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    SubjectRe: [PATCH 6/7] kernfs: Allow kernfs nodes to be deactivated and re-activated
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    On 2022/8/20 08:05, Tejun Heo wrote:
    > Currently, kernfs nodes can be created deactivated and activated later to
    > allow creation of multiple nodes to succeed or fail as a group. Extend this
    > capability so that kernfs nodes can be deactivated and re-activated anytime
    > and however many times. This can be used to toggle interface files for
    > features which can be dynamically turned on and off.
    >
    > kernfs_activate()'s skip conditions are updated so that it doesn't ignore
    > re-activations and suppress re-activations of files which are being removed.
    >
    > Signed-off-by: Tejun Heo <tj@kernel.org>
    > Cc: Chengming Zhou <zhouchengming@bytedance.com>
    > Cc: Johannes Weiner <hannes@cmpxchg.org>
    > ---
    > fs/kernfs/dir.c | 89 ++++++++++++++++++++++++++++++------------
    > include/linux/kernfs.h | 2 +
    > 2 files changed, 65 insertions(+), 26 deletions(-)
    >
    > diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
    > index f857731598cd..6db031362585 100644
    > --- a/fs/kernfs/dir.c
    > +++ b/fs/kernfs/dir.c
    > @@ -483,7 +483,7 @@ static bool kernfs_drain(struct kernfs_node *kn)
    > * worrying about draining.
    > */
    > if (atomic_read(&kn->active) == KN_DEACTIVATED_BIAS &&
    > - kernfs_should_drain_open_files(kn))
    > + !kernfs_should_drain_open_files(kn))
    > return false;
    >
    > up_write(&root->kernfs_rwsem);
    > @@ -1321,14 +1321,15 @@ static struct kernfs_node *kernfs_next_descendant_post(struct kernfs_node *pos,
    > }
    >
    > /**
    > - * kernfs_activate - activate a node which started deactivated
    > + * kernfs_activate - activate a node's subtree
    > * @kn: kernfs_node whose subtree is to be activated
    > *
    > - * If the root has KERNFS_ROOT_CREATE_DEACTIVATED set, a newly created node
    > - * needs to be explicitly activated. A node which hasn't been activated
    > - * isn't visible to userland and deactivation is skipped during its
    > - * removal. This is useful to construct atomic init sequences where
    > - * creation of multiple nodes should either succeed or fail atomically.
    > + * If newly created on a root w/ %KERNFS_ROOT_CREATE_DEACTIVATED or after a
    > + * kernfs_deactivate() call, @kn is deactivated and invisible to userland. This
    > + * function activates all nodes in @kn's inclusive subtree making them visible.
    > + *
    > + * %KERNFS_ROOT_CREATE_DEACTIVATED is useful when constructing init sequences
    > + * where creation of multiple nodes should either succeed or fail atomically.
    > *
    > * The caller is responsible for ensuring that this function is not called
    > * after kernfs_remove*() is invoked on @kn.
    > @@ -1342,7 +1343,7 @@ void kernfs_activate(struct kernfs_node *kn)
    >
    > pos = NULL;
    > while ((pos = kernfs_next_descendant_post(pos, kn))) {
    > - if (pos->flags & KERNFS_ACTIVATED)
    > + if (kernfs_active(pos) || (kn->flags & KERNFS_REMOVING))

    May I ask a question, what's the difference between kernfs_active() and KERNFS_ACTIVATED?

    KERNFS_ACTIVATED is always set when kernfs_activate() and never clear, so I think it means:

    1. !KERNFS_ACTIVATED : allocated but not activated
    2. KERNFS_ACTIVATED && !kernfs_active() : make deactivated by kernfs_deactivate_locked()

    I see most code check kernfs_active(), but two places check KERNFS_ACTIVATED, I'm not sure where
    should check KERNFS_ACTIVATED, or is there any chance we can remove KERNFS_ACTIVATED?

    Thanks!


