Re: [PATCH/RESEND v2 5/5] z3fold: add kref refcounting

[Dan Streetman]

On Wed, Jan 11, 2017 at 12:27 PM, Vitaly Wool <vitalywool@xxxxxxxxx> wrote:
> On Wed, Jan 11, 2017 at 6:08 PM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
>> On Wed, Jan 11, 2017 at 10:06 AM, Vitaly Wool <vitalywool@xxxxxxxxx> 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@xxxxxxxxx>
>>> ---
>>>  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.

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.

>
> ~vitaly