1 files changed, 624 insertions, 0 deletions

diff --git a/drivers/md/dm-vdo/indexer/sparse-cache.c b/drivers/md/dm-vdo/indexer/sparse-cache.c
new file mode 100644
index 000000000000..28920167827c
--- /dev/null
+++ b/drivers/md/dm-vdo/indexer/sparse-cache.c
@@ -0,0 +1,624 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2023 Red Hat
+ */
+
+#include "sparse-cache.h"
+
+#include <linux/cache.h>
+#include <linux/delay.h>
+#include <linux/dm-bufio.h>
+
+#include "logger.h"
+#include "memory-alloc.h"
+#include "permassert.h"
+
+#include "chapter-index.h"
+#include "config.h"
+#include "index.h"
+
+/*
+ * Since the cache is small, it is implemented as a simple array of cache entries. Searching for a
+ * specific virtual chapter is implemented as a linear search. The cache replacement policy is
+ * least-recently-used (LRU). Again, the small size of the cache allows the LRU order to be
+ * maintained by shifting entries in an array list.
+ *
+ * Changing the contents of the cache requires the coordinated participation of all zone threads
+ * via the careful use of barrier messages sent to all the index zones by the triage queue worker
+ * thread. The critical invariant for coordination is that the cache membership must not change
+ * between updates, so that all calls to uds_sparse_cache_contains() from the zone threads must all
+ * receive the same results for every virtual chapter number. To ensure that critical invariant,
+ * state changes such as "that virtual chapter is no longer in the volume" and "skip searching that
+ * chapter because it has had too many cache misses" are represented separately from the cache
+ * membership information (the virtual chapter number).
+ *
+ * As a result of this invariant, we have the guarantee that every zone thread will call
+ * uds_update_sparse_cache() once and exactly once to request a chapter that is not in the cache,
+ * and the serialization of the barrier requests from the triage queue ensures they will all
+ * request the same chapter number. This means the only synchronization we need can be provided by
+ * a pair of thread barriers used only in the uds_update_sparse_cache() call, providing a critical
+ * section where a single zone thread can drive the cache update while all the other zone threads
+ * are known to be blocked, waiting in the second barrier. Outside that critical section, all the
+ * zone threads implicitly hold a shared lock. Inside it, the thread for zone zero holds an
+ * exclusive lock. No other threads may access or modify the cache entries.
+ *
+ * Chapter statistics must only be modified by a single thread, which is also the zone zero thread.
+ * All fields that might be frequently updated by that thread are kept in separate cache-aligned
+ * structures so they will not cause cache contention via "false sharing" with the fields that are
+ * frequently accessed by all of the zone threads.
+ *
+ * The LRU order is managed independently by each zone thread, and each zone uses its own list for
+ * searching and cache membership queries. The zone zero list is used to decide which chapter to
+ * evict when the cache is updated, and its search list is copied to the other threads at that
+ * time.
+ *
+ * The virtual chapter number field of the cache entry is the single field indicating whether a
+ * chapter is a member of the cache or not. The value NO_CHAPTER is used to represent a null or
+ * undefined chapter number. When present in the virtual chapter number field of a
+ * cached_chapter_index, it indicates that the cache entry is dead, and all the other fields of
+ * that entry (other than immutable pointers to cache memory) are undefined and irrelevant. Any
+ * cache entry that is not marked as dead is fully defined and a member of the cache, and
+ * uds_sparse_cache_contains() will always return true for any virtual chapter number that appears
+ * in any of the cache entries.
+ *
+ * A chapter index that is a member of the cache may be excluded from searches between calls to
+ * uds_update_sparse_cache() in two different ways. First, when a chapter falls off the end of the
+ * volume, its virtual chapter number will be less that the oldest virtual chapter number. Since
+ * that chapter is no longer part of the volume, there's no point in continuing to search that
+ * chapter index. Once invalidated, that virtual chapter will still be considered a member of the
+ * cache, but it will no longer be searched for matching names.
