xtc_slab.h

/*-
 * Copyright (c) 2026, The XTC Project
 * @stability stable
 * Use of this source code is governed by the ISC License.
 *
 * src/inc/xtc_slab.h
 *      libumem-inspired slab allocator with per-loop magazines.
 *      Foundation for fixed-size hot-path allocations across xtc.
 *      Replaces ad-hoc per-module pools (lockmgr entries, proc
 *      mailbox envelopes, channel slots, RCU retired nodes, timer
 *      nodes).
 *
 *      Design:
 *        - Each cache owns one or more "chunks" (large mmap regions),
 *          carved into per-object slots.  No per-object malloc.
 *        - Chunks are sub-divided into "slabs" (default 64 KiB each)
 *          tracked on free/partial/full lists.
 *        - Each loop has a magazine -- a small array of recently-freed
 *          object pointers -- for lock-free fast-path alloc/free.
 *          On magazine miss, the cache mutex is taken and the magazine
 *          is refilled from a partial slab.
 *        - On magazine overflow (free), the magazine spills back to
 *          the cache's free list under the mutex.
 *        - Constructor runs once per object lifetime (when first
 *          pulled out of the slab); destructor runs once at cache
 *          destroy or at slab reap.
 *        - Magazines flush on memory pressure; full slabs return to
 *          the OS via munmap.
 *
 *      Two storage modes:
 *        PROCESS_LOCAL -- chunks via mmap MAP_ANONYMOUS; objects are
 *          process-local pointers.
 *        SHARED_MEMORY -- chunks carved from a single user-supplied
 *          region (mmap MAP_SHARED of a tmpfs/posix-shm fd).  Object
 *          handles are offsets (xtc_slab_off_t, a base-relative
 *          offset) so a peer process mapping the same fd at a
 *          different VA can resolve them.  Use xtc_slab_resolve() and
 *          xtc_slab_offset() to convert.
 *
 *          Cross-process support: verified working.  Multiple processes
 *          can attach to the same shm region (via shm_open + mmap) and
 *          allocate AND free concurrently without collision.  The region
 *          contains a 64-byte header at offset 0 holding an atomic cursor
 *          (carves fresh slots) and a cross-process free list threaded
 *          through region-relative offsets (reclaims freed slots), so a
 *          slot freed by any attacher is reusable by any other -- memory
 *          is not bump-only and does not leak until destroy.  First
 *          attacher initializes the header; subsequent attachers verify
 *          magic and the slot stride, then share the cursor and free list.
 *          NOTE: the shm header (the first 64 bytes) is private to the
 *          allocator; do not stash inter-process scratch there.
 *
 *          Typical usage:
 *            1. Process A: shm_open + ftruncate + mmap(MAP_SHARED)
 *            2. Process A: xtc_slab_create with shm_base/shm_size
 *            3. Process B: shm_open + mmap(MAP_SHARED) same path
 *            4. Process B: xtc_slab_create with same shm_base/shm_size
 *            5. Either process can now alloc; the other can resolve
 *               the offset to access the object.
 *
 *      Debug flags (M11.5a):
 *        REDZONE   -- 16-byte guard before+after each object; checked
 *                    on free
 *        AUDIT     -- small ring of recent (alloc, free, who, when)
 *                    events per cache for postmortem
 *        BACKTRACE -- capture backtrace on alloc; combine with AUDIT
 *                    to identify leakers (via the __os_backtrace seam;
 *                    a no-op where no backend is available)
 *
 *      OOM policies:
 *        FAIL    -- return NULL, set XTC_E_RESOURCE in caller
 *        BACKOFF -- short usleep + retry once, then FAIL
 *        ABORT   -- call abort() (debug builds only; never in prod)
 *
 *      Memory-pressure: callers can register `xtc_slab_pressure_listen`
 *      on Linux (/proc/pressure/memory PSI).  When pressure is high,
 *      all caches drop their magazines and reap empty slabs.
 */
 
#ifndef XTC_SLAB_H
#define XTC_SLAB_H
 
#include "xtc_export.h"
 
#include <stddef.h>
#include <stdint.h>
 
#include "xtc.h"
#include "xtc_res.h"
#include "xtc_proc.h"
#include "xtc_loop.h"
 
typedef struct xtc_slab xtc_slab_t;
 
