On Tue, Aug 06, 2013 at 08:42:38AM +0200, Krzysztof Kozlowski wrote:
> Use page reference counter for zbud pages. The ref counter replaces
> zbud_header.under_reclaim flag and ensures that zbud page won't be freed
> when zbud_free() is called during reclaim. It allows implementation of
> additional reclaim paths.
>
> The page count is incremented when:
> - a handle is created and passed to zswap (in zbud_alloc()),
> - user-supplied eviction callback is called (in zbud_reclaim_page()).
I like the idea. I few things below. Also agree with Bob the
s/rebalance/adjust/ for rebalance_lists().
>
> Signed-off-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>
> Signed-off-by: Tomasz Stanislawski <t.stanislaws@xxxxxxxxxxx>
> ---
> mm/zbud.c | 150 +++++++++++++++++++++++++++++++++++--------------------------
> 1 file changed, 86 insertions(+), 64 deletions(-)
>
> diff --git a/mm/zbud.c b/mm/zbud.c
> index ad1e781..a8e986f 100644
> --- a/mm/zbud.c
> +++ b/mm/zbud.c
> @@ -109,7 +109,6 @@ struct zbud_header {
> struct list_head lru;
> unsigned int first_chunks;
> unsigned int last_chunks;
> - bool under_reclaim;
> };
>
> /*****************
> @@ -138,16 +137,9 @@ static struct zbud_header *init_zbud_page(struct page *page)
> zhdr->last_chunks = 0;
> INIT_LIST_HEAD(&zhdr->buddy);
> INIT_LIST_HEAD(&zhdr->lru);
> - zhdr->under_reclaim = 0;
> return zhdr;
> }
>
> -/* Resets the struct page fields and frees the page */
> -static void free_zbud_page(struct zbud_header *zhdr)
> -{
> - __free_page(virt_to_page(zhdr));
> -}
> -
> /*
> * Encodes the handle of a particular buddy within a zbud page
> * Pool lock should be held as this function accesses first|last_chunks
> @@ -188,6 +180,65 @@ static int num_free_chunks(struct zbud_header *zhdr)
> return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks - 1;
> }
>
> +/*
> + * Called after zbud_free() or zbud_alloc().
> + * Checks whether given zbud page has to be:
> + * - removed from buddied/unbuddied/LRU lists completetely (zbud_free).
> + * - moved from buddied to unbuddied list
> + * and to beginning of LRU (zbud_alloc, zbud_free),
> + * - added to buddied list and LRU (zbud_alloc),
> + *
> + * The page must be already removed from buddied/unbuddied lists.
> + * Must be called under pool->lock.
> + */
> +static void rebalance_lists(struct zbud_pool *pool, struct zbud_header *zhdr)
> +{
> + if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
> + /* zbud_free() */
> + list_del(&zhdr->lru);
> + return;
> + } else if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0) {
s/else if/if/ since the if above returns if true.
> + /* zbud_free() or zbud_alloc() */
> + int freechunks = num_free_chunks(zhdr);
> + list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
> + } else {
> + /* zbud_alloc() */
> + list_add(&zhdr->buddy, &pool->buddied);
> + }
> + /* Add/move zbud page to beginning of LRU */
> + if (!list_empty(&zhdr->lru))
> + list_del(&zhdr->lru);
We don't want to reinsert to the LRU list if we have called zbud_free()
on a zbud page that previously had two buddies. This code causes the
zbud page to move to the front of the LRU list which is not what we want.
> + list_add(&zhdr->lru, &pool->lru);
> +}
> +
> +/*
> + * Increases ref count for zbud page.
> + */
> +static void get_zbud_page(struct zbud_header *zhdr)
> +{
> + get_page(virt_to_page(zhdr));
> +}
> +
> +/*
> + * Decreases ref count for zbud page and frees the page if it reaches 0
> + * (no external references, e.g. handles).
> + *
> + * Must be called under pool->lock.
> + *
> + * Returns 1 if page was freed and 0 otherwise.
> + */
> +static int put_zbud_page(struct zbud_pool *pool, struct zbud_header *zhdr)
> +{
> + struct page *page = virt_to_page(zhdr);
> + if (put_page_testzero(page)) {
> + free_hot_cold_page(page, 0);
> + pool->pages_nr--;
> + return 1;
> + }
> + return 0;
> +}
> +
> +
> /*****************
> * API Functions
> *****************/
> @@ -250,7 +301,7 @@ void zbud_destroy_pool(struct zbud_pool *pool)
> int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
> unsigned long *handle)
> {
> - int chunks, i, freechunks;
> + int chunks, i;
> struct zbud_header *zhdr = NULL;
> enum buddy bud;
> struct page *page;
> @@ -273,6 +324,7 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
> bud = FIRST;
> else
> bud = LAST;
> + get_zbud_page(zhdr);
> goto found;
> }
> }
> @@ -284,6 +336,10 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
> return -ENOMEM;
> spin_lock(&pool->lock);
> pool->pages_nr++;
> + /*
> + * We will be using zhdr instead of page, so
> + * don't increase the page count.