+ *
+ * The second mechanism is a heuristic based on keeping track of the number of consecutive search
+ * misses in a given chapter index. Once that count exceeds a threshold, the skip_search flag will
+ * be set to true, causing the chapter to be skipped when searching the entire cache, but still
+ * allowing it to be found when searching for a hook in that specific chapter. Finding a hook will
+ * clear the skip_search flag, once again allowing the non-hook searches to use that cache entry.
+ * Again, regardless of the state of the skip_search flag, the virtual chapter must still
+ * considered to be a member of the cache for uds_sparse_cache_contains().
+ */
+
+#define SKIP_SEARCH_THRESHOLD 20000
+#define ZONE_ZERO 0
+
+/*
+ * These counters are essentially fields of the struct cached_chapter_index, but are segregated
+ * into this structure because they are frequently modified. They are grouped and aligned to keep
+ * them on different cache lines from the chapter fields that are accessed far more often than they
+ * are updated.
+ */
+struct __aligned(L1_CACHE_BYTES) cached_index_counters {
+	u64 consecutive_misses;
+};
+
+struct __aligned(L1_CACHE_BYTES) cached_chapter_index {
+	/*
+	 * The virtual chapter number of the cached chapter index. NO_CHAPTER means this cache
+	 * entry is unused. This field must only be modified in the critical section in
+	 * uds_update_sparse_cache().
+	 */
+	u64 virtual_chapter;
+
+	u32 index_pages_count;
+
+	/*
+	 * These pointers are immutable during the life of the cache. The contents of the arrays
+	 * change when the cache entry is replaced.
+	 */
+	struct delta_index_page *index_pages;
+	struct dm_buffer **page_buffers;
+
+	/*
+	 * If set, skip the chapter when searching the entire cache. This flag is just a
+	 * performance optimization. This flag is mutable between cache updates, but it rarely
+	 * changes and is frequently accessed, so it groups with the immutable fields.
+	 */
+	bool skip_search;
+
+	/*
+	 * The cache-aligned counters change often and are placed at the end of the structure to
+	 * prevent false sharing with the more stable fields above.
+	 */
+	struct cached_index_counters counters;
+};
+
+/*
+ * A search_list represents an ordering of the sparse chapter index cache entry array, from most
+ * recently accessed to least recently accessed, which is the order in which the indexes should be
+ * searched and the reverse order in which they should be evicted from the cache.
+ *
+ * Cache entries that are dead or empty are kept at the end of the list, avoiding the need to even
+ * iterate over them to search, and ensuring that dead entries are replaced before any live entries
+ * are evicted.
+ *
+ * The search list is instantiated for each zone thread, avoiding any need for synchronization. The
+ * structure is allocated on a cache boundary to avoid false sharing of memory cache lines between
+ * zone threads.
+ */
+struct search_list {
+	u8 capacity;
+	u8 first_dead_entry;
+	struct cached_chapter_index *entries[];
+};
+
+struct threads_barrier {
+	/* Lock for this barrier object */
+	struct semaphore lock;
+	/* Semaphore for threads waiting at this barrier */
+	struct semaphore wait;
+	/* Number of threads which have arrived */
+	int arrived;
+	/* Total number of threads using this barrier */
+	int thread_count;
+};
+
+struct sparse_cache {
+	const struct index_geometry *geometry;
+	unsigned int capacity;
+	unsigned int zone_count;
+
+	unsigned int skip_threshold;
+	struct search_list *search_lists[MAX_ZONES];
+	struct cached_chapter_index **scratch_entries;
+
+	struct threads_barrier begin_update_barrier;
+	struct threads_barrier end_update_barrier;
+
+	struct cached_chapter_index chapters[];
+};
+
+static void initialize_threads_barrier(struct threads_barrier *barrier,
+				       unsigned int thread_count)
+{
+	sema_init(&barrier->lock, 1);
+	barrier->arrived = 0;
+	barrier->thread_count = thread_count;
+	sema_init(&barrier->wait, 0);
+}
+
+static inline void __down(struct semaphore *semaphore)
+{
+	/*
+	 * Do not use down(semaphore). Instead use down_interruptible so that
+	 * we do not get 120 second stall messages in kern.log.
