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SubjectRe: [PATCH/RESEND v2 5/5] z3fold: add kref refcounting
On Wed, 11 Jan 2017 13:03:06 -0500
Dan Streetman <ddstreet@ieee.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@gmail.com>
---
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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