Fixes gh-378 "assertion failed"

Do not align maxalloc (the limit on the amount of memory
used by the allocator), it's not necessary.

Make an effort to correctly align large (for 32 bit) prealloc
values (like, a few gigs).

src/lib/small/slab_arena.c

@@ -54,9 +54,10 @@ munmap_checked(void *addr, size_t size)
 static void *
 mmap_checked(size_t size, size_t align, int flags)
 {
-	/* The size must be a power of two. */
-	assert((size & (size - 1)) == 0);
+	/* The alignment must be a power of two. */
 	assert((align & (align - 1)) == 0);
+	/* The size must be a multiple of alignment */
+	assert((size & (align - 1)) == 0);
 	/*
 	 * mmap twice the requested amount to be able to align
 	 * the mapped address.
@@ -101,6 +102,7 @@ pow2round(size_t size)
 #endif
 
 #define MAX(a, b) (a) > (b) ? (a) : (b)
+#define MIN(a, b) (a) < (b) ? (a) : (b)
 
 int
 slab_arena_create(struct slab_arena *arena,
@@ -116,18 +118,13 @@ slab_arena_create(struct slab_arena *arena,
 	 */
 	arena->slab_size = small_round(MAX(slab_size, SLAB_MIN_SIZE));
 
-	if (maxalloc) {
-		arena->maxalloc = small_round(MAX(maxalloc,
-						      arena->slab_size));
-	} else {
-		arena->maxalloc = 0;
-	}
-
-	/* Align arena around a fixed number of slabs. */
-	arena->prealloc = small_align(small_round(prealloc),
-				      arena->slab_size);
-	if (arena->maxalloc < arena->prealloc)
-		arena->prealloc = arena->maxalloc;
+	arena->maxalloc = maxalloc;
+	/** Prealloc can not be greater than maxalloc */
+	prealloc = MIN(prealloc, maxalloc);
+	/** Extremely large sizes can not be aligned properly */
+	prealloc = MIN(prealloc, SIZE_MAX - arena->slab_size);
+	/* Align prealloc around a fixed number of slabs. */
+	arena->prealloc = small_align(prealloc, arena->slab_size);
 
 	arena->used = 0;
 

src/lib/small/slab_arena.h

@@ -36,8 +36,12 @@
 extern "C" {
 #endif /* defined(__cplusplus) */
 
-enum { SLAB_MIN_SIZE = USHRT_MAX,
-       SMALL_UNLIMITED = SIZE_MAX/2 + 1};
+enum {
+	/* Smallest possible slab size. */
+	SLAB_MIN_SIZE = USHRT_MAX,
+	/** The largest allowed amount of memory of a single arena. */
+       SMALL_UNLIMITED = SIZE_MAX/2 + 1
+};
 
 /**
  * slab_arena -- a source of large aligned blocks of memory.
@@ -121,7 +125,11 @@ slab_arena_mprotect(struct slab_arena *arena);
 static inline size_t
 small_align(size_t size, size_t alignment)
 {
-	return (size + alignment - 1) & ~(alignment - 1);
+	/* Must be a power of two */
+	assert((alignment & (alignment - 1)) == 0);
+	/* Bit arithmetics won't work for a large size */
+	assert(size <= SIZE_MAX - alignment);
+	return (size - 1 + alignment) & ~(alignment - 1);
 }
 
 /** Round a number to the nearest power of two. */

test/unit/slab_arena.result

@@ -3,23 +3,23 @@ arena->maxalloc = 0
 arena->used = 0
 arena->slab_size = 65536
 arena->prealloc = 65536
-arena->maxalloc = 65536
+arena->maxalloc = 1
 arena->used = 0
 arena->slab_size = 65536
 arena->prealloc = 65536
-arena->maxalloc = 65536
+arena->maxalloc = 1
 arena->used = 65536
 arena->slab_size = 65536
 going beyond the limit: (nil)
 arena->prealloc = 65536
-arena->maxalloc = 65536
+arena->maxalloc = 1
 arena->used = 65536
 arena->slab_size = 65536
 arena->prealloc = 65536
-arena->maxalloc = 65536
+arena->maxalloc = 1
 arena->used = 65536
 arena->slab_size = 65536
-arena->prealloc = 2097152
-arena->maxalloc = 4194304
+arena->prealloc = 2031616
+arena->maxalloc = 3000000
 arena->used = 0
 arena->slab_size = 65536

test/wal/oom.result

@@ -15,11 +15,11 @@ while true do
     i = i + 1
 end;
 ---
-- error: Failed to allocate 313 bytes in slab allocator for tuple
+- error: Failed to allocate 268 bytes in slab allocator for tuple
 ...
 space:len();
 ---
-- 73
+- 62
 ...
 i = 1;
 ---
@@ -29,11 +29,11 @@ while true do
     i = i + 1
 end;
 ---
-- error: Failed to allocate 314 bytes in slab allocator for tuple
+- error: Failed to allocate 268 bytes in slab allocator for tuple
 ...
 space:len();
 ---
-- 146
+- 124
 ...
 i = 1;
 ---
@@ -43,12 +43,12 @@ while true do
     i = i + 1
 end;
 ---
-- error: Failed to allocate 314 bytes in slab allocator for tuple
+- error: Failed to allocate 265 bytes in slab allocator for tuple
 ...
 --# setopt delimiter ''
 space:len()
 ---
-- 219
+- 185
 ...
 space.index['primary']:get{0}
 ---