+	 */
+	while (down_interruptible(semaphore) != 0) {
+		/*
+		 * If we're called from a user-mode process (e.g., "dmsetup
+		 * remove") while waiting for an operation that may take a
+		 * while (e.g., UDS index save), and a signal is sent (SIGINT,
+		 * SIGUSR2), then down_interruptible will not block. If that
+		 * happens, sleep briefly to avoid keeping the CPU locked up in
+		 * this loop. We could just call cond_resched, but then we'd
+		 * still keep consuming CPU time slices and swamp other threads
+		 * trying to do computational work.
+		 */
+		fsleep(1000);
+	}
+}
+
+static void enter_threads_barrier(struct threads_barrier *barrier)
+{
+	__down(&barrier->lock);
+	if (++barrier->arrived == barrier->thread_count) {
+		/* last thread */
+		int i;
+
+		for (i = 1; i < barrier->thread_count; i++)
+			up(&barrier->wait);
+
+		barrier->arrived = 0;
+		up(&barrier->lock);
+	} else {
+		up(&barrier->lock);
+		__down(&barrier->wait);
+	}
+}
+
+static int __must_check initialize_cached_chapter_index(struct cached_chapter_index *chapter,
+							const struct index_geometry *geometry)
+{
+	int result;
+
+	chapter->virtual_chapter = NO_CHAPTER;
+	chapter->index_pages_count = geometry->index_pages_per_chapter;
+
+	result = vdo_allocate(chapter->index_pages_count, struct delta_index_page,
+			      __func__, &chapter->index_pages);
+	if (result != VDO_SUCCESS)
+		return result;
+
+	return vdo_allocate(chapter->index_pages_count, struct dm_buffer *,
+			    "sparse index volume pages", &chapter->page_buffers);
+}
+
+static int __must_check make_search_list(struct sparse_cache *cache,
+					 struct search_list **list_ptr)
+{
+	struct search_list *list;
+	unsigned int bytes;
+	u8 i;
+	int result;
+
+	bytes = (sizeof(struct search_list) +
+		 (cache->capacity * sizeof(struct cached_chapter_index *)));
+	result = vdo_allocate_cache_aligned(bytes, "search list", &list);
+	if (result != VDO_SUCCESS)
+		return result;
+
+	list->capacity = cache->capacity;
+	list->first_dead_entry = 0;
+
+	for (i = 0; i < list->capacity; i++)
+		list->entries[i] = &cache->chapters[i];
+
+	*list_ptr = list;
+	return UDS_SUCCESS;
+}
+
+int uds_make_sparse_cache(const struct index_geometry *geometry, unsigned int capacity,
+			  unsigned int zone_count, struct sparse_cache **cache_ptr)
+{
+	int result;
+	unsigned int i;
+	struct sparse_cache *cache;
+	unsigned int bytes;
+
+	bytes = (sizeof(struct sparse_cache) + (capacity * sizeof(struct cached_chapter_index)));
+	result = vdo_allocate_cache_aligned(bytes, "sparse cache", &cache);
+	if (result != VDO_SUCCESS)
+		return result;
+
+	cache->geometry = geometry;
+	cache->capacity = capacity;
+	cache->zone_count = zone_count;
+
+	/*
+	 * Scale down the skip threshold since the cache only counts cache misses in zone zero, but
+	 * requests are being handled in all zones.
