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Date: Wed, 11 Jan 2017 13:03:06 -0500
From: Dan Streetman <ddstreet@...e.org>
To: Vitaly Wool <vitalywool@...il.com>
Cc: Linux-MM <linux-mm@...ck.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH/RESEND v2 5/5] z3fold: add kref refcounting
On Wed, Jan 11, 2017 at 12:51 PM, Vitaly Wool <vitalywool@...il.com> wrote:
> On Wed, Jan 11, 2017 at 6:39 PM, Dan Streetman <ddstreet@...e.org> wrote:
>> On Wed, Jan 11, 2017 at 12:27 PM, Vitaly Wool <vitalywool@...il.com> wrote:
>>> On Wed, Jan 11, 2017 at 6:08 PM, Dan Streetman <ddstreet@...e.org> wrote:
>>>> On Wed, Jan 11, 2017 at 10:06 AM, Vitaly Wool <vitalywool@...il.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@...il.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 :)
the header's already rounded up to chunk size, so if there's room then
it won't take any extra memory. but it works either way.
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