Isc Mem Allocator

BIND 9 allocator wrapper (isc_mem / isc_mempool)

Source of truth: lib/isc/include/isc/mem.h (public API), lib/isc/mem.c (implementation), lib/isc/mem_p.h (private init/shutdown), lib/isc/jemalloc_shim.h (non-jemalloc fallback).

Big picture

isc_mem_t (an "mctx") is a thin, reference-counted accounting wrapper over jemalloc's non-standard API: mallocx / sdallocx / rallocx / sallocx. On systems without jemalloc, jemalloc_shim.h emulates those on top of malloc, recovering sizes from malloc_usable_size() / malloc_size(), or as a last resort by prepending a size_info header to every allocation.

Key consequences:

• Allocation never fails. OOM calls oom(), which writes a signal-safe report + backtrace to stderr and abort()s. Never write if (ptr == NULL) after an isc_mem_* call; there is no NULL return.
• Deallocation is size-hinted (sdallocx). Freeing with the wrong size is undefined behavior inside jemalloc, not just a stats bug.
• All contexts share jemalloc arenas (jemalloc_flags is currently always 0), so an mctx is an accounting domain, not a heap: it tracks who owes what, it does not partition memory.
• A global default context isc_g_mctx always exists (created by the library constructor isc__lib_initialize() → isc__mem_initialize() in lib/isc/lib.c). Use it only when nothing more specific fits.

The two allocation families — never mix them

Rules:

• Memory from isc_mem_get must be returned with isc_mem_put using the same mctx and the same size. Memory from isc_mem_allocate must go back via isc_mem_free. Crossing the families corrupts the per-context inuse counter (put charges the requested size, free charges the sallocx size), which detonates later as INSIST(isc_mem_inuse(ctx) == 0) when the context is destroyed. The header's ISC_ATTR_MALLOC_DEALLOCATOR_IDX attributes make compilers warn about some mismatches — treat those warnings as errors.
• Prefer the sized family whenever the caller knows the size (the common case: isc_mem_get(mctx, sizeof(*obj))). Use allocate/free only for variable-length data whose size is inconvenient to carry around.
• The c-prefixed variants are calloc-alikes: ISC_CHECKED_MUL(n, size) (overflow-fatal multiply) plus ISC__MEM_ZERO. ISC__MEM_ZERO is verbatim jemalloc's MALLOCX_ZERO (0x40), RUNTIME_CHECKed at startup. Free a cget'd array with isc_mem_cput(mctx, p, n, size) so the sizes match.
• Size 0 is legal and symmetric: both get and put bump 0 to sizeof(void *) (ADJUST_ZERO_ALLOCATION_SIZE).

The put/free macros NULL the pointer

isc_mem_put, isc_mem_cput, isc_mem_free, isc_mem_putanddetach, and isc_mempool_put are statement macros that end with (p) = NULL;. So:

• The pointer argument must be an assignable lvalue and must not have side effects (macro arguments are expanded more than once — never isc_mem_put(mctx, arr[i++], size)).
• After the call the variable is NULL — code relying on the old pointer value afterwards is a bug even though the memory "was just there".

isc_mem_putanddetach

For objects that hold a reference to their own mctx:

isc_mem_putanddetach(&obj->mctx, obj, sizeof(*obj));

frees obj and then drops the mctx reference in the safe order (equivalent to attach-to-local / detach-member / put / detach-local). This is the standard destructor idiom; use it instead of an open-coded put + detach.

Context lifecycle

• isc_mem_create("name", &mctx) — name is mandatory, copied, shows up in stats channel output and leak dumps. (The create macro also re-assigns isc__mem_malloc = mallocx; that is a deliberate link-order hack for jemalloc — see jemalloc issue #2566 — don't "clean it up".)
• Refcounted via ISC_REFCOUNT_IMPL: isc_mem_attach(src, &dst), isc_mem_detach(&mctx) (NULLs the pointer), isc_mem_ref/unref. The last detach destroys the context.
• Destruction asserts the books balance: INSIST(isc_mem_inuse(ctx) == 0) and that no pools remain. isc_mem_setdestroycheck(mctx, false) disables the leak check — almost never the right fix; a failing inuse INSIST means a leak or a family/size mismatch.
• isc_mem_checkdestroyed(stderr) (called by named/tests at shutdown) arms a library-shutdown check that every context was destroyed; it hits UNREACHABLE() otherwise, after dumping live contexts when debugging is on.
• isc__mem_shutdown() runs rcu_barrier() before checking, so RCU-deferred frees (call_rcu) are flushed first — deferred frees still count as live until the grace period runs.

Statistics and accounting internals

• inuse is striped per thread id: stat_s[ISC_TID_MAX + 1] cacheline-padded slots; ctx->stat = &stat_s[1] so isc_tid() == -1 (ISC_TID_UNKNOWN, i.e. threads outside the loopmgr) indexes stat[-1] legally. Updates are relaxed atomics on the caller's own stripe.
• A stripe can go negative (thread A frees what thread B allocated) — that's why stripes are signed. Only the sum (isc_mem_inuse(), which walks -1..isc_tid_count()) is meaningful.
• isc_mem_inuse() is O(threads) and iterates all stripes — fine for water-mark checks, don't put it in per-packet hot paths gratuitously.