> + */
> zhdr = init_zbud_page(page);
> bud = FIRST;
>
> @@ -293,19 +349,7 @@ found:
> else
> zhdr->last_chunks = chunks;
>
> - if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0) {
> - /* 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);
> - }
> -
> - /* Add/move zbud page to beginning of LRU */
> - if (!list_empty(&zhdr->lru))
> - list_del(&zhdr->lru);
> - list_add(&zhdr->lru, &pool->lru);
> + rebalance_lists(pool, zhdr);
>
> *handle = encode_handle(zhdr, bud);
> spin_unlock(&pool->lock);
> @@ -326,10 +370,10 @@ found:
> void zbud_free(struct zbud_pool *pool, unsigned long handle)
> {
> struct zbud_header *zhdr;
> - int freechunks;
>
> spin_lock(&pool->lock);
> zhdr = handle_to_zbud_header(handle);
> + BUG_ON(zhdr->last_chunks == 0 && zhdr->first_chunks == 0);
Not sure we need this. Maybe, at most, VM_BUG_ON()?
>
> /* If first buddy, handle will be page aligned */
> if ((handle - ZHDR_SIZE_ALIGNED) & ~PAGE_MASK)
> @@ -337,26 +381,9 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle)
> else
> zhdr->first_chunks = 0;
>
> - if (zhdr->under_reclaim) {
> - /* zbud page is under reclaim, reclaim will free */
> - spin_unlock(&pool->lock);
> - return;
> - }
> -
> - /* Remove from existing buddy list */
> list_del(&zhdr->buddy);
> -
> - if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
> - /* zbud page is empty, free */
> - list_del(&zhdr->lru);
> - free_zbud_page(zhdr);
> - pool->pages_nr--;
> - } else {
> - /* Add to unbuddied list */
> - freechunks = num_free_chunks(zhdr);
> - list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
> - }
> -
> + rebalance_lists(pool, zhdr);
> + put_zbud_page(pool, zhdr);
> spin_unlock(&pool->lock);
> }
>
> @@ -400,7 +427,7 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle)
> */
> int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> {
> - int i, ret, freechunks;
> + int i, ret;
> struct zbud_header *zhdr;
> unsigned long first_handle = 0, last_handle = 0;
>
> @@ -411,11 +438,24 @@ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> return -EINVAL;
> }
> for (i = 0; i < retries; i++) {
> + if (list_empty(&pool->lru)) {
> + /*
> + * LRU was emptied during evict calls in previous
> + * iteration but put_zbud_page() returned 0 meaning
> + * that someone still holds the page. This may
> + * happen when some other mm mechanism increased
> + * the page count.
> + * In such case we succedded with reclaim.
> + */
> + return 0;
> + }
> zhdr = list_tail_entry(&pool->lru, struct zbud_header, lru);
> + BUG_ON(zhdr->first_chunks == 0 && zhdr->last_chunks == 0);
Again here.
Thanks,
Seth
> + /* Move this last element to beginning of LRU */
> list_del(&zhdr->lru);
> - list_del(&zhdr->buddy);
> + list_add(&zhdr->lru, &pool->lru);
> /* Protect zbud page against free */
> - zhdr->under_reclaim = true;
> + get_zbud_page(zhdr);
> /*
> * We need encode the handles before unlocking, since we can
> * race with free that will set (first|last)_chunks to 0
> @@ -441,28 +481,10 @@ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> }
> next:
> spin_lock(&pool->lock);
> - zhdr->under_reclaim = false;
> - if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
> - /*
> - * Both buddies are now free, free the zbud page and
> - * return success.
> - */
> - free_zbud_page(zhdr);
> - pool->pages_nr--;
> + if (put_zbud_page(pool, zhdr)) {
> spin_unlock(&pool->lock);
> return 0;
> - } else if (zhdr->first_chunks == 0 ||
> - zhdr->last_chunks == 0) {
> - /* 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);
> }
> -
> - /* add to beginning of LRU */
> - list_add(&zhdr->lru, &pool->lru);
> }
> spin_unlock(&pool->lock);
> return -EAGAIN;
> --
> 1.7.9.5
>
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