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New page table representation where all 4 states can have a skip count
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@kg
kg committed May 7, 2024
1 parent c78da3e commit a030fc6
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Showing 2 changed files with 113 additions and 88 deletions.
180 changes: 104 additions & 76 deletions src/mono/mono/utils/mono-wasm-pagemgr.c
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Original file line number Diff line number Diff line change
Expand Up @@ -10,13 +10,29 @@
#include <mono/utils/mono-os-mutex.h>

// #define MWPM_LOGGING
// #define MWPM_STATS

typedef enum {
MWPM_UNINITIALIZED = 0,
MWPM_INITIALIZING = 1,
MWPM_INITIALIZED = 2
} init_state;

typedef enum {
MWPM_MARK_DEAD_PAGES,
MWPM_MARK_NEW_PAGES,
MWPM_FREE_TO_ALLOCATED,
MWPM_FREE_TO_ALLOCATED_ZEROED,
MWPM_ALLOCATED_TO_FREE,
} page_action;

#define is_page_free(state) (state & MWPM_FREE_BIT)
#define is_page_owned(state) (state & MWPM_STATE_MASK)
#define is_page_in_use(state) ((state & MWPM_STATE_MASK) == MWPM_ALLOCATED)
#define get_page_skip_count(state) (state & MWPM_SKIP_MASK) + 1

typedef uint8_t mwpm_page_state;

static mono_mutex_t mutex;
static uint8_t page_table[MWPM_MAX_PAGES];
static gint32 is_initialized = MWPM_UNINITIALIZED;
Expand Down Expand Up @@ -54,49 +70,74 @@ last_page_of_range (void *addr, size_t size) {
return first_page + page_count_rounded_up - 1;
}

// returns the number of pages in the range that were successfully transitioned.
static uint32_t
transition_page_states (mwpm_page_state from_state, mwpm_page_state to_state, uint32_t first_page, uint32_t page_count) {
static inline mwpm_page_state
encode_page_state (uint8_t bits, uint32_t skip_count) {
if (skip_count > MWPM_SKIP_MASK)
skip_count = MWPM_SKIP_MASK;

return (bits & MWPM_STATE_MASK) | (skip_count & MWPM_SKIP_MASK);
}

static void
transition_page_states (page_action action, uint32_t first_page, uint32_t page_count) {
if (page_count == 0)
return 0;
return;

if (first_page >= MWPM_MAX_PAGES)
return 0;
g_assert (first_page < MWPM_MAX_PAGES);

uint32_t last_page = first_page + (page_count - 1);
g_assert (last_page >= first_page);

if (last_page >= MWPM_MAX_PAGES)
return 0;
g_assert (last_page < MWPM_MAX_PAGES);

uint32_t result = 0;
// POSIX specifies that munmap () on an address range that isn't mapped has no,
// effect, so we need to make sure that it's harmless to try and unmap pages we
// don't control. We can't use memset since it might trample UNKNOWN pages.
for (uint32_t i = first_page; i <= last_page; i++) {
for (uint32_t i = first_page, skip_value = page_count - 1; i <= last_page; i++) {
mwpm_page_state page_state = page_table[i];
// Normalize skip data
if (page_state > MWPM_UNKNOWN)
page_state = MWPM_UNKNOWN;

if (page_state != from_state)
continue;

page_table[i] = to_state;
result++;
// TODO: Remove the duplication in here
switch (action) {
case MWPM_MARK_DEAD_PAGES:
g_assert (!is_page_owned (page_state));
page_table[i] = encode_page_state (MWPM_EXTERNAL, skip_value--);
break;
case MWPM_MARK_NEW_PAGES:
g_assert (!is_page_owned (page_state));
page_table[i] = encode_page_state (MWPM_FREE_ZEROED, skip_value--);
break;
case MWPM_FREE_TO_ALLOCATED:
g_assert (is_page_free (page_state));
page_table[i] = encode_page_state (MWPM_ALLOCATED, skip_value--);
break;
case MWPM_FREE_TO_ALLOCATED_ZEROED:
g_assert (is_page_free (page_state));
page_table[i] = encode_page_state (MWPM_ALLOCATED, skip_value--);
if (!(page_state & MWPM_META_BIT))
// TODO: Don't recalculate the address from scratch each time
memset (address_from_page_index (i), 0, MWPM_PAGE_SIZE);
break;
case MWPM_ALLOCATED_TO_FREE:
// FIXME: Can we generate correct skip_value here? This is used
// by munmap, which is valid to call even on pages that are not mapped
if (is_page_in_use (page_state))
page_table[i] = encode_page_state (MWPM_FREE_DIRTY, 0);
break;
default:
g_assert_not_reached ();
break;
}
}

