[RFC PATCH v2 2/7] mm: zswap: remove page reclaim logic from zbud

Domenico Cerasuolo
2023-06-06T14:57:51Z

With the recent enhancement to zswap enabling direct page writeback, the
need for the shrink code in zbud has become obsolete. As a result, this
commit removes the page reclaim logic from zbud entirely.

Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
---
mm/zbud.c | 164 +-----------------------------------------------------
1 file changed, 2 insertions(+), 162 deletions(-)

diff --git a/mm/zbud.c b/mm/zbud.c
index 3acd26193920..19bc662ef5e9 100644
--- a/mm/zbud.c
+++ b/mm/zbud.c
@@ -83,11 +83,7 @@ struct zbud_pool;
* its free region.
* @buddied: list tracking the zbud pages that contain two buddies;
* these zbud pages are full
- * @lru: list tracking the zbud pages in LRU order by most recently
- * added buddy.
* @pages_nr: number of zbud pages in the pool.
- * @zpool: zpool driver
- * @zpool_ops: zpool operations structure with an evict callback
*
* This structure is allocated at pool creation time and maintains metadata
* pertaining to a particular zbud pool.
@@ -102,26 +98,20 @@ struct zbud_pool {
struct list_head buddied;
struct list_head unbuddied[NCHUNKS];
};
- struct list_head lru;
u64 pages_nr;
- struct zpool *zpool;
- const struct zpool_ops *zpool_ops;
};

/*
* struct zbud_header - zbud page metadata occupying the first chunk of each
* zbud page.
* @buddy: links the zbud page into the unbuddied/buddied lists in the pool
- * @lru: links the zbud page into the lru list in the pool
* @first_chunks: the size of the first buddy in chunks, 0 if free
* @last_chunks: the size of the last buddy in chunks, 0 if free
*/
struct zbud_header {
struct list_head buddy;
- struct list_head lru;
unsigned int first_chunks;
unsigned int last_chunks;
- bool under_reclaim;
};

/*****************
@@ -149,8 +139,6 @@ static struct zbud_header *init_zbud_page(struct page *page)
zhdr->first_chunks = 0;
zhdr->last_chunks = 0;
INIT_LIST_HEAD(&zhdr->buddy);
- INIT_LIST_HEAD(&zhdr->lru);
- zhdr->under_reclaim = false;
return zhdr;
}

@@ -221,7 +209,6 @@ static struct zbud_pool *zbud_create_pool(gfp_t gfp)
for_each_unbuddied_list(i, 0)
INIT_LIST_HEAD(&pool->unbuddied[i]);
INIT_LIST_HEAD(&pool->buddied);
- INIT_LIST_HEAD(&pool->lru);
pool->pages_nr = 0;
return pool;
}
@@ -310,11 +297,6 @@ static int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
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);
-
*handle = encode_handle(zhdr, bud);
spin_unlock(&pool->lock);

@@ -325,11 +307,6 @@ static int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
* zbud_free() - frees the allocation associated with the given handle
* @pool: pool in which the allocation resided
* @handle: handle associated with the allocation returned by zbud_alloc()
- *
- * In the case that the zbud page in which the allocation resides is under
- * reclaim, as indicated by the PG_reclaim flag being set, this function
- * only sets the first|last_chunks to 0. The page is actually freed
- * once both buddies are evicted (see zbud_reclaim_page() below).
*/
static void zbud_free(struct zbud_pool *pool, unsigned long handle)
{
@@ -345,18 +322,11 @@ static 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 {
@@ -368,110 +338,6 @@ static void zbud_free(struct zbud_pool *pool, unsigned long handle)
spin_unlock(&pool->lock);
}

