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    SubjectRe: [PATCH/RESEND v2 5/5] z3fold: add kref refcounting
    On Wed, Jan 11, 2017 at 6:39 PM, Dan Streetman <ddstreet@ieee.org> wrote:
    > On Wed, Jan 11, 2017 at 12:27 PM, Vitaly Wool <vitalywool@gmail.com> wrote:
    >> On Wed, Jan 11, 2017 at 6:08 PM, Dan Streetman <ddstreet@ieee.org> wrote:
    >>> On Wed, Jan 11, 2017 at 10:06 AM, Vitaly Wool <vitalywool@gmail.com> wrote:
    >>>> With both coming and already present locking optimizations,
    >>>> introducing kref to reference-count z3fold objects is the right
    >>>> thing to do. Moreover, it makes buddied list no longer necessary,
    >>>> and allows for a simpler handling of headless pages.
    >>>>
    >>>> Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
    >>>> ---
    >>>> mm/z3fold.c | 145 ++++++++++++++++++++++++++----------------------------------
    >>>> 1 file changed, 62 insertions(+), 83 deletions(-)
    >>>>
    >>>> diff --git a/mm/z3fold.c b/mm/z3fold.c
    >>>> index 4325bde..59cb14f 100644
    >>>> --- a/mm/z3fold.c
    >>>> +++ b/mm/z3fold.c
    >>>> @@ -52,6 +52,7 @@ enum buddy {
    >>>> * z3fold page, except for HEADLESS pages
    >>>> * @buddy: links the z3fold page into the relevant list in the pool
    >>>> * @page_lock: per-page lock
    >>>> + * @refcount: reference cound for the z3fold page
    >>>> * @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
    >>>> @@ -60,6 +61,7 @@ enum buddy {
    >>>> struct z3fold_header {
    >>>> struct list_head buddy;
    >>>> spinlock_t page_lock;
    >>>> + struct kref refcount;
    >>>> unsigned short first_chunks;
    >>>> unsigned short middle_chunks;
    >>>> unsigned short last_chunks;
    >>>> @@ -95,8 +97,6 @@ struct z3fold_header {
    >>>> * @unbuddied: array of lists tracking z3fold pages that contain 2- buddies;
    >>>> * the lists each z3fold page is added to depends on the size of
    >>>> * its free region.
    >>>> - * @buddied: list tracking the z3fold pages that contain 3 buddies;
    >>>> - * these z3fold pages are full
    >>>> * @lru: list tracking the z3fold pages in LRU order by most recently
    >>>> * added buddy.
    >>>> * @pages_nr: number of z3fold pages in the pool.
    >>>> @@ -109,7 +109,6 @@ struct z3fold_header {
    >>>> struct z3fold_pool {
    >>>> spinlock_t lock;
    >>>> struct list_head unbuddied[NCHUNKS];
    >>>> - struct list_head buddied;
    >>>> struct list_head lru;
    >>>> atomic64_t pages_nr;
    >>>> const struct z3fold_ops *ops;
    >>>> @@ -121,8 +120,7 @@ struct z3fold_pool {
    >>>> * Internal z3fold page flags
    >>>> */
    >>>> enum z3fold_page_flags {
    >>>> - UNDER_RECLAIM = 0,
    >>>> - PAGE_HEADLESS,
    >>>> + PAGE_HEADLESS = 0,
    >>>> MIDDLE_CHUNK_MAPPED,
    >>>> };
    >>>>
    >>>> @@ -146,11 +144,11 @@ static struct z3fold_header *init_z3fold_page(struct page *page)
    >>>> struct z3fold_header *zhdr = page_address(page);
    >>>>
    >>>> INIT_LIST_HEAD(&page->lru);
    >>>> - clear_bit(UNDER_RECLAIM, &page->private);
    >>>> clear_bit(PAGE_HEADLESS, &page->private);
    >>>> clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
    >>>>
    >>>> spin_lock_init(&zhdr->page_lock);
    >>>> + kref_init(&zhdr->refcount);
    >>>> zhdr->first_chunks = 0;
    >>>> zhdr->middle_chunks = 0;
    >>>> zhdr->last_chunks = 0;
    >>>> @@ -161,9 +159,21 @@ static struct z3fold_header *init_z3fold_page(struct page *page)
    >>>> }
    >>>>
    >>>> /* Resets the struct page fields and frees the page */
    >>>> -static void free_z3fold_page(struct z3fold_header *zhdr)
    >>>> +static void free_z3fold_page(struct page *page)
    >>>> {
    >>>> - __free_page(virt_to_page(zhdr));
    >>>> + __free_page(page);
    >>>> +}
    >>>> +
    >>>> +static void release_z3fold_page(struct kref *ref)
    >>>> +{
    >>>> + struct z3fold_header *zhdr = container_of(ref, struct z3fold_header,