/* Offset handle for shared-memory caches (a base-relative offset). */
typedef int64_t xtc_slab_off_t;
#define XTC_SLAB_OFF_NONE  ((xtc_slab_off_t)-1)
 
/* Per-object lifecycle hooks. */
typedef int  (*xtc_slab_ctor_fn)(void *obj, void *user);
typedef void (*xtc_slab_dtor_fn)(void *obj, void *user);
 
/* Memory-pressure callback signature.  level: 0=normal 1=warning
 * 2=critical.  Caller may reap, drop caches, etc. */
typedef void (*xtc_slab_pressure_fn)(int level, void *user);
 
/* Opaque handle for a stoppable pressure listener (see
 * xtc_slab_pressure_listen_ex / xtc_slab_pressure_stop). */
typedef struct psi_listener xtc_slab_pressure_t;
 
/* Storage mode. */
typedef enum xtc_slab_mode {
        XTC_SLAB_PROCESS_LOCAL = 0,
        XTC_SLAB_SHARED_MEMORY = 1
} xtc_slab_mode_t;
 
/* OOM policy. */
typedef enum xtc_slab_oom_policy {
        XTC_SLAB_OOM_FAIL    = 0,    /* return NULL, increment rejects */
        XTC_SLAB_OOM_BACKOFF = 1,    /* brief usleep + retry once */
        XTC_SLAB_OOM_ABORT   = 2     /* call abort() (debug builds) */
} xtc_slab_oom_policy_t;
 
/* Debug flags (bit OR'd into opts.flags). */
#define XTC_SLAB_REDZONE    (1u << 0)
#define XTC_SLAB_AUDIT      (1u << 1)
#define XTC_SLAB_BACKTRACE  (1u << 2)
#define XTC_SLAB_NO_MAGAZINE (1u << 3)   /* always go through cache lock */
 
typedef struct xtc_slab_opts {
        size_t                 size;          /* sizeof(xtc_slab_opts_t) */
        const char            *name;          /* required, for stats/diag */
        size_t                 obj_size;      /* required */
        size_t                 align;          /* default cache-line, 64 */
        size_t                 chunk_size;     /* mmap unit; default 64 KiB */
        int                    magazine_size;  /* per-loop cache; default 16 */
        xtc_slab_ctor_fn       ctor;            /* optional */
        xtc_slab_dtor_fn       dtor;            /* optional */
        void                  *cb_user;
        unsigned               flags;
        xtc_slab_oom_policy_t  oom_policy;
        xtc_res_t             *res;             /* optional accounting */
 
        /* Shared-memory mode only: caller-supplied base + size of an
         * already-mapped region.  When set, the cache carves chunks
         * from this region instead of mmap'ing fresh ones.  Released
         * via shm_unlink when the cache is destroyed. */
        xtc_slab_mode_t        mode;
        void                  *shm_base;
        size_t                 shm_size;
} xtc_slab_opts_t;
 
#define XTC_SLAB_OPTS_DEFAULT { \
        .size = sizeof(xtc_slab_opts_t), \
        .name = "anon", \
        .obj_size = 0, \
        .align = 64, \
        .chunk_size = 64 * 1024, \
        .magazine_size = 16, \
        .ctor = NULL, .dtor = NULL, .cb_user = NULL, \
        .flags = 0, \
        .oom_policy = XTC_SLAB_OOM_FAIL, \
        .res = NULL, \
        .mode = XTC_SLAB_PROCESS_LOCAL, \
        .shm_base = NULL, \
        .shm_size = 0 \
}
 
typedef struct xtc_slab_stats {
        size_t   size;              /* sizeof(xtc_slab_stats_t): XTC_SLAB_STATS_INIT */
        uint64_t alloc_fast;        /* magazine hit */
        uint64_t alloc_slow;        /* magazine miss -> cache lock */
        uint64_t free_fast;
        uint64_t free_slow;
        uint64_t n_inuse;
        uint64_t n_free;            /* available in slabs */
        uint64_t n_chunks;
        uint64_t bytes_inuse;
        uint64_t bytes_total;
        uint64_t reaps;
        uint64_t oom_fails;
        uint64_t redzone_violations;
} xtc_slab_stats_t;
 