    > continue;
    >
    > WARN_ON_ONCE(pos->parent && RB_EMPTY_NODE(&pos->rb));
    > @@ -1355,6 +1356,58 @@ void kernfs_activate(struct kernfs_node *kn)
    > up_write(&root->kernfs_rwsem);
    > }
    >
    > +static void kernfs_deactivate_locked(struct kernfs_node *kn, bool removing)
    > +{
    > + struct kernfs_root *root = kernfs_root(kn);
    > + struct kernfs_node *pos;
    > +
    > + lockdep_assert_held_write(&root->kernfs_rwsem);
    > +
    > + /* prevent any new usage under @kn by deactivating all nodes */
    > + pos = NULL;
    > + while ((pos = kernfs_next_descendant_post(pos, kn))) {
    > + if (kernfs_active(pos))
    > + atomic_add(KN_DEACTIVATED_BIAS, &pos->active);
    > + if (removing)
    > + pos->flags |= KERNFS_REMOVING;
    > + }
    > +
    > + /*
    > + * No new active usage can be created. Drain existing ones. As
    > + * kernfs_drain() may drop kernfs_rwsem temporarily, pin @pos so that it
    > + * doesn't go away underneath us.
    > + *
    > + * If kernfs_rwsem was released, restart from the beginning. Forward
    > + * progress is guaranteed as a drained node is guaranteed to stay
    > + * drained. In the unlikely case that the loop restart ever becomes a
    > + * problem, we should be able to work around by batching up the
    > + * draining.
    > + */
    > + pos = NULL;
    > + while ((pos = kernfs_next_descendant_post(pos, kn))) {
    > + kernfs_get(pos);
    > + if (kernfs_drain(pos))
    > + pos = NULL;
    > + kernfs_put(pos);
    > + }
    > +}
    > +
    > +/**
    > + * kernfs_deactivate - deactivate a node's subtree
    > + * @kn: kernfs_node whose subtree is to be deactivated
    > + *
    > + * Deactivate @kn's inclusive subtree. On return, the subtree is invisible to
    > + * userland and there are no in-flight file operations.
    > + */
    > +void kernfs_deactivate(struct kernfs_node *kn)
    > +{
    > + struct kernfs_root *root = kernfs_root(kn);
    > +
    > + down_write(&root->kernfs_rwsem);
    > + kernfs_deactivate_locked(kn, false);
    > + up_write(&root->kernfs_rwsem);
    > +}
    > +
    > static void __kernfs_remove(struct kernfs_node *kn)
    > {
    > struct kernfs_node *pos;
    > @@ -1374,26 +1427,12 @@ static void __kernfs_remove(struct kernfs_node *kn)
    >
    > pr_debug("kernfs %s: removing\n", kn->name);
    >
    > - /* prevent any new usage under @kn by deactivating all nodes */
    > - pos = NULL;
    > - while ((pos = kernfs_next_descendant_post(pos, kn)))
    > - if (kernfs_active(pos))
    > - atomic_add(KN_DEACTIVATED_BIAS, &pos->active);
    > + kernfs_deactivate_locked(kn, true);
    >
    > - /* deactivate and unlink the subtree node-by-node */
    > + /* unlink the subtree node-by-node */
    > do {
    > pos = kernfs_leftmost_descendant(kn);
    >
    > - /*
    > - * kernfs_drain() may drop kernfs_rwsem temporarily and @pos's
    > - * base ref could have been put by someone else by the time
    > - * the function returns. Make sure it doesn't go away
    > - * underneath us.
    > - */
    > - kernfs_get(pos);
    > -
    > - kernfs_drain(pos);
    > -
    > /*
    > * kernfs_unlink_sibling() succeeds once per node. Use it
    > * to decide who's responsible for cleanups.
    > @@ -1410,8 +1449,6 @@ static void __kernfs_remove(struct kernfs_node *kn)
    >
    > kernfs_put(pos);
    > }
    > -
    > - kernfs_put(pos);
    > } while (pos != kn);
    > }
    >
    > diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
    > index 367044d7708c..657eea1395b6 100644
    > --- a/include/linux/kernfs.h
    > +++ b/include/linux/kernfs.h
    > @@ -112,6 +112,7 @@ enum kernfs_node_flag {
    > KERNFS_SUICIDED = 0x0800,
    > KERNFS_EMPTY_DIR = 0x1000,
    > KERNFS_HAS_RELEASE = 0x2000,
    > + KERNFS_REMOVING = 0x4000,
    > };
    >
    > /* @flags for kernfs_create_root() */
    > @@ -429,6 +430,7 @@ struct kernfs_node *kernfs_create_link(struct kernfs_node *parent,
    > const char *name,
    > struct kernfs_node *target);
    > void kernfs_activate(struct kernfs_node *kn);
    > +void kernfs_deactivate(struct kernfs_node *kn);
    > void kernfs_remove(struct kernfs_node *kn);
    > void kernfs_break_active_protection(struct kernfs_node *kn);
    > void kernfs_unbreak_active_protection(struct kernfs_node *kn);

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