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Subject[PATCH/RESEND v2 4/5] z3fold: use per-page spinlock
Most of z3fold operations are in-page, such as modifying z3fold page
header or moving z3fold objects within a page. Taking per-pool spinlock
to protect per-page objects is therefore suboptimal, and the idea of
having a per-page spinlock (or rwlock) has been around for some time.

This patch implements spinlock-based per-page locking mechanism which
is lightweight enough to normally fit ok into the z3fold header.

Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
---
mm/z3fold.c | 148 +++++++++++++++++++++++++++++++++++++++++++-----------------
1 file changed, 106 insertions(+), 42 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index fca3310..4325bde 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -51,6 +51,7 @@ enum buddy {
* struct z3fold_header - z3fold page metadata occupying the first chunk of each
* z3fold page, except for HEADLESS pages
* @buddy: links the z3fold page into the relevant list in the pool
+ * @page_lock: per-page lock
* @first_chunks: the size of the first buddy in chunks, 0 if free
* @middle_chunks: the size of the middle buddy in chunks, 0 if free
* @last_chunks: the size of the last buddy in chunks, 0 if free
@@ -58,6 +59,7 @@ enum buddy {
*/
struct z3fold_header {
struct list_head buddy;
+ spinlock_t page_lock;
unsigned short first_chunks;
unsigned short middle_chunks;
unsigned short last_chunks;
@@ -148,6 +150,7 @@ static struct z3fold_header *init_z3fold_page(struct page *page)
clear_bit(PAGE_HEADLESS, &page->private);
clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);

+ spin_lock_init(&zhdr->page_lock);
zhdr->first_chunks = 0;
zhdr->middle_chunks = 0;
zhdr->last_chunks = 0;
@@ -163,6 +166,19 @@ static void free_z3fold_page(struct z3fold_header *zhdr)
__free_page(virt_to_page(zhdr));
}

+/* Lock a z3fold page */
+static inline void z3fold_page_lock(struct z3fold_header *zhdr)
+{
+ spin_lock(&zhdr->page_lock);
+}
+
+/* Unlock a z3fold page */
+static inline void z3fold_page_unlock(struct z3fold_header *zhdr)
+{
+ spin_unlock(&zhdr->page_lock);
+}
+
+
/*
* Encodes the handle of a particular buddy within a z3fold page
* Pool lock should be held as this function accesses first_num
@@ -351,50 +367,60 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
bud = HEADLESS;
else {
chunks = size_to_chunks(size);
- spin_lock(&pool->lock);

/* First, try to find an unbuddied z3fold page. */
zhdr = NULL;
for_each_unbuddied_list(i, chunks) {
- if (!list_empty(&pool->unbuddied[i])) {
- zhdr = list_first_entry(&pool->unbuddied[i],
+ spin_lock(&pool->lock);
+ zhdr = list_first_entry_or_null(&pool->unbuddied[i],
struct z3fold_header, buddy);
- page = virt_to_page(zhdr);
- if (zhdr->first_chunks == 0) {
- if (zhdr->middle_chunks != 0 &&
- chunks >= zhdr->start_middle)
- bud = LAST;
- else
- bud = FIRST;
- } else if (zhdr->last_chunks == 0)
+ if (!zhdr) {
+ spin_unlock(&pool->lock);
+ continue;
+ }
+ list_del_init(&zhdr->buddy);
+ spin_unlock(&pool->lock);
+
+ page = virt_to_page(zhdr);
+ z3fold_page_lock(zhdr);
+ if (zhdr->first_chunks == 0) {
+ if (zhdr->middle_chunks != 0 &&
+ chunks >= zhdr->start_middle)
bud = LAST;
- else if (zhdr->middle_chunks == 0)
- bud = MIDDLE;
- else {
- pr_err("No free chunks in unbuddied\n");
- WARN_ON(1);
- continue;
- }
- list_del(&zhdr->buddy);
- goto found;
+ else
+ bud = FIRST;
+ } else if (zhdr->last_chunks == 0)
+ bud = LAST;
+ else if (zhdr->middle_chunks == 0)
+ bud = MIDDLE;
+ else {
+ spin_lock(&pool->lock);
+ list_add(&zhdr->buddy, &pool->buddied);
+ spin_unlock(&pool->lock);
+ z3fold_page_unlock(zhdr);
+ pr_err("No free chunks in unbuddied\n");
+ WARN_ON(1);
+ continue;
}
+ goto found;
}
bud = FIRST;
- spin_unlock(&pool->lock);
}