Water marks / overmem

• isc_mem_setwater(mctx, hiwater, lowater) (0,0 or isc_mem_clearwater() disables). Used by the resolver/cache to bound cache memory.
• isc_mem_isovermem() is probabilistic: false below lowater, true above hiwater, and in between returns true with probability ramping linearly 0→1 (8-bit resolution, isc_random8()). This deliberately spreads cache cleaning over many inserts instead of a thundering herd at the mark — do not "fix" the randomness, and don't expect two consecutive calls to agree.

Returning memory to the OS

• Each thread counts bytes it frees (freed_bytes, thread-local); every 16 MiB it triggers mem_purge(): jemalloc arena.<all>.decay (or glibc malloc_trim(0)), rate-limited via CAS on last_purge to once per second globally.
• Init-time jemalloc tuning: background_thread = true, dirty_decay_ms = 10000 applied to existing and future arenas. Failures are ignored on purpose (the volumetric purge covers it).

Debugging facilities

Compile-time gate: ISC_MEM_TRACKLINES (set automatically by -Ddeveloper=enabled meson builds) compiles in per-call __func__/__FILE__/__LINE__ plumbing. Without it, the runtime flags below are inert no-ops. ISC_MEM_TRACE additionally turns attach/detach into traced refcounting.

Runtime flags (a context copies the global default at creation; isc_mem_debugon()/debugoff() adjust the default and isc_g_mctx; isc_mem_setdebugging() sets one context but requires inuse == 0):

• ISC_MEM_DEBUGTRACE — print every alloc/free (add ptr size func file line mctx / del ...) to stderr.
• ISC_MEM_DEBUGRECORD — record every live allocation in a 512-bucket hash table; freeing something never allocated hits UNREACHABLE(); leaks are dumped (print_active) with file:line when the context is destroyed. This is the tool for "inuse != 0 at destroy" hunts.
• ISC_MEM_DEBUGUSAGE — log when usage crosses the water marks.

Each flag can be enabled by simply setting the environment variable of the same name (existence is checked, not the value) before start; the file name in each record is copied, not pointed to, so plugins can be unloaded safely.

Observability: isc_mem_stats(mctx, fp) (pool table + active allocations), isc__mem_printactive() (unit tests), and the statistics channel renders all contexts via isc_mem_renderxml() / isc_mem_renderjson() (id, name, references, inuse, pool count, water marks).

isc_mempool — fixed-size free-list pools

isc_mempool_t batches fixed-size items on top of an mctx to cut allocator round-trips (used for message buffers etc.):

isc_mempool_create(mctx, sizeof(item_t), "items", &pool);
isc_mempool_setfillcount(pool, 32);  /* items grabbed per refill, default 1 */
isc_mempool_setfreemax(pool, 32);    /* free-list cap, default 1 */
...
item_t *it = isc_mempool_get(pool);   /* never NULL */
isc_mempool_put(pool, it);            /* NULLs 'it' */
...
isc_mempool_destroy(&pool);

Critical facts:

• No locking whatsoever. The struct comment says "always unlocked"; the caller must confine a pool to one thread or provide external locking. Getters (getallocated, getfreecount, ...) return garbage under concurrent mutation.
• get: pops the free list; if empty, grabs fillcount items from the mctx in one loop. put: pushes back on the free list unless freecount >= freemax, in which case the item goes straight back to the mctx.
• Item size is silently raised to sizeof(element) (one pointer) because free items are chained through their own storage — a pool of very small items wastes the difference.
• Under AddressSanitizer, fillcount is forced to 1 and freemax to 0 so every get/put reaches the real allocator and poisoning/use-after-free detection works. Don't assume pooling behavior in ASAN builds.
• isc_mempool_destroy() requires every item returned: outstanding items log UNEXPECTED_ERROR("mempool %s leaked memory") and fail a REQUIRE.
• The pool holds a reference on its mctx and is linked on the context's pools list (isc_mem_stats prints them); a context cannot be destroyed while its pools exist.
• Pool items are charged to the mctx when fetched from it (i.e. items sitting on the pool free list still count as inuse for the mctx).

Review checklist

When touching allocation code, check:

1. get↔put / allocate↔free pairing, same mctx, same size (or matched n, size pairs for cget/cput). Grep for the struct's free sites when a size or family changes.
2. Realloc sizing: isc_mem_reget needs the correct old size; on the non-jemalloc path it manually zeroes the growth for creget, so a wrong old size also breaks zeroing.
3. No NULL checks / error paths after allocation — remove dead OOM handling.
4. Put-macro arguments: lvalue, no side effects, and nothing reads the pointer after the macro (it's NULL now).
5. Destructor idiom: last-ref objects use isc_mem_putanddetach.
6. Mempools: single-thread confinement is actually guaranteed; destroy path returns every item first.
7. New long-lived subsystems get their own named mctx (visible in stats channel), not isc_g_mctx.