#include <stdint.h>
+#ifdef __cplusplus
#include <vector>
+#include <boost/array.hpp>
#include <boost/utility.hpp>
#include <boost/unordered_map.hpp>
#include <boost/unordered_set.hpp>
+#endif
#include <xbt.h>
-#include "mc_private.h"
#include "mc_mmu.h"
+#include "mc_forward.h"
+
+#ifndef MC_PAGE_STORE_H
+#define MC_PAGE_STORE_H
+
+struct s_mc_pages_store;
-#ifndef MC_PAGE_SNAPSHOT_H
-#define MC_PAGE_SNAPSHOT_H
+#ifdef __cplusplus
-/** @brief Manager for snapshot pages
+/** @brief Storage for snapshot memory pages
+ *
+ * The first (lower) layer of the per-page snapshot mechanism is a page
+ * store: it's responsibility is to store immutable shareable
+ * reference-counted memory pages independently of the snapshoting
+ * logic. Snapshot management and representation, soft-dirty tracking is
+ * handled to an higher layer. READMORE
+ *
+ * Data structure:
+ *
+ * * A pointer (`memory_`) to a (currently anonymous) `mmap()`ed memory
+ * region holding the memory pages (the address of the first page).
+ *
+ * We want to keep this memory region aligned on the memory pages (so
+ * that we might be able to create non-linear memory mappings on those
+ * pages in the future) and be able to expand it without coyping the
+ * data (there will be a lot of pages here): we will be able to
+ * efficiently expand the memory mapping using `mremap()`, moving it
+ * to another virtual address if necessary.
+ *
+ * Because we will move this memory mapping on the virtual address
+ * space, only the index of the page will be stored in the snapshots
+ * and the page will always be looked up by going through `memory`:
+ *
+ * void* page = (char*) page_store->memory + page_index << pagebits;
+ *
+ * * The number of pages mapped in virtual memory (`capacity_`). Once all
+ * those pages are used, we need to expand the page store with
+ * `mremap()`.
+ *
+ * * A reference count for each memory page `page_counts_`. Each time a
+ * snapshot references a page, the counter is incremented. If a
+ * snapshot is freed, the reference count is decremented. When the
+ * reference count, of a page reaches 0 it is added to a list of available
+ * pages (`free_pages_`).
+ *
+ * * A list of free pages `free_pages_` which can be reused. This avoids having
+ * to scan the reference count list to find a free page.
+ *
+ * * When we are expanding the memory map we do not want to add thousand of page
+ * to the `free_pages_` list and remove them just afterwards. The `top_index_`
+ * field is an index after which all pages are free and are not in the `free_pages_`
+ * list.
+ *
+ * * When we are adding a page, we need to check if a page with the same
+ * content is already in the page store in order to reuse it. For this
+ * reason, we maintain an index (`hash_index_`) mapping the hash of a
+ * page to the list of page indices with this hash.
+ * We use a fast (non cryptographic) hash so there may be conflicts:
+ * we must be able to store multiple indices for the same hash.
*
- * Page management: the free pages are stored as a simple linked-list.
- * The number of the first page if stored in `free_pages`.
- * Each free page store the number of the next free page.
*/
struct s_mc_pages_store {
-private: // Types
+public: // Types
+#ifdef MC_PAGE_STORE_MD4
+ typedef boost::array<uint64_t,2> hash_type;
+#else
typedef uint64_t hash_type;
- typedef boost ::unordered_set<size_t> page_set_type;
+#endif
+private: // Types
+#ifdef MC_PAGE_STORE_MD4
+ // We are using a secure hash to identify a page.
+ // We assume there will not be any collision: we need to map a hash
+ // to a single page index.
+ typedef boost::unordered_map<hash_type, size_t> pages_map_type;
+#else
+ // We are using a cheap hash to index a page.
+ // We should expect collision and we need to associate multiple page indices
+ // to the same hash.