return result;
}

static void
print_stats () {
#ifdef MWPM_LOGGING
#if defined(MWPM_LOGGING) || defined(MWPM_STATS)
uint32_t in_use = 0, free = 0, unallocated = 0,
max_run = 0, current_run = 0;

for (uint32_t i = first_controlled_page_index; i <= last_controlled_page_index; i++) {
switch (page_table[i]) {
switch (page_table[i] & MWPM_STATE_MASK) {
case MWPM_ALLOCATED:
in_use++;
current_run = 0;
Expand Down Expand Up @@ -125,28 +166,6 @@ print_stats () {
#endif
}

static void
optimize_unknown_pages (uint8_t *start, uint8_t *end) {
g_assert (end > start);

uint32_t first_page = first_page_from_address (start),
page_count = page_count_from_size (end - start);

for (uint32_t i = 0, skip_count = page_count - 1; i < page_count; i++, skip_count--) {
uint32_t j = i + first_page, skip_value = MWPM_UNKNOWN + skip_count;
if (skip_value > 255)
skip_value = 255;
g_assert (page_table[j] >= MWPM_UNKNOWN);
g_print (
"#%u = %u ",
j, skip_value
);
page_table[j] = skip_value;
}

g_print ("\n");
}

static void *
acquire_new_pages_initialized (uint32_t page_count) {
if (page_count < 1)
Expand Down Expand Up @@ -175,7 +194,11 @@ acquire_new_pages_initialized (uint32_t page_count) {
recovered_bytes = allocation - prev_waste_start;
allocation = prev_waste_start;
} else {
optimize_unknown_pages (prev_waste_end, allocation);
// Update the dead pages that were allocated by someone else via sbrk()
// so that they have skip data
uint32_t first_dead_page = first_page_from_address (prev_waste_end),
dead_page_count = page_count_from_size (allocation - prev_waste_end);
transition_page_states (MWPM_MARK_DEAD_PAGES, first_dead_page, dead_page_count);
}

uint8_t *result = allocation;
Expand Down Expand Up @@ -205,9 +228,8 @@ acquire_new_pages_initialized (uint32_t page_count) {
}

// g_print ("mwpm allocated %u bytes (%u pages) starting at @%u (%u recovered)\n", (uint32_t)bytes, page_count, (uint32_t)allocation, recovered_bytes);
uint32_t pages_transitioned = transition_page_states (MWPM_UNKNOWN, MWPM_FREE_ZEROED, first_page_index, page_count);
transition_page_states (MWPM_MARK_NEW_PAGES, first_page_index, page_count);
print_stats ();
g_assert (pages_transitioned == page_count);
last_controlled_page_index = last_page_index;
return result;
}
Expand All @@ -217,7 +239,23 @@ free_pages_initialized (uint32_t first_page, uint32_t page_count) {
// expected behavior: freeing UNKNOWN pages leaves them unknown.
// freeing FREE_ZEROED pages leaves them zeroed.
// freeing ALLOCATED or FREE_DIRTY pages makes them FREE_DIRTY.
transition_page_states (MWPM_ALLOCATED, MWPM_FREE_DIRTY, first_page, page_count);
transition_page_states (MWPM_ALLOCATED_TO_FREE, first_page, page_count);
}

static inline const char *
get_state_name (uint8_t state) {
switch (state & MWPM_STATE_MASK) {
case MWPM_EXTERNAL:
return "external";
case MWPM_FREE_DIRTY:
return "dirty";
case MWPM_FREE_ZEROED:
return "zeroed";
case MWPM_ALLOCATED:
return "in use";
default:
g_assert_not_reached ();
}
}

static uint32_t
Expand All @@ -233,22 +271,27 @@ find_n_free_pages_in_range (uint32_t start_scan_where, uint32_t end_scan_where,
if (j > last_controlled_page_index)
break;

// TODO: If we find a free page with a skip count in it, that would indicate
// that there are N sequential free pages left we can claim without doing
// the scan below.

