Re: [PATCH 2/4] xfs: prepare inode for i_gclist detection

Darrick J. Wong
2024-02-01T19:15:34Z

On Thu, Feb 01, 2024 at 11:30:14AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> We currently don't initialise the inode->i_gclist member because it
> it not necessary for a pure llist_add/llist_del_all producer-
> consumer usage pattern. However, for lazy removal from the inodegc
> list, we need to be able to determine if the inode is already on an
> inodegc list before we queue it.
>
> We can do this detection by using llist_on_list(), but this requires
> that we initialise the llist_node before we use it, and we
> re-initialise it when we remove it from the llist.
>
> Because we already serialise the inodegc list add with inode state
> changes under the ip->i_flags_lock, we can do the initialisation on
> list removal atomically with the state change. We can also do the
> check of whether the inode is already on a inodegc list inside the
> state change region on insert.
>
> This gives us the ability to use llist_on_list(ip->i_gclist) to
> determine if the inode needs to be queued for inactivation without
> having to depend on inode state flags.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_icache.c | 21 ++++++++++++++++++---
> 1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index 425b55526386..2dd1559aade2 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -114,6 +114,7 @@ xfs_inode_alloc(
> spin_lock_init(&ip->i_ioend_lock);
> ip->i_next_unlinked = NULLAGINO;
> ip->i_prev_unlinked = 0;
> + init_llist_node(&ip->i_gclist);
>
> return ip;
> }
> @@ -1875,10 +1876,16 @@ xfs_inodegc_worker(
> llist_for_each_entry_safe(ip, n, node, i_gclist) {
> int error;
>
> - /* Switch state to inactivating. */
> + /*
> + * Switch state to inactivating and remove the inode from the
> + * gclist. This allows the use of llist_on_list() in the queuing
> + * code to determine if the inode is already on an inodegc
> + * queue.
> + */
> spin_lock(&ip->i_flags_lock);
> ip->i_flags |= XFS_INACTIVATING;
> ip->i_flags &= ~XFS_NEED_INACTIVE;
> + init_llist_node(&ip->i_gclist);
> spin_unlock(&ip->i_flags_lock);
>
> error = xfs_inodegc_inactivate(ip);
> @@ -2075,11 +2082,20 @@ xfs_inodegc_queue(
> trace_xfs_inode_set_need_inactive(ip);
>
> /*
> - * Put the addition of the inode to the gc list under the
> + * The addition of the inode to the gc list is done under the
> * ip->i_flags_lock so that the state change and list addition are
> * atomic w.r.t. lookup operations under the ip->i_flags_lock.
> + * The removal is also done under the ip->i_flags_lock and so this
> + * allows us to safely use llist_on_list() here to determine if the
> + * inode is already queued on an inactivation queue.
> */
> spin_lock(&ip->i_flags_lock);
> + ip->i_flags |= XFS_NEED_INACTIVE;

Oh, I see, the flag setting line moves back.

Other than that everything makes sense here...
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> +
> + if (llist_on_list(&ip->i_gclist)) {
> + spin_unlock(&ip->i_flags_lock);
> + return;
> + }
>
> cpu_nr = get_cpu();
> gc = this_cpu_ptr(mp->m_inodegc);
> @@ -2088,7 +2104,6 @@ xfs_inodegc_queue(
> WRITE_ONCE(gc->items, items + 1);
> shrinker_hits = READ_ONCE(gc->shrinker_hits);
>
> - ip->i_flags |= XFS_NEED_INACTIVE;
> spin_unlock(&ip->i_flags_lock);
>
> /*
> --
> 2.43.0
>
>