    >>>> + refcount);
    >>>> + struct page *page = virt_to_page(zhdr);
    >>>> + if (!list_empty(&zhdr->buddy))
    >>>> + list_del(&zhdr->buddy);
    >>>> + if (!list_empty(&page->lru))
    >>>> + list_del(&page->lru);
    >>>> + free_z3fold_page(page);
    >>>> }
    >>>>
    >>>> /* Lock a z3fold page */
    >>>> @@ -257,7 +267,6 @@ static struct z3fold_pool *z3fold_create_pool(gfp_t gfp,
    >>>> spin_lock_init(&pool->lock);
    >>>> for_each_unbuddied_list(i, 0)
    >>>> INIT_LIST_HEAD(&pool->unbuddied[i]);
    >>>> - INIT_LIST_HEAD(&pool->buddied);
    >>>> INIT_LIST_HEAD(&pool->lru);
    >>>> atomic64_set(&pool->pages_nr, 0);
    >>>> pool->ops = ops;
    >>>> @@ -378,6 +387,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
    >>>> spin_unlock(&pool->lock);
    >>>> continue;
    >>>> }
    >>>> + kref_get(&zhdr->refcount);
    >>>> list_del_init(&zhdr->buddy);
    >>>> spin_unlock(&pool->lock);
    >>>>
    >>>> @@ -394,10 +404,12 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
    >>>> else if (zhdr->middle_chunks == 0)
    >>>> bud = MIDDLE;
    >>>> else {
    >>>> + z3fold_page_unlock(zhdr);
    >>>> spin_lock(&pool->lock);
    >>>> - list_add(&zhdr->buddy, &pool->buddied);
    >>>> + if (kref_put(&zhdr->refcount,
    >>>> + release_z3fold_page))
    >>>> + atomic64_dec(&pool->pages_nr);
    >>>> spin_unlock(&pool->lock);
    >>>> - z3fold_page_unlock(zhdr);
    >>>> pr_err("No free chunks in unbuddied\n");
    >>>> WARN_ON(1);
    >>>> continue;
    >>>> @@ -438,9 +450,6 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
    >>>> /* Add to unbuddied list */
    >>>> freechunks = num_free_chunks(zhdr);
    >>>> list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
    >>>> - } else {
    >>>> - /* Add to buddied list */
    >>>> - list_add(&zhdr->buddy, &pool->buddied);
    >>>> }
    >>>>
    >>>> headless:
    >>>> @@ -504,52 +513,29 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
    >>>> }
    >>>> }
    >>>>
    >>>> - if (test_bit(UNDER_RECLAIM, &page->private)) {
    >>>> - /* z3fold page is under reclaim, reclaim will free */
    >>>> - if (bud != HEADLESS)
    >>>> - z3fold_page_unlock(zhdr);
    >>>> - return;
    >>>> - }
    >>>> -
    >>>> - /* Remove from existing buddy list */
    >>>> - 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 */
    >>>> + if (bud == HEADLESS) {
    >>>> 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);
    >>>> + free_z3fold_page(page);
    >>>> atomic64_dec(&pool->pages_nr);
    >>>> } else {
    >>>> - z3fold_compact_page(zhdr);
    >>>> - /* Add to the unbuddied list */
    >>>> + if (zhdr->first_chunks != 0 || zhdr->middle_chunks != 0 ||
    >>>> + zhdr->last_chunks != 0) {
    >>>> + z3fold_compact_page(zhdr);
    >>>> + /* Add to the unbuddied list */
    >>>> + spin_lock(&pool->lock);
    >>>> + if (!list_empty(&zhdr->buddy))
    >>>> + list_del(&zhdr->buddy);
    >>>> + freechunks = num_free_chunks(zhdr);
    >>>> + list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
    >>>> + spin_unlock(&pool->lock);
    >>>> + }
    >>>> + z3fold_page_unlock(zhdr);
    >>>> spin_lock(&pool->lock);
    >>>> - freechunks = num_free_chunks(zhdr);
    >>>> - list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
    >>>> + if (kref_put(&zhdr->refcount, release_z3fold_page))
    >>>> + atomic64_dec(&pool->pages_nr);
    >>>> spin_unlock(&pool->lock);
    >>>> - z3fold_page_unlock(zhdr);
    >>>> }
    >>>>
    >>>> }
    >>>> @@ -608,13 +594,13 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
    >>>> return -EINVAL;
    >>>> }
    >>>> page = list_last_entry(&pool->lru, struct page, lru);
    >>>> - list_del(&page->lru);
    >>>> + list_del_init(&page->lru);
    >>>>
    >>>> - /* Protect z3fold page against free */
    >>>> - set_bit(UNDER_RECLAIM, &page->private);
    >>>> zhdr = page_address(page);