+	 */
+	cache->skip_threshold = (SKIP_SEARCH_THRESHOLD / zone_count);
+
+	initialize_threads_barrier(&cache->begin_update_barrier, zone_count);
+	initialize_threads_barrier(&cache->end_update_barrier, zone_count);
+
+	for (i = 0; i < capacity; i++) {
+		result = initialize_cached_chapter_index(&cache->chapters[i], geometry);
+		if (result != UDS_SUCCESS)
+			goto out;
+	}
+
+	for (i = 0; i < zone_count; i++) {
+		result = make_search_list(cache, &cache->search_lists[i]);
+		if (result != UDS_SUCCESS)
+			goto out;
+	}
+
+	/* purge_search_list() needs some temporary lists for sorting. */
+	result = vdo_allocate(capacity * 2, struct cached_chapter_index *,
+			      "scratch entries", &cache->scratch_entries);
+	if (result != VDO_SUCCESS)
+		goto out;
+
+	*cache_ptr = cache;
+	return UDS_SUCCESS;
+out:
+	uds_free_sparse_cache(cache);
+	return result;
+}
+
+static inline void set_skip_search(struct cached_chapter_index *chapter,
+				   bool skip_search)
+{
+	/* Check before setting to reduce cache line contention. */
+	if (READ_ONCE(chapter->skip_search) != skip_search)
+		WRITE_ONCE(chapter->skip_search, skip_search);
+}
+
+static void score_search_hit(struct cached_chapter_index *chapter)
+{
+	chapter->counters.consecutive_misses = 0;
+	set_skip_search(chapter, false);
+}
+
+static void score_search_miss(struct sparse_cache *cache,
+			      struct cached_chapter_index *chapter)
+{
+	chapter->counters.consecutive_misses++;
+	if (chapter->counters.consecutive_misses > cache->skip_threshold)
+		set_skip_search(chapter, true);
+}
+
+static void release_cached_chapter_index(struct cached_chapter_index *chapter)
+{
+	unsigned int i;
+
+	chapter->virtual_chapter = NO_CHAPTER;
+	if (chapter->page_buffers == NULL)
+		return;
+
+	for (i = 0; i < chapter->index_pages_count; i++) {
+		if (chapter->page_buffers[i] != NULL)
+			dm_bufio_release(vdo_forget(chapter->page_buffers[i]));
+	}
+}
+
+void uds_free_sparse_cache(struct sparse_cache *cache)
+{
+	unsigned int i;
+
+	if (cache == NULL)
+		return;
+
+	vdo_free(cache->scratch_entries);
+
+	for (i = 0; i < cache->zone_count; i++)
+		vdo_free(cache->search_lists[i]);
+
+	for (i = 0; i < cache->capacity; i++) {
+		release_cached_chapter_index(&cache->chapters[i]);
+		vdo_free(cache->chapters[i].index_pages);
+		vdo_free(cache->chapters[i].page_buffers);
+	}
+
+	vdo_free(cache);
+}
+
+/*
+ * Take the indicated element of the search list and move it to the start, pushing the pointers
+ * previously before it back down the list.
+ */
+static inline void set_newest_entry(struct search_list *search_list, u8 index)
+{
+	struct cached_chapter_index *newest;
+
+	if (index > 0) {
+		newest = search_list->entries[index];
+		memmove(&search_list->entries[1], &search_list->entries[0],
+			index * sizeof(struct cached_chapter_index *));
+		search_list->entries[0] = newest;
+	}
+
+	/*
+	 * This function may have moved a dead chapter to the front of the list for reuse, in which
+	 * case the set of dead chapters becomes smaller.
+	 */
+	if (search_list->first_dead_entry <= index)
+		search_list->first_dead_entry++;
+}
+
+bool uds_sparse_cache_contains(struct sparse_cache *cache, u64 virtual_chapter,
+			       unsigned int zone_number)
+{
+	struct search_list *search_list;
+	struct cached_chapter_index *chapter;
+	u8 i;
+
+	/*
+	 * The correctness of the barriers depends on the invariant that between calls to
+	 * uds_update_sparse_cache(), the answers this function returns must never vary: the result
+	 * for a given chapter must be identical across zones. That invariant must be maintained
+	 * even if the chapter falls off the end of the volume, or if searching it is disabled
+	 * because of too many search misses.