/* Couldn't find unbuddied z3fold page, create new one */
page = alloc_page(gfp);
if (!page)
return -ENOMEM;
- spin_lock(&pool->lock);
+
atomic64_inc(&pool->pages_nr);
zhdr = init_z3fold_page(page);

if (bud == HEADLESS) {
set_bit(PAGE_HEADLESS, &page->private);
+ spin_lock(&pool->lock);
goto headless;
}
+ z3fold_page_lock(zhdr);

found:
if (bud == FIRST)
@@ -406,6 +432,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
zhdr->start_middle = zhdr->first_chunks + ZHDR_CHUNKS;
}

+ spin_lock(&pool->lock);
if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0 ||
zhdr->middle_chunks == 0) {
/* Add to unbuddied list */
@@ -425,6 +452,8 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,

*handle = encode_handle(zhdr, bud);
spin_unlock(&pool->lock);
+ if (bud != HEADLESS)
+ z3fold_page_unlock(zhdr);

return 0;
}
@@ -446,7 +475,6 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
struct page *page;
enum buddy bud;

- spin_lock(&pool->lock);
zhdr = handle_to_z3fold_header(handle);
page = virt_to_page(zhdr);

@@ -454,6 +482,7 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
/* HEADLESS page stored */
bud = HEADLESS;
} else {
+ z3fold_page_lock(zhdr);
bud = handle_to_buddy(handle);

switch (bud) {
@@ -470,37 +499,59 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
default:
pr_err("%s: unknown bud %d\n", __func__, bud);
WARN_ON(1);
- spin_unlock(&pool->lock);
+ z3fold_page_unlock(zhdr);
return;
}
}

if (test_bit(UNDER_RECLAIM, &page->private)) {
/* z3fold page is under reclaim, reclaim will free */
- spin_unlock(&pool->lock);
+ if (bud != HEADLESS)
+ z3fold_page_unlock(zhdr);
return;
}

/* Remove from existing buddy list */
- if (bud != HEADLESS)
- list_del(&zhdr->buddy);
+ if (bud != HEADLESS) {
+ spin_lock(&pool->lock);
+ /*
+ * this object may have been removed from its list by
+ * z3fold_alloc(). In that case we just do nothing,
+ * z3fold_alloc() will allocate an object and add the page
+ * to the relevant list.
+ */
+ if (!list_empty(&zhdr->buddy)) {
+ list_del(&zhdr->buddy);
+ } else {
+ spin_unlock(&pool->lock);
+ z3fold_page_unlock(zhdr);
+ return;
+ }
+ spin_unlock(&pool->lock);
+ }

if (bud == HEADLESS ||
(zhdr->first_chunks == 0 && zhdr->middle_chunks == 0 &&
zhdr->last_chunks == 0)) {
/* z3fold page is empty, free */
+ spin_lock(&pool->lock);
list_del(&page->lru);
+ spin_unlock(&pool->lock);
clear_bit(PAGE_HEADLESS, &page->private);
+ if (bud != HEADLESS)
+ z3fold_page_unlock(zhdr);
free_z3fold_page(zhdr);
atomic64_dec(&pool->pages_nr);
} else {
z3fold_compact_page(zhdr);
/* Add to the unbuddied list */
+ spin_lock(&pool->lock);
freechunks = num_free_chunks(zhdr);
list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
+ spin_unlock(&pool->lock);
+ z3fold_page_unlock(zhdr);
}

- spin_unlock(&pool->lock);
}

/**
@@ -547,12 +598,15 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
unsigned long first_handle = 0, middle_handle = 0, last_handle = 0;

spin_lock(&pool->lock);
- if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||
- retries == 0) {
+ if (!pool->ops || !pool->ops->evict || retries == 0) {
spin_unlock(&pool->lock);
return -EINVAL;
}
for (i = 0; i < retries; i++) {
+ if (list_empty(&pool->lru)) {
+ spin_unlock(&pool->lock);
+ return -EINVAL;
+ }
page = list_last_entry(&pool->lru, struct page, lru);
list_del(&page->lru);

