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Date: Thu, 12 Jan 2017 12:49:24 -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 3:51 PM, Vitaly Wool <vitalywool@...il.com> wrote:
> On Wed, 11 Jan 2017 13:03:06 -0500
> Dan Streetman <ddstreet@...e.org> wrote:
>
> <snip>
>> >>> 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.
>
> So let's have it like this then:
>
>
> 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>
Reviewed-by: Dan Streetman <ddstreet@...e.org>
> ---
> mm/z3fold.c | 151 ++++++++++++++++++++++++++----------------------------------
> 1 file changed, 66 insertions(+), 85 deletions(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 4325bde..f0d2eaf 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 +641,19 @@ 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))
> - 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);
> + if (test_bit(PAGE_HEADLESS, &page->private)) {
> + if (ret == 0) {
> + free_z3fold_page(page);
> + return 0;
> } else {
> + spin_lock(&pool->lock);
> + }
> + } else {
> + z3fold_page_lock(zhdr);
> + 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);
> @@ -687,13 +662,19 @@ 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);
> + return 0;
> + }
> + }
>
> - spin_lock(&pool->lock);
> - /* add to beginning of LRU */
> + /*
> + * Add to the beginning of LRU.
> + * Pool lock has to be kept here to ensure the page has
> + * not already been released
> + */
> list_add(&page->lru, &pool->lru);
> }
> spin_unlock(&pool->lock);
> --
> 2.4.2
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