Slab outline

kern/slab.h

@@ -0,0 +1,121 @@
+#ifndef SLAB_H
+#define SLAB_H
+
+/*
+ * To understand this code, some prerequisite knowledge on memory and allocators are
+ * required. You should probably know what a free-list is, and maybe how to implement a bump allocator.
+ *
+ * See James Shackleford's excellent presentation on the Linux slab allocator:
+ *      https://www.youtube.com/watch?v=pFi-JKgoX-I
+ *
+ * In the Linux source tree, the slub implementation resides in:
+ *      mm/slub.c
+ *
+ * To see the slab caches on a linux system:
+ *      $ sudo cat /proc/slabinfo
+ * In order of columns:
+ *      name, active obj, total n obj, obj size, obj per slab, pages per slab
+ * For more info about how to decode this, see:
+ *      $ man 5 slabinfo
+ *
+ * The way kernel memory allocation works in modern unixes is essentially outlined by:
+ *      kmalloc() -> slub/slab/slob -> binary-buddy
+ *
+ * Where kmalloc redirects the requests to one of its appropriately sized slab caches,
+ * which in turn checks for an available cache, and if not found, allocates a new one
+ * using the binary-buddy allocator, which only allocates fixed size objects (usually pages)
+ */
+
+#include <stddef.h>
+#include <stdint.h>
+
+#define KERN_TOP_ADDR
+#define USER_TOP_ADDR
+
+#define POISON_FREE   0x6b
+#define POISON_END    0x6a
+#define POISON_IN_USE 0x5a
+
+#define RED_INACTIVE 0xbb
+#define RED_ACTIVE   0xcc
+
+/* Canonical addresses are addresses that point to unmapped memory (neither user nor kernel space) */
+#define IS_CANONICAL_ADDRESS(ptr) (ptr < KERN_TOP_ADDR && ptr > USER_TOP_ADDR)
+
+/* 'bin size', 'order' */
+#define SIZE_TO_KMALLOC_CACHE_INDEX(size) (0)
+struct kmem_cache *kmalloc_caches[10];
+
+/* GetFreePages_t: The type used for flags in various allocation related functions */
+typedef uint32_t gfp_t;
+
+/* Kmalloc manages an internal array of slab caches, sizes 8 bytes to 2^13 bytes */
+void *kzalloc(size_t size); // Same but zeroes memory
+void *kmalloc(size_t size) {
+    /* If the order (size) is too large to fit inside one of the slab caches, go directly to frame (page, buddy)
+     * allocator */
+    struct kmem_cache *suitable_cache = kmalloc_caches[SIZE_TO_KMALLOC_CACHE_INDEX(size)];
+    /* Do operations on kmem cache */
+    (void)suitable_cache;
+    return NULL;
+}
+
+/* For big allocations, not physically contiguous */
+void *vmalloc(size_t size);
+
+// SLAB_HWCACHE_ALIGN
+// SLAB_CACHE_DMA
+// SLAB_PANIC
+
+/* This struct exists only conceptually, as the poison is variable in length */
+/* The struct within each slab */
+struct kmem_free_obj {
+    /* FreePointer, pointer to the next free pointer in the slab, if null then full, essentially a linked list */
+    /* Poison, inserted to see if someone reads or writes to a free area, checked by comparison, these values are known
+     */
+    /* Pad, to keep things aligned*/
+};
+
+/* This struct exists only conceptually */
+struct kmem_allocated_obj {
+    char payload[128]; /* Sized for demonstration */
+    char red_zone[16]; /* Sized for demonstration */
+};
+
+/* Per cpu slab cache, to avoid worrying about locking */
+struct kmem_cache_cpu {
+    /* FreeList, points to the _first free object_, which will be equal to the Page pointer if the page is empty */
+    /* Page, the page allocated by buddy */
+    /* Page_amt, there may be multiple pages */
+};
+
+/* Essentially a list head for pages */
+struct kmem_page {
+    /* Page */
+    /* FirstFree */
+    /* Slabsize? */
+    /* Next */
+    /* Prev? */
+};
+
+/* Global (as in cross CPU) pointers to pages in various states */
+/* When kmem_cache_cpu->page gets full, it gets swapped into the full linked list */
+/* Operations on this struct requires locking */
+struct kmem_cache_node {
+    /* Partial, linked list of kmem_page, moved from full once something inside a full page is free'd */
+    /* Full, linked list of kmem_page */
+};
+
+struct kmem_cache {
+    const char             *name;
+    struct kmem_cache_cpu  *cpu_slab;
+    struct kmem_cache_node *node;
+};
+
+// KMEM_CACHE macro
+// kmem_cache_create(name, size, alignment, flags, constructor)
+// kmem_cache_destroy(*kmem_cache)
+// kmem_cache_alloc(*kmem_cache, flags (GFP_KERNEL|GFP_ATOMIC|GFP_ZERO))
+// kmem_cache_free(*kmem_cache, *obj) <- Optionally checks redzone
+
+#endif // SLAB_H