@@ -561,6 +615,8 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
zhdr = page_address(page);
if (!test_bit(PAGE_HEADLESS, &page->private)) {
list_del(&zhdr->buddy);
+ spin_unlock(&pool->lock);
+ z3fold_page_lock(zhdr);
/*
* We need encode the handles before unlocking, since
* we can race with free that will set
@@ -575,13 +631,13 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
middle_handle = encode_handle(zhdr, MIDDLE);
if (zhdr->last_chunks)
last_handle = encode_handle(zhdr, LAST);
+ z3fold_page_unlock(zhdr);
} else {
first_handle = encode_handle(zhdr, HEADLESS);
last_handle = middle_handle = 0;
+ spin_unlock(&pool->lock);
}

- spin_unlock(&pool->lock);
-
/* Issue the eviction callback(s) */
if (middle_handle) {
ret = pool->ops->evict(pool, middle_handle);
@@ -599,7 +655,8 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
goto next;
}
next:
- spin_lock(&pool->lock);
+ if (!test_bit(PAGE_HEADLESS, &page->private))
+ z3fold_page_lock(zhdr);
clear_bit(UNDER_RECLAIM, &page->private);
if ((test_bit(PAGE_HEADLESS, &page->private) && ret == 0) ||
(zhdr->first_chunks == 0 && zhdr->last_chunks == 0 &&
@@ -608,26 +665,34 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
* All buddies are now free, free the z3fold page and
* return success.
*/
- clear_bit(PAGE_HEADLESS, &page->private);
+ if (!test_and_clear_bit(PAGE_HEADLESS, &page->private))
+ z3fold_page_unlock(zhdr);
free_z3fold_page(zhdr);
atomic64_dec(&pool->pages_nr);
- spin_unlock(&pool->lock);
return 0;
} else if (!test_bit(PAGE_HEADLESS, &page->private)) {
if (zhdr->first_chunks != 0 &&
zhdr->last_chunks != 0 &&
zhdr->middle_chunks != 0) {
/* Full, add to buddied list */
+ spin_lock(&pool->lock);
list_add(&zhdr->buddy, &pool->buddied);
+ spin_unlock(&pool->lock);
} else {
z3fold_compact_page(zhdr);
/* add to unbuddied list */
+ spin_lock(&pool->lock);
freechunks = num_free_chunks(zhdr);
list_add(&zhdr->buddy,
&pool->unbuddied[freechunks]);
+ spin_unlock(&pool->lock);
}
}

+ if (!test_bit(PAGE_HEADLESS, &page->private))
+ z3fold_page_unlock(zhdr);
+
+ spin_lock(&pool->lock);
/* add to beginning of LRU */
list_add(&page->lru, &pool->lru);
}
@@ -652,7 +717,6 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
void *addr;
enum buddy buddy;

- spin_lock(&pool->lock);
zhdr = handle_to_z3fold_header(handle);
addr = zhdr;
page = virt_to_page(zhdr);
@@ -660,6 +724,7 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
if (test_bit(PAGE_HEADLESS, &page->private))
goto out;

+ z3fold_page_lock(zhdr);
buddy = handle_to_buddy(handle);
switch (buddy) {
case FIRST:
@@ -678,8 +743,9 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
addr = NULL;
break;
}
+
+ z3fold_page_unlock(zhdr);
out:
- spin_unlock(&pool->lock);
return addr;
}

@@ -694,19 +760,17 @@ static void z3fold_unmap(struct z3fold_pool *pool, unsigned long handle)
struct page *page;
enum buddy buddy;

- spin_lock(&pool->lock);
zhdr = handle_to_z3fold_header(handle);
page = virt_to_page(zhdr);

- if (test_bit(PAGE_HEADLESS, &page->private)) {
- spin_unlock(&pool->lock);
+ if (test_bit(PAGE_HEADLESS, &page->private))
return;
- }

+ z3fold_page_lock(zhdr);
buddy = handle_to_buddy(handle);
if (buddy == MIDDLE)
clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
- spin_unlock(&pool->lock);
+ z3fold_page_unlock(zhdr);
}

/**
--
2.4.2
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