+ typedef boost::unordered_set<size_t> page_set_type;
typedef boost::unordered_map<hash_type, page_set_type> pages_map_type;
+#endif
private: // Fields:
- /** First page */
+ /** First page
+ *
+ * mc_page_store_get_page expects that this is the first field.
+ * */
void* memory_;
/** Number of available pages in virtual memory */
size_t capacity_;
public: // Methods
- /** @brief Decrement the refcount for a given page */
- void unref_page(size_t pageno) {
- if ((--this->page_counts_[pageno]) == 0) {
- this->remove_page(pageno);
- }
- }
-
- /** @brief Increment the refcount for a given page */
- void ref_page(size_t pageno) {
- ++this->page_counts_[pageno];
- }
+ /** @brief Decrement the reference count for a given page
+ *
+ * Decrement the reference count of this page. Used when a snapshot is
+ * destroyed.
+ *
+ * If the reference count reaches zero, the page is recycled:
+ * it is added to the `free_pages_` list and removed from the `hash_index_`.
+ *
+ * */
+ void unref_page(size_t pageno);
- /** @brief Store a page
+ /** @brief Increment the refcount for a given page
*
- * Either allocate a new page in the store or reuse
- * a shared page if is is already in the page store.
- */
+ * This method used to increase a reference count of a page if we know
+ * that the content of a page is the same as a page already in the page
+ * store.
+ *
+ * This will be the case if a page if soft clean: we know that is has not
+ * changed since the previous cnapshot/restoration and we can avoid
+ * hashing the page, comparing byte-per-byte to candidates.
+ * */
+ void ref_page(size_t pageno);
+
+ /** @brief Store a page in the page store */
size_t store_page(void* page);
/** @brief Get a page from its page number
* @param Number of the memory page in the store
* @return Start of the page
*/
- const void* get_page(size_t pageno) const {
- return mc_page_from_number(this->memory_, pageno);
- }
+ const void* get_page(size_t pageno) const;
public: // Debug/test methods
/** @brief Get the number of references for a page */
- size_t get_ref(size_t pageno) {
- return this->page_counts_[pageno];
- }
+ size_t get_ref(size_t pageno);
/** @brief Get the number of used pages */
- size_t size() {
- return this->top_index_ - this->free_pages_.size();
- }
+ size_t size();
/** @brief Get the capacity of the page store
*
* The capacity is expanded by a system call (mremap).
* */
- size_t capacity() {
- return this->capacity_;
- }
+ size_t capacity();
};
+inline __attribute__((always_inline))
+void s_mc_pages_store::unref_page(size_t pageno) {
+ if ((--this->page_counts_[pageno]) == 0) {
+ this->remove_page(pageno);
+ }
+}
+
+inline __attribute__((always_inline))
+void s_mc_pages_store::ref_page(size_t pageno) {
+ ++this->page_counts_[pageno];
+}
+
+inline __attribute__((always_inline))
+const void* s_mc_pages_store::get_page(size_t pageno) const {
+ return mc_page_from_number(this->memory_, pageno);
+}
+
+inline __attribute__((always_inline))
+size_t s_mc_pages_store::get_ref(size_t pageno) {
+ return this->page_counts_[pageno];
+}
+
+inline __attribute__((always_inline))
+size_t s_mc_pages_store::size() {
+ return this->top_index_ - this->free_pages_.size();
+}
+
+inline __attribute__((always_inline))
+size_t s_mc_pages_store::capacity() {
+ return this->capacity_;
+}
+
#endif
+SG_BEGIN_DECL()
+
+mc_pages_store_t mc_pages_store_new(void);
+void mc_pages_store_delete(mc_pages_store_t store);
+
+/**
+ */
+static inline __attribute__((always_inline))
+const void* mc_page_store_get_page(mc_pages_store_t page_store, size_t pageno)
+{
+ // This is page_store->memory_:
+ void* memory = *(void**)page_store;
+ return mc_page_from_number(memory, pageno);
+}
+
+SG_END_DECL()
+
+#endif