    >>>> if (!test_bit(PAGE_HEADLESS, &page->private)) {
    >>>> - list_del(&zhdr->buddy);
    >>>> + if (!list_empty(&zhdr->buddy))
    >>>> + list_del_init(&zhdr->buddy);
    >>>> + kref_get(&zhdr->refcount);
    >>>> spin_unlock(&pool->lock);
    >>>> z3fold_page_lock(zhdr);
    >>>> /*
    >>>> @@ -655,30 +641,18 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
    >>>> goto next;
    >>>> }
    >>>> next:
    >>>> - if (!test_bit(PAGE_HEADLESS, &page->private))
    >>>> + if (test_bit(PAGE_HEADLESS, &page->private)) {
    >>>> + if (ret == 0) {
    >>>> + free_z3fold_page(page);
    >>>> + return 0;
    >>>> + }
    >>>> + } else {
    >>>> + int freed;
    >>>> 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 &&
    >>>> - zhdr->middle_chunks == 0)) {
    >>>> - /*
    >>>> - * All buddies are now free, free the z3fold page and
    >>>> - * return success.
    >>>> - */
    >>>> - if (!test_and_clear_bit(PAGE_HEADLESS, &page->private))
    >>>> - z3fold_page_unlock(zhdr);
    >>>> - free_z3fold_page(zhdr);
    >>>> - atomic64_dec(&pool->pages_nr);
    >>>> - 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 {
    >>>> + if ((zhdr->first_chunks || zhdr->last_chunks ||
    >>>> + zhdr->middle_chunks) &&
    >>>> + !(zhdr->first_chunks && zhdr->last_chunks &&
    >>>> + zhdr->middle_chunks)) {
    >>>> z3fold_compact_page(zhdr);
    >>>> /* add to unbuddied list */
    >>>> spin_lock(&pool->lock);
    >>>> @@ -687,10 +661,15 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
    >>>> &pool->unbuddied[freechunks]);
    >>>> spin_unlock(&pool->lock);
    >>>> }
    >>>> - }
    >>>> -
    >>>> - if (!test_bit(PAGE_HEADLESS, &page->private))
    >>>> z3fold_page_unlock(zhdr);
    >>>> + spin_lock(&pool->lock);
    >>>> + freed = kref_put(&zhdr->refcount, release_z3fold_page);
    >>>> + spin_unlock(&pool->lock);
    >>>> + if (freed) {
    >>>> + atomic64_dec(&pool->pages_nr);
    >>>> + return 0;
    >>>> + }
    >>>
    >>> you still can't do this - put the kref and then add it back to the lru
    >>> later. freed here is only useful for knowing that it was freed - you
    >>> can't assume that !freed means it's still valid for you to use.
    >>
    >> Oh right, thanks. I can however do something like
    >> ...
    >> spin_lock(&pool->lock);
    >> if (kref_put(&zhdr->refcount, release_z3fold_page)) {
    >> atomic64_dec(&pool->pages_nr);
    >> spin_unlock(&pool->lock);
    >> return 0;
    >> }
    >> }
    >>
    >> /* add to beginning of LRU */
    >> list_add(&page->lru, &pool->lru);
    >> }
    >> ...
    >>
    >> provided that I take the lock for the headless case above. That will
    >> work won't it?
    >
    > in this specific case - since every single kref_put in the driver is
    > protected by the pool lock - yeah, you can do that, since you know
    > that specific kref_put didn't free the page and no other kref_put
    > could happen since you're holding the pool lock.
    >
    > but that specific requirement isn't made obvious by the code, and I
    > can see how a future patch could release the pool lock between the
    > kref_put and lru list add, without realizing that introduces a bug.
    > isn't it better to just add it to the lru list before you put the
    > kref? just a suggestion.

    That would require to add it to the lru separately for headless pages
    above, I don't think it makes things better or less error prone.
    I would rather add a comment before list_add.

    > i'd also suggest the pages_nr dec go into the page release function,
    > instead of checking every kref_put return value; that's easier and
    > less prone to forgetting to check one of the kref_puts.

    That will mean z3fold_header should contain a pointer to its pool. I'm
    not sure it is worth it but I can do that :)

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