+	 */
+	search_list = cache->search_lists[zone_number];
+	for (i = 0; i < search_list->first_dead_entry; i++) {
+		chapter = search_list->entries[i];
+
+		if (virtual_chapter == chapter->virtual_chapter) {
+			if (zone_number == ZONE_ZERO)
+				score_search_hit(chapter);
+
+			set_newest_entry(search_list, i);
+			return true;
+		}
+	}
+
+	return false;
+}
+
+/*
+ * Re-sort cache entries into three sets (active, skippable, and dead) while maintaining the LRU
+ * ordering that already existed. This operation must only be called during the critical section in
+ * uds_update_sparse_cache().
+ */
+static void purge_search_list(struct search_list *search_list,
+			      struct sparse_cache *cache, u64 oldest_virtual_chapter)
+{
+	struct cached_chapter_index **entries;
+	struct cached_chapter_index **skipped;
+	struct cached_chapter_index **dead;
+	struct cached_chapter_index *chapter;
+	unsigned int next_alive = 0;
+	unsigned int next_skipped = 0;
+	unsigned int next_dead = 0;
+	unsigned int i;
+
+	entries = &search_list->entries[0];
+	skipped = &cache->scratch_entries[0];
+	dead = &cache->scratch_entries[search_list->capacity];
+
+	for (i = 0; i < search_list->first_dead_entry; i++) {
+		chapter = search_list->entries[i];
+		if ((chapter->virtual_chapter < oldest_virtual_chapter) ||
+		    (chapter->virtual_chapter == NO_CHAPTER))
+			dead[next_dead++] = chapter;
+		else if (chapter->skip_search)
+			skipped[next_skipped++] = chapter;
+		else
+			entries[next_alive++] = chapter;
+	}
+
+	memcpy(&entries[next_alive], skipped,
+	       next_skipped * sizeof(struct cached_chapter_index *));
+	memcpy(&entries[next_alive + next_skipped], dead,
+	       next_dead * sizeof(struct cached_chapter_index *));
+	search_list->first_dead_entry = next_alive + next_skipped;
+}
+
+static int __must_check cache_chapter_index(struct cached_chapter_index *chapter,
+					    u64 virtual_chapter,
+					    const struct volume *volume)
+{
+	int result;
+
+	release_cached_chapter_index(chapter);
+
+	result = uds_read_chapter_index_from_volume(volume, virtual_chapter,
+						    chapter->page_buffers,
+						    chapter->index_pages);
+	if (result != UDS_SUCCESS)
+		return result;
+
+	chapter->counters.consecutive_misses = 0;
+	chapter->virtual_chapter = virtual_chapter;
+	chapter->skip_search = false;
+
+	return UDS_SUCCESS;
+}
+
+static inline void copy_search_list(const struct search_list *source,
+				    struct search_list *target)
+{
+	*target = *source;
+	memcpy(target->entries, source->entries,
+	       source->capacity * sizeof(struct cached_chapter_index *));
+}
+
+/*
+ * Update the sparse cache to contain a chapter index. This function must be called by all the zone
+ * threads with the same chapter number to correctly enter the thread barriers used to synchronize
+ * the cache updates.
+ */
+int uds_update_sparse_cache(struct index_zone *zone, u64 virtual_chapter)
+{
+	int result = UDS_SUCCESS;
+	const struct uds_index *index = zone->index;
+	struct sparse_cache *cache = index->volume->sparse_cache;
+
+	if (uds_sparse_cache_contains(cache, virtual_chapter, zone->id))
+		return UDS_SUCCESS;
+
+	/*
+	 * Wait for every zone thread to reach its corresponding barrier request and invoke this
+	 * function before starting to modify the cache.
+	 */
+	enter_threads_barrier(&cache->begin_update_barrier);
+
+	/*
+	 * This is the start of the critical section: the zone zero thread is captain, effectively
+	 * holding an exclusive lock on the sparse cache. All the other zone threads must do
+	 * nothing between the two barriers. They will wait at the end_update_barrier again for the
+	 * captain to finish the update.