-/**
- * zbud_reclaim_page() - evicts allocations from a pool page and frees it
- * @pool: pool from which a page will attempt to be evicted
- * @retries: number of pages on the LRU list for which eviction will
- * be attempted before failing
- *
- * zbud reclaim is different from normal system reclaim in that the reclaim is
- * done from the bottom, up. This is because only the bottom layer, zbud, has
- * information on how the allocations are organized within each zbud page. This
- * has the potential to create interesting locking situations between zbud and
- * the user, however.
- *
- * To avoid these, this is how zbud_reclaim_page() should be called:
- *
- * The user detects a page should be reclaimed and calls zbud_reclaim_page().
- * zbud_reclaim_page() will remove a zbud page from the pool LRU list and call
- * the user-defined eviction handler with the pool and handle as arguments.
- *
- * If the handle can not be evicted, the eviction handler should return
- * non-zero. zbud_reclaim_page() will add the zbud page back to the
- * appropriate list and try the next zbud page on the LRU up to
- * a user defined number of retries.
- *
- * If the handle is successfully evicted, the eviction handler should
- * return 0 _and_ should have called zbud_free() on the handle. zbud_free()
- * contains logic to delay freeing the page if the page is under reclaim,
- * as indicated by the setting of the PG_reclaim flag on the underlying page.
- *
- * If all buddies in the zbud page are successfully evicted, then the
- * zbud page can be freed.
- *
- * Returns: 0 if page is successfully freed, otherwise -EINVAL if there are
- * no pages to evict or an eviction handler is not registered, -EAGAIN if
- * the retry limit was hit.
- */
-static int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
-{
- int i, ret, freechunks;
- struct zbud_header *zhdr;
- unsigned long first_handle = 0, last_handle = 0;
-
- spin_lock(&pool->lock);
- if (list_empty(&pool->lru)) {
- spin_unlock(&pool->lock);
- return -EINVAL;
- }
- for (i = 0; i < retries; i++) {
- zhdr = list_last_entry(&pool->lru, struct zbud_header, lru);
- list_del(&zhdr->lru);
- list_del(&zhdr->buddy);
- /* Protect zbud page against free */
- zhdr->under_reclaim = true;
- /*
- * We need encode the handles before unlocking, since we can
- * race with free that will set (first|last)_chunks to 0
- */
- first_handle = 0;
- last_handle = 0;
- if (zhdr->first_chunks)
- first_handle = encode_handle(zhdr, FIRST);
- if (zhdr->last_chunks)
- last_handle = encode_handle(zhdr, LAST);
- spin_unlock(&pool->lock);
-
- /* Issue the eviction callback(s) */
- if (first_handle) {
- ret = pool->zpool_ops->evict(pool->zpool, first_handle);
- if (ret)
- goto next;
- }
- if (last_handle) {
- ret = pool->zpool_ops->evict(pool->zpool, last_handle);
- if (ret)
- goto next;
- }
-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--;
- 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;
-}
-
/**
* zbud_map() - maps the allocation associated with the given handle
* @pool: pool in which the allocation resides
@@ -518,14 +384,7 @@ static void *zbud_zpool_create(const char *name, gfp_t gfp,
const struct zpool_ops *zpool_ops,
struct zpool *zpool)
{
- struct zbud_pool *pool;
-
- pool = zbud_create_pool(gfp);
- if (pool) {
- pool->zpool = zpool;
- pool->zpool_ops = zpool_ops;
- }
- return pool;
+ return zbud_create_pool(gfp);
}

static void zbud_zpool_destroy(void *pool)
@@ -543,25 +402,6 @@ static void zbud_zpool_free(void *pool, unsigned long handle)
zbud_free(pool, handle);
}

-static int zbud_zpool_shrink(void *pool, unsigned int pages,
- unsigned int *reclaimed)
-{
- unsigned int total = 0;
- int ret = -EINVAL;
-
- while (total < pages) {
- ret = zbud_reclaim_page(pool, 8);
- if (ret < 0)
- break;
- total++;
- }
-
- if (reclaimed)
- *reclaimed = total;
-
- return ret;
-}
-
static void *zbud_zpool_map(void *pool, unsigned long handle,
enum zpool_mapmode mm)
{
@@ -585,7 +425,7 @@ static struct zpool_driver zbud_zpool_driver = {
.destroy = zbud_zpool_destroy,
.malloc = zbud_zpool_malloc,
.free = zbud_zpool_free,
- .shrink = zbud_zpool_shrink,
+ .shrink = NULL,
.map = zbud_zpool_map,
.unmap = zbud_zpool_unmap,
.total_size = zbud_zpool_total_size,
--
2.34.1