Re: [PATCH/RESEND v2 4/5] z3fold: use per-page spinlock

From: Dan Streetman
Date: Wed Jan 11 2017 - 11:43:15 EST

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In reply to: Vitaly Wool: "[PATCH/RESEND v2 4/5] z3fold: use per-page spinlock"
Next in thread: Vitaly Wool: "[PATCH/RESEND v2 2/5] z3fold: fix header size related issues"
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On Wed, Jan 11, 2017 at 10:06 AM, Vitaly Wool <vitalywool@xxxxxxxxx> wrote:
> 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@xxxxxxxxx>

I still have a bit of concern on the coordination between the page and
pool locks...but I don't see anything specifically wrong so

Reviewed-by: Dan Streetman <ddstreet@xxxxxxxx>

> ---
> 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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