+	 */
+
+	if (zone->id == ZONE_ZERO) {
+		unsigned int z;
+		struct search_list *list = cache->search_lists[ZONE_ZERO];
+
+		purge_search_list(list, cache, zone->oldest_virtual_chapter);
+
+		if (virtual_chapter >= index->oldest_virtual_chapter) {
+			set_newest_entry(list, list->capacity - 1);
+			result = cache_chapter_index(list->entries[0], virtual_chapter,
+						     index->volume);
+		}
+
+		for (z = 1; z < cache->zone_count; z++)
+			copy_search_list(list, cache->search_lists[z]);
+	}
+
+	/*
+	 * This is the end of the critical section. All cache invariants must have been restored.
+	 */
+	enter_threads_barrier(&cache->end_update_barrier);
+	return result;
+}
+
+void uds_invalidate_sparse_cache(struct sparse_cache *cache)
+{
+	unsigned int i;
+
+	for (i = 0; i < cache->capacity; i++)
+		release_cached_chapter_index(&cache->chapters[i]);
+}
+
+static inline bool should_skip_chapter(struct cached_chapter_index *chapter,
+				       u64 oldest_chapter, u64 requested_chapter)
+{
+	if ((chapter->virtual_chapter == NO_CHAPTER) ||
+	    (chapter->virtual_chapter < oldest_chapter))
+		return true;
+
+	if (requested_chapter != NO_CHAPTER)
+		return requested_chapter != chapter->virtual_chapter;
+	else
+		return READ_ONCE(chapter->skip_search);
+}
+
+static int __must_check search_cached_chapter_index(struct cached_chapter_index *chapter,
+						    const struct index_geometry *geometry,
+						    const struct index_page_map *index_page_map,
+						    const struct uds_record_name *name,
+						    u16 *record_page_ptr)
+{
+	u32 physical_chapter =
+		uds_map_to_physical_chapter(geometry, chapter->virtual_chapter);
+	u32 index_page_number =
+		uds_find_index_page_number(index_page_map, name, physical_chapter);
+	struct delta_index_page *index_page =
+		&chapter->index_pages[index_page_number];
+
+	return uds_search_chapter_index_page(index_page, geometry, name,
+					     record_page_ptr);
+}
+
+int uds_search_sparse_cache(struct index_zone *zone, const struct uds_record_name *name,
+			    u64 *virtual_chapter_ptr, u16 *record_page_ptr)
+{
+	int result;
+	struct volume *volume = zone->index->volume;
+	struct sparse_cache *cache = volume->sparse_cache;
+	struct cached_chapter_index *chapter;
+	struct search_list *search_list;
+	u8 i;
+	/* Search the entire cache unless a specific chapter was requested. */
+	bool search_one = (*virtual_chapter_ptr != NO_CHAPTER);
+
+	*record_page_ptr = NO_CHAPTER_INDEX_ENTRY;
+	search_list = cache->search_lists[zone->id];
+	for (i = 0; i < search_list->first_dead_entry; i++) {
+		chapter = search_list->entries[i];
+
+		if (should_skip_chapter(chapter, zone->oldest_virtual_chapter,
+					*virtual_chapter_ptr))
+			continue;
+
+		result = search_cached_chapter_index(chapter, cache->geometry,
+						     volume->index_page_map, name,
+						     record_page_ptr);
+		if (result != UDS_SUCCESS)
+			return result;
+
+		if (*record_page_ptr != NO_CHAPTER_INDEX_ENTRY) {
+			/*
+			 * In theory, this might be a false match while a true match exists in
+			 * another chapter, but that's a very rare case and not worth the extra
+			 * search complexity.
+			 */
+			set_newest_entry(search_list, i);
+			if (zone->id == ZONE_ZERO)
+				score_search_hit(chapter);
+
+			*virtual_chapter_ptr = chapter->virtual_chapter;
+			return UDS_SUCCESS;
+		}
+
+		if (zone->id == ZONE_ZERO)
+			score_search_miss(cache, chapter);
+
+		if (search_one)
+			break;
+	}
+
+	return UDS_SUCCESS;
+}