b/fs/xfs/xfs_zone_space_resv.c
> @@ -0,0 +1,244 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2023 Christoph Hellwig.
> + * Copyright (c) 2024, Western Digital Corporation or its affiliates.
> + */
> +#include "xfs.h"
> +#include "xfs_shared.h"
> +#include "xfs_format.h"
> +#include "xfs_trans_resv.h"
> +#include "xfs_mount.h"
> +#include "xfs_inode.h"
> +#include "xfs_rtbitmap.h"
> +#include "xfs_zone_alloc.h"
> +#include "xfs_zone_priv.h"
> +#include "xfs_zones.h"
> +
> +/*
> + * Note: the zoned allocator does not support a rtextsize > 1, so this code and
> + * the allocator itself uses file system blocks interchangable with realtime
> + * extents without doing the otherwise required conversions.
> + */
> +
> +/*
> + * Per-task space reservation.
> + *
> + * Tasks that need to wait for GC to free up space allocate one of these
> + * on-stack and adds it to the per-mount zi_reclaim_reservations lists.
> + * The GC thread will then wake the tasks in order when space becomes available.
> + */
> +struct xfs_zone_reservation {
> +	struct list_head	entry;
> +	struct task_struct	*task;
> +	xfs_filblks_t		count_fsb;
> +};
> +
> +/*
> + * Calculate the number of reserved blocks.
> + *
> + * XC_FREE_RTEXTENTS counts the user available capacity, to which the file
> + * system can be filled, while XC_FREE_RTAVAILABLE counts the blocks instantly
> + * available for writes without waiting for GC.
> + *
> + * For XC_FREE_RTAVAILABLE only the smaller reservation required for GC and
> + * block zeroing is excluded from the user capacity, while XC_FREE_RTEXTENTS
> + * is further restricted by at least one zone as well as the optional
> + * persistently reserved blocks.  This allows the allocator to run more
> + * smoothly by not always triggering GC.

Hmm, so _RTAVAILABLE really means _RTNOGC?  That makes sense.

> + */
> +uint64_t
> +xfs_zoned_default_resblks(
> +	struct xfs_mount	*mp,
> +	enum xfs_free_counter	ctr)
> +{
> +	switch (ctr) {
> +	case XC_FREE_RTEXTENTS:
> +		return (uint64_t)XFS_RESERVED_ZONES *
> +			mp->m_groups[XG_TYPE_RTG].blocks +
> +			mp->m_sb.sb_rtreserved;
> +	case XC_FREE_RTAVAILABLE:
> +		return (uint64_t)XFS_GC_ZONES *
> +			mp->m_groups[XG_TYPE_RTG].blocks;
> +	default:
> +		ASSERT(0);
> +		return 0;
> +	}
> +}
> +
> +void
> +xfs_zoned_resv_wake_all(
> +	struct xfs_mount		*mp)
> +{
> +	struct xfs_zone_info		*zi = mp->m_zone_info;
> +	struct xfs_zone_reservation	*reservation;
> +
> +	spin_lock(&zi->zi_reservation_lock);
> +	list_for_each_entry(reservation, &zi->zi_reclaim_reservations, entry)
> +		wake_up_process(reservation->task);
> +	spin_unlock(&zi->zi_reservation_lock);
> +}
> +
> +void
> +xfs_zoned_add_available(
> +	struct xfs_mount		*mp,
> +	xfs_filblks_t			count_fsb)
> +{
> +	struct xfs_zone_info		*zi = mp->m_zone_info;
> +	struct xfs_zone_reservation	*reservation;
> +
> +	if (list_empty_careful(&zi->zi_reclaim_reservations)) {
> +		xfs_add_freecounter(mp, XC_FREE_RTAVAILABLE, count_fsb);
> +		return;
> +	}
> +
> +	spin_lock(&zi->zi_reservation_lock);
> +	xfs_add_freecounter(mp, XC_FREE_RTAVAILABLE, count_fsb);
> +	count_fsb = xfs_sum_freecounter(mp, XC_FREE_RTAVAILABLE);
> +	list_for_each_entry(reservation, &zi->zi_reclaim_reservations, entry) {
> +		if (reservation->count_fsb > count_fsb)
> +			break;
> +		wake_up_process(reservation->task);
> +		count_fsb -= reservation->count_fsb;
> +
> +	}
> +	spin_unlock(&zi->zi_reservation_lock);
> +}
> +
> +static int
> +xfs_zoned_space_wait_error(
> +	struct xfs_mount		*mp)
> +{
> +	if (xfs_is_shutdown(mp))
> +		return -EIO;
> +	if (fatal_signal_pending(current))
> +		return -EINTR;
> +	return 0;
> +}
> +
> +static int
> +xfs_zoned_reserve_available(
> +	struct xfs_inode		*ip,
> +	xfs_filblks_t			count_fsb,
> +	unsigned int			flags)
> +{
> +	struct xfs_mount		*mp = ip->i_mount;
> +	struct xfs_zone_info		*zi = mp->m_zone_info;
> +	struct xfs_zone_reservation	reservation = {
> +		.task		= current,
> +		.count_fsb	= count_fsb,
> +	};
> +	int				error;
> +
> +	/*
> +	 * If there are no waiters, try to directly grab the available blocks
> +	 * from the percpu counter.
> +	 *
> +	 * If the caller wants to dip into the reserved pool also bypass the
> +	 * wait list.  This relies on the fact that we have a very graciously
> +	 * sized reserved pool that always has enough space.  If the reserved
> +	 * allocations fail we're in trouble.
> +	 */
> +	if (likely(list_empty_careful(&zi->zi_reclaim_reservations) ||
> +	    (flags & XFS_ZR_RESERVED))) {
> +		error = xfs_dec_freecounter(mp, XC_FREE_RTAVAILABLE, count_fsb,
> +				flags & XFS_ZR_RESERVED);
> +		if (error != -ENOSPC)
> +			return error;
> +	}
> +
> +	if (flags & XFS_ZR_NOWAIT)
> +		return -EAGAIN;
> +
> +	spin_lock(&zi->zi_reservation_lock);
> +	list_add_tail(&reservation.entry, &zi->zi_reclaim_reservations);
> +	while ((error = xfs_zoned_space_wait_error(mp)) == 0) {
> +		set_current_state(TASK_KILLABLE);
> +
> +		error = xfs_dec_freecounter(mp, XC_FREE_RTAVAILABLE, count_fsb,
> +				flags & XFS_ZR_RESERVED);
> +		if (error != -ENOSPC)
> +			break;
> +
> +		spin_unlock(&zi->zi_reservation_lock);
> +		schedule();
> +		spin_lock(&zi->zi_reservation_lock);
> +	}
> +	list_del(&reservation.entry);
> +	spin_unlock(&zi->zi_reservation_lock);

