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SubjectRe: [PATCH/RESEND 5/5] z3fold: add kref refcounting
On Wed, Jan 4, 2017 at 7:42 PM, Dan Streetman <ddstreet@ieee.org> wrote:
> On Sun, Dec 25, 2016 at 7:40 PM, 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 | 137 ++++++++++++++++++++++++++++++------------------------------
>> 1 file changed, 68 insertions(+), 69 deletions(-)
>>
>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> index 729a2da..4593493 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;
>> raw_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;
>> @@ -162,9 +161,21 @@ 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);
>
> wait, a page shouldn't ever be on a buddy or lru list if it's free,
> should it? these checks are bugs if they're true aren't they?
> Relying on the release function to remove a page from its buddy and/or
> lru list (and hoping no other code already took it off and it using
> it) seems very error-prone.

Why? We manage not to care if page is on a list or not when it's
become unused. When we release a page we don't actually know if anyone
else is using it and that's ok.

>> + free_z3fold_page(page);
>> }
>>
>> /* Lock a z3fold page */
>> @@ -256,9 +267,9 @@ static struct z3fold_pool *z3fold_create_pool(gfp_t gfp,
>> if (!pool)
>> return NULL;
>> spin_lock_init(&pool->lock);
>> + kref_init(&zhdr->refcount);
>> 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;
>> @@ -383,7 +394,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>> spin_lock(&pool->lock);
>> zhdr = list_first_entry_or_null(&pool->unbuddied[i],
>> struct z3fold_header, buddy);
>> - if (!zhdr) {
>> + if (!zhdr || !kref_get_unless_zero(&zhdr->refcount)) {
>
> if we can't rely on the kref to be safe under the pool lock, the kref
> isn't very useful is it? seems like it just makes things more
> complicated.

No, this is just an overkill check. I tried this patch with the direct
kref_get() instead and it's ok.

> the kref should be assumed to be safe while holding the pool lock, or
> whatever lock protects the list(s) the object is on, otherwise it
> seems likely that use-after-free problems will result...but this goes
> back to my concern about relying on the freeing function to remove
> objects from their lists.

As long as kref_put() is called with the pool lock held it is no problem.

>> spin_unlock(&pool->lock);
>> continue;
>> }
>> @@ -403,10 +414,12 @@ 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;
>> @@ -447,9 +460,6 @@ 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:
>> @@ -515,50 +525,39 @@ 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)
>> + 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_lock(&pool->lock);
>> + if (kref_put(&zhdr->refcount, release_z3fold_page))
>> + atomic64_dec(&pool->pages_nr);
>> spin_unlock(&pool->lock);
>> - z3fold_page_unlock(zhdr);
>> - return;
>> }
>> - spin_unlock(&pool->lock);
>> + return;
>> }
>>
>> - 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);
>> }
>>
>> }
>> @@ -617,13 +616,15 @@ 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);
>
> UNDER_RECLAIM shouldn't be needed anymore when kref counting is used,
> and with the separate pool and page locks, z3fold_free and
> z3fold_reclaim can race to set/check this bit anyway (it's set under
> the pool lock, but checked under the page lock).

True; I'll refactor the patch to get rid of it.

>> 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);
>> /*
>> @@ -664,30 +665,26 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>> goto next;
>> }
>> next:
>> - 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 &&
>> - 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))
>> + if (test_bit(PAGE_HEADLESS, &page->private)) {
>> + if (ret == 0) {
>> + free_z3fold_page(page);
>> + return 0;
>> + }
>> + } else {
>> + z3fold_page_lock(zhdr);
>> + if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0 &&
>> + zhdr->middle_chunks == 0) {
>> 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);
>> + if (kref_put(&zhdr->refcount,
>> + release_z3fold_page))
>> + atomic64_dec(&pool->pages_nr);
>> spin_unlock(&pool->lock);
>> - } else {
>> + return 0;
>> + } else if (zhdr->first_chunks == 0 ||
>> + zhdr->last_chunks == 0 ||
>> + zhdr->middle_chunks == 0) {
>> z3fold_compact_page(zhdr);
>> /* add to unbuddied list */
>> spin_lock(&pool->lock);
>> @@ -696,10 +693,12 @@ 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);
>> + if (kref_put(&zhdr->refcount, release_z3fold_page))
>> + atomic64_dec(&pool->pages_nr);
>> + spin_unlock(&pool->lock);
>> + }
>
> you can't put the zhdr above, and then go on to add the page to the
> lru list; you don't have the kref anymore to allow you to do that.

I don't think so since I do kref_get() in the beginning of reclaim
function so strictly speaking kref_put should not release the page
here.
I can add an extra check though to just return 0 in case it did.

>>
>> spin_lock(&pool->lock);
>> /* add to beginning of LRU */
>> --
>> 2.4.2

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