--- /dev/null
+/* Copyright (c) 2015-2018. The SimGrid Team. All rights reserved. */
+
+/* This program is free software; you can redistribute it and/or modify it
+ * under the terms of the license (GNU LGPL) which comes with this package. */
+
+#ifndef SIMGRID_MC_PAGESTORE_HPP
+#define SIMGRID_MC_PAGESTORE_HPP
+
+#include <cstdint>
+#include <vector>
+
+#include <unordered_map>
+#include <unordered_set>
+
+#include "src/mc/mc_forward.hpp"
+#include "src/mc/mc_mmu.hpp"
+
+#ifndef XBT_ALWAYS_INLINE
+#define XBT_ALWAYS_INLINE inline __attribute__((always_inline))
+#endif
+
+namespace simgrid {
+namespace mc {
+
+/** @brief Storage for snapshot memory pages
+ *
+ * The first (lower) layer of the per-page snapshot mechanism is a page store:
+ * its responsibility is to store immutable sharable reference-counted memory
+ * pages independently of the snapshotting logic. Snapshot management and
+ * representation 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.
+ *
+ */
+class PageStore {
+public: // Types
+ typedef std::uint64_t hash_type;
+
+private:
+ // Types
+ // 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 std::unordered_set<std::size_t> page_set_type;
+ typedef std::unordered_map<hash_type, page_set_type> pages_map_type;
+
+ // Fields:
+ /** First page */
+ void* memory_;
+ /** Number of available pages in virtual memory */
+ std::size_t capacity_;
+ /** Top of the used pages (index of the next available page) */
+ std::size_t top_index_;
+ /** Page reference count */
+ std::vector<std::uint64_t> page_counts_;
+ /** Index of available pages before the top */
+ std::vector<std::size_t> free_pages_;
+ /** Index from page hash to page index */
+ pages_map_type hash_index_;
+
+ // Methods
+ void resize(std::size_t size);
+ std::size_t alloc_page();
+ void remove_page(std::size_t pageno);
+
+public:
+ // Constructors
+ PageStore(PageStore const&) = delete;
+ PageStore& operator=(PageStore const&) = delete;
+ explicit PageStore(std::size_t size);
+ ~PageStore();
+
+ // Methods
+
+ /** @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(std::size_t pageno);
+
+ /** @brief Increment the refcount for a given page
+ *
+ * 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 */
+ std::size_t store_page(void* page);
+
+ /** @brief Get a page from its page number
+ *
+ * @param pageno Number of the memory page in the store
+ * @return Start of the page
+ */
+ const void* get_page(std::size_t pageno) const;
+
+ // Debug/test methods
+
+ /** @brief Get the number of references for a page */
+ std::size_t get_ref(std::size_t pageno);
+
+ /** @brief Get the number of used pages */
+ std::size_t size();
+
+ /** @brief Get the capacity of the page store
+ *
+ * The capacity is expanded by a system call (mremap).
+ * */
+ std::size_t capacity();
+};
+
+XBT_ALWAYS_INLINE void PageStore::unref_page(std::size_t pageno)
+{
+ if ((--this->page_counts_[pageno]) == 0)
+ this->remove_page(pageno);
+}
+
+XBT_ALWAYS_INLINE void PageStore::ref_page(size_t pageno)
+{
+ ++this->page_counts_[pageno];
+}
+
+XBT_ALWAYS_INLINE const void* PageStore::get_page(std::size_t pageno) const
+{
+ return (void*)simgrid::mc::mmu::join(pageno, (std::uintptr_t)this->memory_);
+}
+
+XBT_ALWAYS_INLINE std::size_t PageStore::get_ref(std::size_t pageno)
+{
+ return this->page_counts_[pageno];
+}
+
+XBT_ALWAYS_INLINE std::size_t PageStore::size()
+{
+ return this->top_index_ - this->free_pages_.size();
+}
+
+XBT_ALWAYS_INLINE std::size_t PageStore::capacity()
+{
+ return this->capacity_;
+}
+
+} // namespace mc
+} // namespace simgrid
+
+#endif