Hmm.  So if I'm understanding correctly, threads wanting to write to a
file try to locklessly reserve space from RTAVAILABLE.  If they can't
get space because the zone is nearly full / needs gc / etc then everyone
gets to wait FIFO style in the reclaim_reservations list.  They can be
woken up from the wait if either (a) someone gives back reserved space
or (b) the copygc empties out this zone.

Or if the thread isn't willing to wait, we skip the fifo and either fail
up to userspace or just move on to the next zone?

I think I understand the general idea, but I don't quite know when we're
going to use the greedy algorithm.  Later I see XFS_ZR_GREEDY gets used
from the buffered write path, but there doesn't seem to be an obvious
reason why?

--D

> +
> +	__set_current_state(TASK_RUNNING);
> +	return error;
> +}
> +
> +/*
> + * Implement greedy space allocation for short writes by trying to grab all
> + * that is left after locking out other threads from trying to do the same.
> + *
> + * This isn't exactly optimal and can hopefully be replaced by a proper
> + * percpu_counter primitive one day.
> + */
> +static int
> +xfs_zoned_reserve_extents_greedy(
> +	struct xfs_inode		*ip,
> +	xfs_filblks_t			*count_fsb,
> +	unsigned int			flags)
> +{
> +	struct xfs_mount		*mp = ip->i_mount;
> +	struct xfs_zone_info		*zi = mp->m_zone_info;
> +	s64				len = *count_fsb;
> +	int				error = -ENOSPC;
> +
> +	spin_lock(&zi->zi_reservation_lock);
> +	len = min(len, xfs_sum_freecounter(mp, XC_FREE_RTEXTENTS));
> +	if (len > 0) {
> +		*count_fsb = len;
> +		error = xfs_dec_freecounter(mp, XC_FREE_RTEXTENTS, *count_fsb,
> +				flags & XFS_ZR_RESERVED);
> +	}
> +	spin_unlock(&zi->zi_reservation_lock);
> +	return error;
> +}
> +
> +int
> +xfs_zoned_space_reserve(
> +	struct xfs_inode		*ip,
> +	xfs_filblks_t			count_fsb,
> +	unsigned int			flags,
> +	struct xfs_zone_alloc_ctx	*ac)
> +{
> +	struct xfs_mount		*mp = ip->i_mount;
> +	int				error;
> +
> +	ASSERT(ac->reserved_blocks == 0);
> +	ASSERT(ac->open_zone == NULL);
> +
> +	error = xfs_dec_freecounter(mp, XC_FREE_RTEXTENTS, count_fsb,
> +			flags & XFS_ZR_RESERVED);
> +	if (error == -ENOSPC && (flags & XFS_ZR_GREEDY) && count_fsb > 1)
> +		error = xfs_zoned_reserve_extents_greedy(ip, &count_fsb, flags);
> +	if (error)
> +		return error;
> +
> +	error = xfs_zoned_reserve_available(ip, count_fsb, flags);
> +	if (error) {
> +		xfs_add_freecounter(mp, XC_FREE_RTEXTENTS, count_fsb);
> +		return error;
> +	}
> +	ac->reserved_blocks = count_fsb;
> +	return 0;
> +}
> +
> +void
> +xfs_zoned_space_unreserve(
> +	struct xfs_inode		*ip,
> +	struct xfs_zone_alloc_ctx	*ac)
> +{
> +	if (ac->reserved_blocks > 0) {
> +		struct xfs_mount	*mp = ip->i_mount;
> +
> +		xfs_zoned_add_available(mp, ac->reserved_blocks);
> +		xfs_add_freecounter(mp, XC_FREE_RTEXTENTS, ac->reserved_blocks);
> +	}
> +	if (ac->open_zone)
> +		xfs_open_zone_put(ac->open_zone);
> +}
> -- 
> 2.45.2
> 
>