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dm-writecache: Reduce metadata memory use by 8 bytes per cache block

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wc_entry.index removed: This can be computed by pointer arithmetic
without storage.
wc_entry.age reduced to 32 bits: It only needs to be big enough to hold
max_age, which is limited to well below 32-bit range.

Reduces wc_entry size from 56 bytes to 48 bytes (from 72 to 64 bytes if
DM_WRITECACHE_HANDLE_HARDWARE_ERRORS enabled).

Signed-off-by: Henry Wong <henry@stuffedcow.net>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
This commit is contained in:
Henry Wong 2026-09-07 14:46:47 -07:00 committed by Mikulas Patocka
parent c69cc51136
commit 28e78fd0d2
1 changed files with 18 additions and 21 deletions
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39
drivers/md/dm-writecache.c
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@ -81,15 +81,9 @@ struct wc_memory_superblock {
struct wc_entry {
struct rb_node rb_node;
struct list_head lru;
u32 age; // jiffies
unsigned short wc_list_contiguous;
#if BITS_PER_LONG == 64
bool write_in_progress : 1;
unsigned long index : 47;
#else
bool write_in_progress;
unsigned long index;
#endif
unsigned long age;
#ifdef DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
uint64_t original_sector;
uint64_t seq_count;
@ -389,20 +383,28 @@ static struct wc_memory_superblock *sb(struct dm_writecache *wc)
return wc->memory_map;
}
static inline unsigned long wc_entry_index(const struct wc_entry *base, const struct wc_entry *entry)
{
return entry - base;
}
static struct wc_memory_entry *memory_entry(struct dm_writecache *wc, struct wc_entry *e)
{
return &sb(wc)->entries[e->index];
const unsigned long index = wc_entry_index(wc->entries, e);
return &sb(wc)->entries[index];
}
static void *memory_data(struct dm_writecache *wc, struct wc_entry *e)
{
return (char *)wc->block_start + (e->index << wc->block_size_bits);
const unsigned long index = wc_entry_index(wc->entries, e);
return (char *)wc->block_start + (index << wc->block_size_bits);
}
static sector_t cache_sector(struct dm_writecache *wc, struct wc_entry *e)
{
const unsigned long index = wc_entry_index(wc->entries, e);
return wc->start_sector + wc->metadata_sectors +
((sector_t)e->index << (wc->block_size_bits - SECTOR_SHIFT));
((sector_t)index << (wc->block_size_bits - SECTOR_SHIFT));
}
static uint64_t read_original_sector(struct dm_writecache *wc, struct wc_entry *e)
@ -668,7 +670,7 @@ static void writecache_insert_entry(struct dm_writecache *wc, struct wc_entry *i
rb_link_node(&ins->rb_node, parent, node);
rb_insert_color(&ins->rb_node, &wc->tree);
list_add(&ins->lru, &wc->lru);
ins->age = jiffies;
ins->age = (u32)jiffies;
}
static void writecache_unlink(struct dm_writecache *wc, struct wc_entry *e)
@ -969,7 +971,6 @@ static int writecache_alloc_entries(struct dm_writecache *wc)
for (b = 0; b < wc->n_blocks; b++) {
struct wc_entry *e = &wc->entries[b];
e->index = b;
e->write_in_progress = false;
cond_resched();
}
@ -1986,7 +1987,7 @@ static void writecache_writeback(struct work_struct *work)
while (!list_empty(&wc->lru) &&
(wc->writeback_all ||
wc->freelist_size + wc->writeback_size <= wc->freelist_low_watermark ||
(jiffies - container_of(wc->lru.prev, struct wc_entry, lru)->age >=
((u32)(jiffies - container_of(wc->lru.prev, struct wc_entry, lru)->age) >=
wc->max_age - wc->max_age / MAX_AGE_DIV))) {
n_walked++;
@ -2107,7 +2108,6 @@ static int calculate_memory_size(uint64_t device_size, unsigned int block_size,
size_t *n_blocks_p, size_t *n_metadata_blocks_p)
{
uint64_t n_blocks, offset;
struct wc_entry e;
n_blocks = device_size;
do_div(n_blocks, block_size + sizeof(struct wc_memory_entry));
@ -2126,11 +2126,6 @@ static int calculate_memory_size(uint64_t device_size, unsigned int block_size,
n_blocks--;
}
/* check if the bit field overflows */
e.index = n_blocks;
if (e.index != n_blocks)
return -EFBIG;
if (n_blocks_p)
*n_blocks_p = n_blocks;
if (n_metadata_blocks_p)
@ -2444,13 +2439,15 @@ static int writecache_ctr(struct dm_target *ti, unsigned int argc, char **argv)
wc->autocommit_time_set = true;
} else if (!strcasecmp(string, "max_age") && opt_params >= 1) {
unsigned int max_age_msecs;
unsigned long max_age_jiffies;
string = dm_shift_arg(&as), opt_params--;
if (sscanf(string, "%u%c", &max_age_msecs, &dummy) != 1)
goto invalid_optional;
if (max_age_msecs > 86400000)
max_age_jiffies = msecs_to_jiffies(max_age_msecs);
if (max_age_jiffies >= min(MAX_JIFFY_OFFSET, (7 << 28))) // 7/8ths of 1 << 31
goto invalid_optional;
wc->max_age = msecs_to_jiffies(max_age_msecs);
wc->max_age = max_age_jiffies;
wc->max_age_set = true;
wc->max_age_value = max_age_msecs;
} else if (!strcasecmp(string, "cleaner")) {