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Date: Wed, 11 Jan 2017 18:51:44 +0100
From: Vitaly Wool <vitalywool@...il.com>
To: Dan Streetman <ddstreet@...e.org>
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 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 :)
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