Implement a very simple cookie state machine to handle lookup, invalidation, withdrawal, relinquishment and, to be added later, commit on LRU discard.
Three cache methods are provided: ->lookup_cookie() to look up and, if necessary, create a data storage object; ->withdraw_cookie() to free the resources associated with that object and potentially delete it; and ->prepare_to_write(), to do prepare for changes to the cached data to be modified locally. Changes ======= ver #3: - Fix a race between LRU discard and relinquishment whereby the former would override the latter and thus the latter would never happen[1]. ver #2: - Don't hold n_accesses elevated whilst cache is bound to a cookie, but rather add a flag that prevents the state machine from being queued when n_accesses reaches 0. Signed-off-by: David Howells <dhowe...@redhat.com> Reviewed-by: Jeff Layton <jlay...@kernel.org> cc: linux-cachefs@redhat.com Link: https://lore.kernel.org/r/599331.1639410...@warthog.procyon.org.uk/ [1] Link: https://lore.kernel.org/r/163819599657.215744.15799615296912341745.st...@warthog.procyon.org.uk/ # v1 Link: https://lore.kernel.org/r/163906903925.143852.1805855338154353867.st...@warthog.procyon.org.uk/ # v2 Link: https://lore.kernel.org/r/163967105456.1823006.14730395299835841776.st...@warthog.procyon.org.uk/ # v3 --- fs/fscache/cookie.c | 313 +++++++++++++++++++++++++++++++++++----- include/linux/fscache-cache.h | 27 +++ include/trace/events/fscache.h | 4 + 3 files changed, 300 insertions(+), 44 deletions(-) diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c index 04d2127bd354..336046de08ee 100644 --- a/fs/fscache/cookie.c +++ b/fs/fscache/cookie.c @@ -15,7 +15,8 @@ struct kmem_cache *fscache_cookie_jar; -static void fscache_drop_cookie(struct fscache_cookie *cookie); +static void fscache_cookie_worker(struct work_struct *work); +static void fscache_unhash_cookie(struct fscache_cookie *cookie); #define fscache_cookie_hash_shift 15 static struct hlist_bl_head fscache_cookie_hash[1 << fscache_cookie_hash_shift]; @@ -62,6 +63,19 @@ static void fscache_free_cookie(struct fscache_cookie *cookie) kmem_cache_free(fscache_cookie_jar, cookie); } +static void __fscache_queue_cookie(struct fscache_cookie *cookie) +{ + if (!queue_work(fscache_wq, &cookie->work)) + fscache_put_cookie(cookie, fscache_cookie_put_over_queued); +} + +static void fscache_queue_cookie(struct fscache_cookie *cookie, + enum fscache_cookie_trace where) +{ + fscache_get_cookie(cookie, where); + __fscache_queue_cookie(cookie); +} + /* * Initialise the access gate on a cookie by setting a flag to prevent the * state machine from being queued when the access counter transitions to 0. @@ -98,9 +112,8 @@ void fscache_end_cookie_access(struct fscache_cookie *cookie, trace_fscache_access(cookie->debug_id, refcount_read(&cookie->ref), n_accesses, why); if (n_accesses == 0 && - !test_bit(FSCACHE_COOKIE_NO_ACCESS_WAKE, &cookie->flags)) { - // PLACEHOLDER: Need to poke the state machine - } + !test_bit(FSCACHE_COOKIE_NO_ACCESS_WAKE, &cookie->flags)) + fscache_queue_cookie(cookie, fscache_cookie_get_end_access); } EXPORT_SYMBOL(fscache_end_cookie_access); @@ -171,35 +184,58 @@ static inline void wake_up_cookie_state(struct fscache_cookie *cookie) wake_up_var(&cookie->state); } +/* + * Change the state a cookie is at and wake up anyone waiting for that. Impose + * an ordering between the stuff stored in the cookie and the state member. + * Paired with fscache_cookie_state(). + */ static void __fscache_set_cookie_state(struct fscache_cookie *cookie, enum fscache_cookie_state state) { - cookie->state = state; + smp_store_release(&cookie->state, state); } -/* - * Change the state a cookie is at and wake up anyone waiting for that - but - * only if the cookie isn't already marked as being in a cleanup state. - */ -void fscache_set_cookie_state(struct fscache_cookie *cookie, - enum fscache_cookie_state state) +static void fscache_set_cookie_state(struct fscache_cookie *cookie, + enum fscache_cookie_state state) { - bool changed = false; - spin_lock(&cookie->lock); - switch (cookie->state) { - case FSCACHE_COOKIE_STATE_RELINQUISHING: - break; - default: - __fscache_set_cookie_state(cookie, state); - changed = true; - break; - } + __fscache_set_cookie_state(cookie, state); spin_unlock(&cookie->lock); - if (changed) - wake_up_cookie_state(cookie); + wake_up_cookie_state(cookie); +} + +/** + * fscache_cookie_lookup_negative - Note negative lookup + * @cookie: The cookie that was being looked up + * + * Note that some part of the metadata path in the cache doesn't exist and so + * we can release any waiting readers in the certain knowledge that there's + * nothing for them to actually read. + * + * This function uses no locking and must only be called from the state machine. + */ +void fscache_cookie_lookup_negative(struct fscache_cookie *cookie) +{ + set_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags); + fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_CREATING); +} +EXPORT_SYMBOL(fscache_cookie_lookup_negative); + +/** + * fscache_caching_failed - Report that a failure stopped caching on a cookie + * @cookie: The cookie that was affected + * + * Tell fscache that caching on a cookie needs to be stopped due to some sort + * of failure. + * + * This function uses no locking and must only be called from the state machine. + */ +void fscache_caching_failed(struct fscache_cookie *cookie) +{ + clear_bit(FSCACHE_COOKIE_IS_CACHING, &cookie->flags); + fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_FAILED); } -EXPORT_SYMBOL(fscache_set_cookie_state); +EXPORT_SYMBOL(fscache_caching_failed); /* * Set the index key in a cookie. The cookie struct has space for a 16-byte @@ -291,10 +327,10 @@ static struct fscache_cookie *fscache_alloc_cookie( refcount_set(&cookie->ref, 1); cookie->debug_id = atomic_inc_return(&fscache_cookie_debug_id); - cookie->state = FSCACHE_COOKIE_STATE_QUIESCENT; spin_lock_init(&cookie->lock); INIT_LIST_HEAD(&cookie->commit_link); - INIT_WORK(&cookie->work, NULL /* PLACEHOLDER */); + INIT_WORK(&cookie->work, fscache_cookie_worker); + __fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_QUIESCENT); write_lock(&fscache_cookies_lock); list_add_tail(&cookie->proc_link, &fscache_cookies); @@ -417,6 +453,192 @@ struct fscache_cookie *__fscache_acquire_cookie( } EXPORT_SYMBOL(__fscache_acquire_cookie); +/* + * Prepare a cache object to be written to. + */ +static void fscache_prepare_to_write(struct fscache_cookie *cookie) +{ + cookie->volume->cache->ops->prepare_to_write(cookie); +} + +/* + * Look up a cookie in the cache. + */ +static void fscache_perform_lookup(struct fscache_cookie *cookie) +{ + enum fscache_access_trace trace = fscache_access_lookup_cookie_end_failed; + bool need_withdraw = false; + + _enter(""); + + if (!cookie->volume->cache_priv) { + fscache_create_volume(cookie->volume, true); + if (!cookie->volume->cache_priv) { + fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_QUIESCENT); + goto out; + } + } + + if (!cookie->volume->cache->ops->lookup_cookie(cookie)) { + if (cookie->state != FSCACHE_COOKIE_STATE_FAILED) + fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_QUIESCENT); + need_withdraw = true; + _leave(" [fail]"); + goto out; + } + + fscache_see_cookie(cookie, fscache_cookie_see_active); + fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_ACTIVE); + trace = fscache_access_lookup_cookie_end; + +out: + fscache_end_cookie_access(cookie, trace); + if (need_withdraw) + fscache_withdraw_cookie(cookie); + fscache_end_volume_access(cookie->volume, cookie, trace); +} + +/* + * Perform work upon the cookie, such as committing its cache state, + * relinquishing it or withdrawing the backing cache. We're protected from the + * cache going away under us as object withdrawal must come through this + * non-reentrant work item. + */ +static void fscache_cookie_state_machine(struct fscache_cookie *cookie) +{ + enum fscache_cookie_state state; + bool wake = false; + + _enter("c=%x", cookie->debug_id); + +again: + spin_lock(&cookie->lock); +again_locked: + state = cookie->state; + switch (state) { + case FSCACHE_COOKIE_STATE_QUIESCENT: + /* The QUIESCENT state is jumped to the LOOKING_UP state by + * fscache_use_cookie(). + */ + + if (atomic_read(&cookie->n_accesses) == 0 && + test_bit(FSCACHE_COOKIE_DO_RELINQUISH, &cookie->flags)) { + __fscache_set_cookie_state(cookie, + FSCACHE_COOKIE_STATE_RELINQUISHING); + wake = true; + goto again_locked; + } + break; + + case FSCACHE_COOKIE_STATE_LOOKING_UP: + spin_unlock(&cookie->lock); + fscache_init_access_gate(cookie); + fscache_perform_lookup(cookie); + goto again; + + case FSCACHE_COOKIE_STATE_ACTIVE: + if (test_and_clear_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags)) { + spin_unlock(&cookie->lock); + fscache_prepare_to_write(cookie); + spin_lock(&cookie->lock); + } + fallthrough; + + case FSCACHE_COOKIE_STATE_FAILED: + if (atomic_read(&cookie->n_accesses) != 0) + break; + if (test_bit(FSCACHE_COOKIE_DO_RELINQUISH, &cookie->flags)) { + __fscache_set_cookie_state(cookie, + FSCACHE_COOKIE_STATE_RELINQUISHING); + wake = true; + goto again_locked; + } + if (test_bit(FSCACHE_COOKIE_DO_WITHDRAW, &cookie->flags)) { + __fscache_set_cookie_state(cookie, + FSCACHE_COOKIE_STATE_WITHDRAWING); + wake = true; + goto again_locked; + } + break; + + case FSCACHE_COOKIE_STATE_RELINQUISHING: + case FSCACHE_COOKIE_STATE_WITHDRAWING: + if (cookie->cache_priv) { + spin_unlock(&cookie->lock); + cookie->volume->cache->ops->withdraw_cookie(cookie); + spin_lock(&cookie->lock); + } + + switch (state) { + case FSCACHE_COOKIE_STATE_RELINQUISHING: + fscache_see_cookie(cookie, fscache_cookie_see_relinquish); + fscache_unhash_cookie(cookie); + __fscache_set_cookie_state(cookie, + FSCACHE_COOKIE_STATE_DROPPED); + wake = true; + goto out; + case FSCACHE_COOKIE_STATE_WITHDRAWING: + fscache_see_cookie(cookie, fscache_cookie_see_withdraw); + break; + default: + BUG(); + } + + clear_bit(FSCACHE_COOKIE_NEEDS_UPDATE, &cookie->flags); + clear_bit(FSCACHE_COOKIE_DO_WITHDRAW, &cookie->flags); + clear_bit(FSCACHE_COOKIE_DO_LRU_DISCARD, &cookie->flags); + clear_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags); + set_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags); + __fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_QUIESCENT); + wake = true; + goto again_locked; + + case FSCACHE_COOKIE_STATE_DROPPED: + break; + + default: + WARN_ONCE(1, "Cookie %x in unexpected state %u\n", + cookie->debug_id, state); + break; + } + +out: + spin_unlock(&cookie->lock); + if (wake) + wake_up_cookie_state(cookie); + _leave(""); +} + +static void fscache_cookie_worker(struct work_struct *work) +{ + struct fscache_cookie *cookie = container_of(work, struct fscache_cookie, work); + + fscache_see_cookie(cookie, fscache_cookie_see_work); + fscache_cookie_state_machine(cookie); + fscache_put_cookie(cookie, fscache_cookie_put_work); +} + +/* + * Wait for the object to become inactive. The cookie's work item will be + * scheduled when someone transitions n_accesses to 0 - but if someone's + * already done that, schedule it anyway. + */ +static void __fscache_withdraw_cookie(struct fscache_cookie *cookie) +{ + int n_accesses; + bool unpinned; + + unpinned = test_and_clear_bit(FSCACHE_COOKIE_NO_ACCESS_WAKE, &cookie->flags); + + /* Need to read the access count after unpinning */ + n_accesses = atomic_read(&cookie->n_accesses); + if (unpinned) + trace_fscache_access(cookie->debug_id, refcount_read(&cookie->ref), + n_accesses, fscache_access_cache_unpin); + if (n_accesses == 0) + fscache_queue_cookie(cookie, fscache_cookie_get_end_access); +} + /* * Remove a cookie from the hash table. */ @@ -432,21 +654,27 @@ static void fscache_unhash_cookie(struct fscache_cookie *cookie) hlist_bl_del(&cookie->hash_link); clear_bit(FSCACHE_COOKIE_IS_HASHED, &cookie->flags); hlist_bl_unlock(h); + fscache_stat(&fscache_n_relinquishes_dropped); } -/* - * Finalise a cookie after all its resources have been disposed of. - */ -static void fscache_drop_cookie(struct fscache_cookie *cookie) +static void fscache_drop_withdraw_cookie(struct fscache_cookie *cookie) { - spin_lock(&cookie->lock); - __fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_DROPPED); - spin_unlock(&cookie->lock); - wake_up_cookie_state(cookie); + __fscache_withdraw_cookie(cookie); +} - fscache_unhash_cookie(cookie); - fscache_stat(&fscache_n_relinquishes_dropped); +/** + * fscache_withdraw_cookie - Mark a cookie for withdrawal + * @cookie: The cookie to be withdrawn. + * + * Allow the cache backend