#define XTC_SLAB_STATS_INIT { .size = sizeof(xtc_slab_stats_t) }
 
/*
 * PUBLIC: int  xtc_slab_create __P((const xtc_slab_opts_t *, xtc_slab_t **));
 * PUBLIC: void xtc_slab_destroy __P((xtc_slab_t *));
 *
 * PUBLIC: void *xtc_slab_alloc __P((xtc_slab_t *));
 * PUBLIC: void  xtc_slab_free __P((xtc_slab_t *, void *));
 *
 * PUBLIC: int  xtc_slab_reap __P((xtc_slab_t *));
 * PUBLIC: int  xtc_slab_stat __P((const xtc_slab_t *, xtc_slab_stats_t *));
 *
 * PUBLIC: xtc_slab_off_t xtc_slab_offset __P((const xtc_slab_t *, const void *));
 * PUBLIC: void *xtc_slab_resolve __P((const xtc_slab_t *, xtc_slab_off_t));
 *
 * PUBLIC: int  xtc_slab_pressure_listen __P((const char *, xtc_slab_pressure_fn, void *));
 * PUBLIC: int  xtc_slab_pressure_listen_ex __P((const char *, xtc_slab_pressure_fn, void *, xtc_slab_pressure_t **));
 * PUBLIC: int  xtc_slab_pressure_stop __P((xtc_slab_pressure_t *));
 * PUBLIC: int  xtc_slab_reap_all __P((void));
 */
 
XTC_API int   xtc_slab_create(const xtc_slab_opts_t *opts, xtc_slab_t **out);
XTC_API void  xtc_slab_destroy(xtc_slab_t *slab);
 
XTC_API void *xtc_slab_alloc(xtc_slab_t *slab);
XTC_API void  xtc_slab_free(xtc_slab_t *slab, void *obj);
 
/* Drop all magazines + return empty slabs to the OS. */
XTC_API int   xtc_slab_reap(xtc_slab_t *slab);
 
XTC_API int   xtc_slab_stat(const xtc_slab_t *slab, xtc_slab_stats_t *out);
 
/* Shared-memory helpers -- convert between process-local pointer
 * and a stable offset.  In PROCESS_LOCAL mode these are still
 * meaningful but only valid within this process. */
XTC_API xtc_slab_off_t xtc_slab_offset(const xtc_slab_t *slab, const void *p);
XTC_API void          *xtc_slab_resolve(const xtc_slab_t *slab, xtc_slab_off_t off);
 
/* Install a process-wide memory-pressure listener.  On Linux, opens
 * /proc/pressure/memory in poll mode at the "some" trigger (>10 ms
 * stall in 1s); on platforms without PSI, returns XTC_E_NOSYS.  When
 * pressure fires, all registered slab caches receive a reap call.
 * `psi_path` is "/proc/pressure/memory" by default (NULL = use it).
 */
XTC_API int   xtc_slab_pressure_listen(const char *psi_path,
                                       xtc_slab_pressure_fn fn,
                                       void *user);
 
/* Like xtc_slab_pressure_listen, but returns an opaque handle so the
 * listener can be stopped (its background thread joined and its fds
 * closed) with xtc_slab_pressure_stop.  On platforms without PSI,
 * returns XTC_E_NOSYS and leaves *out NULL.  *out must be non-NULL. */
XTC_API int   xtc_slab_pressure_listen_ex(const char *psi_path,
                                          xtc_slab_pressure_fn fn,
                                          void *user,
                                          xtc_slab_pressure_t **out);
 
/* Stop a listener created by xtc_slab_pressure_listen_ex: signal its
 * thread, join it, close its fds, and free the handle.  Idempotent on
 * NULL (returns XTC_E_INVAL).  After this returns the handle is
 * invalid. */
XTC_API int   xtc_slab_pressure_stop(xtc_slab_pressure_t *handle);
 
/* Fan a reap across every cache currently registered.  Returns the
 * total number of objects reaped. */
XTC_API int   xtc_slab_reap_all(void);
 
/* Spawn an xtc_proc that calls xtc_slab_reap_all() every
 * `interval_ns` and logs the count via xtc_log.  The proc runs
 * forever; supervise it or rely on loop teardown to kill it.
 * Returns the proc's pid (XTC_PID_NONE on failure). */
int   xtc_slab_reaper_spawn(xtc_loop_t *loop, int64_t interval_ns,
                            xtc_pid_t *out_pid);
 
#endif /* XTC_SLAB_H */