// Scan backwards from the last candidate page to look for any non-free pages
// the first non-free page we find is the next place we will search from.
for (; j >= i; j--) {
mwpm_page_state page_state = page_table[j];
if (page_state > MWPM_UNKNOWN) {

if (!is_page_free (page_state)) {
// Skip multiple pages
uint32_t skip_count = page_state - MWPM_UNKNOWN;
uint32_t skip_count = get_page_skip_count (page_state);
i = j + skip_count;
g_print (
"scan skipping %u unknown page(s); new page is #%u with state %u\n",
skip_count, i, page_table[i]
);
found_obstruction = 1;
break;
} else if (page_state >= MWPM_ALLOCATED) {
i = j + 1;
#ifdef MWPM_LOGGING
if (skip_count > 1)
g_print (
"scan skipping %u %s page(s); new page is #%u with state %s\n",
skip_count, get_state_name (page_state),
i, get_state_name (page_table[i])
);
#endif
found_obstruction = 1;
break;
}
Expand Down Expand Up @@ -282,7 +325,7 @@ mwpm_init () {
mono_os_mutex_init_recursive (&mutex);
// Set the entire page table to 'unknown state'. As we acquire pages from sbrk, we will
// set those respective ranges in the table to a known state.
memset (page_table, MWPM_UNKNOWN, sizeof(page_table));
memset (page_table, MWPM_EXTERNAL, sizeof(page_table));
void *first_controlled_page_address = acquire_new_pages_initialized (MWPM_MINIMUM_PAGE_COUNT);
g_assert (first_controlled_page_address);
first_controlled_page_index = first_page_from_address (first_controlled_page_address);
Expand Down Expand Up @@ -339,23 +382,8 @@ mwpm_alloc_range (size_t size, uint8_t zeroed) {
if (!result)
goto exit;

uint32_t first_result_page = first_page_from_address (result),
zeroed_pages = transition_page_states (MWPM_FREE_ZEROED, MWPM_ALLOCATED, first_result_page, page_count),
nonzeroed_pages = 0;
// FIXME: Do this in one pass instead of two
if (zeroed_pages != page_count) {
// g_print ("only %u of %u page(s) were zeroed\n", zeroed_pages, page_count);
// If we got here, not all of the pages in our allocation were in FREE_ZEROED state, so we need to
// zero at least one of them.
if (zeroed) {
// g_print ("mwpm zeroing %u bytes at %u\n", size, (uint32_t)result);
// FIXME: Only zero the dirty pages instead of the whole region.
memset (result, 0, size);
}
}
nonzeroed_pages = transition_page_states (MWPM_FREE_DIRTY, MWPM_ALLOCATED, first_result_page, page_count);

g_assert ((nonzeroed_pages + zeroed_pages) == page_count);
uint32_t first_result_page = first_page_from_address (result);
transition_page_states (zeroed ? MWPM_FREE_TO_ALLOCATED_ZEROED : MWPM_FREE_TO_ALLOCATED, first_result_page, page_count);

#ifdef MWPM_LOGGING
g_print ("mwpm allocated %u bytes at %u\n", size, (uint32_t)result);
Expand Down
21 changes: 9 additions & 12 deletions src/mono/mono/utils/mono-wasm-pagemgr.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -56,18 +56,15 @@
// 256 12281 256
#define MWPM_MINIMUM_PAGE_COUNT 32

typedef enum {
// We control this page, it's unused, and it's zeroed (fresh pages)
MWPM_FREE_ZEROED = 0,
// We control this page, it's unused, and it contains garbage
MWPM_FREE_DIRTY = 1,
// We control this page, it's in use
MWPM_ALLOCATED = 2,
// We don't know the state of this page
MWPM_UNKNOWN = 3,
// Values above MWPM_UNKNOWN indicate that the page is followed by more
// UNKNOWN pages and you can skip them by subtracting 3
} mwpm_page_state;
#define MWPM_FREE_BIT 0b10000000
#define MWPM_META_BIT 0b01000000
#define MWPM_STATE_MASK 0b11000000
#define MWPM_SKIP_MASK 0b00111111

#define MWPM_FREE_ZEROED (uint8_t)(MWPM_FREE_BIT | MWPM_META_BIT)
#define MWPM_FREE_DIRTY (uint8_t)(MWPM_FREE_BIT)
#define MWPM_ALLOCATED (uint8_t)(MWPM_META_BIT)
#define MWPM_EXTERNAL (uint8_t)(0)

// Allocate enough pages to hold size bytes of data, optionally ensuring they are zeroed.
// Zeroing memory on wasm is somewhat expensive, so use this option wisely!
Expand Down

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