to withdraw the backing for a cookie for its own + * reasons, even if that cookie is in active use. + */ +void fscache_withdraw_cookie(struct fscache_cookie *cookie) +{ + set_bit(FSCACHE_COOKIE_DO_WITHDRAW, &cookie->flags); + fscache_drop_withdraw_cookie(cookie); } +EXPORT_SYMBOL(fscache_withdraw_cookie); /* * Allow the netfs to release a cookie back to the cache. @@ -473,12 +701,13 @@ void __fscache_relinquish_cookie(struct fscache_cookie *cookie, bool retire) ASSERTCMP(atomic_read(&cookie->volume->n_cookies), >, 0); atomic_dec(&cookie->volume->n_cookies); - set_bit(FSCACHE_COOKIE_DO_RELINQUISH, &cookie->flags); - - if (test_bit(FSCACHE_COOKIE_HAS_BEEN_CACHED, &cookie->flags)) - ; // PLACEHOLDER: Do something here if the cookie was cached - else - fscache_drop_cookie(cookie); + if (test_bit(FSCACHE_COOKIE_HAS_BEEN_CACHED, &cookie->flags)) { + set_bit(FSCACHE_COOKIE_DO_RELINQUISH, &cookie->flags); + fscache_drop_withdraw_cookie(cookie); + } else { + fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_DROPPED); + fscache_unhash_cookie(cookie); + } fscache_put_cookie(cookie, fscache_cookie_put_relinquish); } EXPORT_SYMBOL(__fscache_relinquish_cookie); diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h index 936ef731bbc7..ae6a75976450 100644 --- a/include/linux/fscache-cache.h +++ b/include/linux/fscache-cache.h @@ -57,6 +57,15 @@ struct fscache_cache_ops { /* Free the cache's data attached to a volume */ void (*free_volume)(struct fscache_volume *volume); + + /* Look up a cookie in the cache */ + bool (*lookup_cookie)(struct fscache_cookie *cookie); + + /* Withdraw an object without any cookie access counts held */ + void (*withdraw_cookie)(struct fscache_cookie *cookie); + + /* Prepare to write to a live cache object */ + void (*prepare_to_write)(struct fscache_cookie *cookie); }; extern struct workqueue_struct *fscache_wq; @@ -72,6 +81,7 @@ extern int fscache_add_cache(struct fscache_cache *cache, void *cache_priv); extern void fscache_withdraw_cache(struct fscache_cache *cache); extern void fscache_withdraw_volume(struct fscache_volume *volume); +extern void fscache_withdraw_cookie(struct fscache_cookie *cookie); extern void fscache_io_error(struct fscache_cache *cache); @@ -85,8 +95,21 @@ extern void fscache_put_cookie(struct fscache_cookie *cookie, enum fscache_cookie_trace where); extern void fscache_end_cookie_access(struct fscache_cookie *cookie, enum fscache_access_trace why); -extern void fscache_set_cookie_state(struct fscache_cookie *cookie, - enum fscache_cookie_state state); +extern void fscache_cookie_lookup_negative(struct fscache_cookie *cookie); +extern void fscache_caching_failed(struct fscache_cookie *cookie); + +/** + * fscache_cookie_state - Read the state of a cookie + * @cookie: The cookie to query + * + * Get the state of a cookie, imposing an ordering between the cookie contents + * and the state value. Paired with fscache_set_cookie_state(). + */ +static inline +enum fscache_cookie_state fscache_cookie_state(struct fscache_cookie *cookie) +{ + return smp_load_acquire(&cookie->state); +} /** * fscache_get_key - Get a pointer to the cookie key diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h index 1d576bd8112e..030c97bb9c8b 100644 --- a/include/trace/events/fscache.h +++ b/include/trace/events/fscache.h @@ -68,6 +68,8 @@ enum fscache_access_trace { fscache_access_acquire_volume_end, fscache_access_cache_pin, fscache_access_cache_unpin, + fscache_access_lookup_cookie_end, + fscache_access_lookup_cookie_end_failed, fscache_access_relinquish_volume, fscache_access_relinquish_volume_end, fscache_access_unlive, @@ -124,6 +126,8 @@ enum fscache_access_trace { EM(fscache_access_acquire_volume_end, "END acq_vol") \ EM(fscache_access_cache_pin, "PIN cache ") \ EM(fscache_access_cache_unpin, "UNPIN cache ") \ + EM(fscache_access_lookup_cookie_end, "END lookup ") \ + EM(fscache_access_lookup_cookie_end_failed,"END lookupf") \ EM(fscache_access_relinquish_volume, "BEGIN rlq_vol") \ EM(fscache_access_relinquish_volume_end,"END rlq_vol") \ E_(fscache_access_unlive, "END unlive ") -- Linux-cachefs mailing list Linux-cachefs@redhat.com https://listman.redhat.com/mailman/listinfo/linux-cachefs
