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Date: Mon, 5 Jun 2023 16:37:46 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Domenico Cerasuolo <cerasuolodomenico@...il.com>
Cc: vitaly.wool@...sulko.com, minchan@...nel.org,
senozhatsky@...omium.org, yosryahmed@...gle.com,
linux-mm@...ck.org, ddstreet@...e.org, sjenning@...hat.com,
hannes@...xchg.org, akpm@...ux-foundation.org,
linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [RFC PATCH 4/7] mm: zswap: remove page reclaim logic from zsmalloc
On Mon, Jun 5, 2023 at 1:54 AM Domenico Cerasuolo
<cerasuolodomenico@...il.com> wrote:
>
> With the recent enhancement to zswap enabling direct page writeback, the
> need for the shrink code in zsmalloc has become obsolete. As a result,
> this commit removes the page reclaim logic from zsmalloc entirely.
>
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@...il.com>
> ---
> mm/zsmalloc.c | 291 +-------------------------------------------------
> 1 file changed, 2 insertions(+), 289 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 02f7f414aade..c87a60514f21 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -250,13 +250,6 @@ struct zs_pool {
> /* Compact classes */
> struct shrinker shrinker;
>
> -#ifdef CONFIG_ZPOOL
> - /* List tracking the zspages in LRU order by most recently added object */
> - struct list_head lru;
> - struct zpool *zpool;
> - const struct zpool_ops *zpool_ops;
> -#endif
> -
> #ifdef CONFIG_ZSMALLOC_STAT
> struct dentry *stat_dentry;
> #endif
> @@ -279,13 +272,6 @@ struct zspage {
> unsigned int freeobj;
> struct page *first_page;
> struct list_head list; /* fullness list */
> -
> -#ifdef CONFIG_ZPOOL
> - /* links the zspage to the lru list in the pool */
> - struct list_head lru;
> - bool under_reclaim;
> -#endif
> -
> struct zs_pool *pool;
> rwlock_t lock;
> };
> @@ -393,14 +379,7 @@ static void *zs_zpool_create(const char *name, gfp_t gfp,
> * different contexts and its caller must provide a valid
> * gfp mask.
> */
> - struct zs_pool *pool = zs_create_pool(name);
> -
> - if (pool) {
> - pool->zpool = zpool;
> - pool->zpool_ops = zpool_ops;
> - }
> -
> - return pool;
> + return zs_create_pool(name);
> }
>
> static void zs_zpool_destroy(void *pool)
> @@ -422,27 +401,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
> zs_free(pool, handle);
> }
>
> -static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries);
> -
> -static int zs_zpool_shrink(void *pool, unsigned int pages,
> - unsigned int *reclaimed)
> -{
> - unsigned int total = 0;
> - int ret = -EINVAL;
> -
> - while (total < pages) {
> - ret = zs_reclaim_page(pool, 8);
> - if (ret < 0)
> - break;
> - total++;
> - }
> -
> - if (reclaimed)
> - *reclaimed = total;
> -
> - return ret;
> -}
> -
> static void *zs_zpool_map(void *pool, unsigned long handle,
> enum zpool_mapmode mm)
> {
> @@ -481,7 +439,7 @@ static struct zpool_driver zs_zpool_driver = {
> .malloc_support_movable = true,
> .malloc = zs_zpool_malloc,
> .free = zs_zpool_free,
> - .shrink = zs_zpool_shrink,
> + .shrink = NULL,
> .map = zs_zpool_map,
> .unmap = zs_zpool_unmap,
> .total_size = zs_zpool_total_size,
> @@ -884,14 +842,6 @@ static inline bool obj_allocated(struct page *page, void *obj, unsigned long *ph
> return obj_tagged(page, obj, phandle, OBJ_ALLOCATED_TAG);
> }
>
> -#ifdef CONFIG_ZPOOL
> -static bool obj_stores_deferred_handle(struct page *page, void *obj,
> - unsigned long *phandle)
> -{
> - return obj_tagged(page, obj, phandle, OBJ_DEFERRED_HANDLE_TAG);
> -}
> -#endif
> -
> static void reset_page(struct page *page)
> {
> __ClearPageMovable(page);
> @@ -1006,9 +956,6 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
> }
>
> remove_zspage(class, zspage, ZS_INUSE_RATIO_0);
> -#ifdef CONFIG_ZPOOL
> - list_del(&zspage->lru);
> -#endif
> __free_zspage(pool, class, zspage);
> }
>
> @@ -1054,11 +1001,6 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
> off %= PAGE_SIZE;
> }
>
> -#ifdef CONFIG_ZPOOL
> - INIT_LIST_HEAD(&zspage->lru);
> - zspage->under_reclaim = false;
> -#endif
> -
> set_freeobj(zspage, 0);
> }
>
> @@ -1525,13 +1467,6 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
> /* We completely set up zspage so mark them as movable */
> SetZsPageMovable(pool, zspage);
> out:
> -#ifdef CONFIG_ZPOOL
> - /* Add/move zspage to beginning of LRU */
> - if (!list_empty(&zspage->lru))
> - list_del(&zspage->lru);
> - list_add(&zspage->lru, &pool->lru);
> -#endif
> -
> spin_unlock(&pool->lock);
>
> return handle;
> @@ -1600,20 +1535,6 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
> class = zspage_class(pool, zspage);
>
> class_stat_dec(class, ZS_OBJS_INUSE, 1);
> -
> -#ifdef CONFIG_ZPOOL
> - if (zspage->under_reclaim) {
> - /*
> - * Reclaim needs the handles during writeback. It'll free
> - * them along with the zspage when it's done with them.
> - *
> - * Record current deferred handle in the object's header.
> - */
> - obj_free(class->size, obj, &handle);
> - spin_unlock(&pool->lock);
> - return;
> - }
> -#endif
> obj_free(class->size, obj, NULL);
>
> fullness = fix_fullness_group(class, zspage);
> @@ -1890,23 +1811,6 @@ static void lock_zspage(struct zspage *zspage)
> }
> #endif /* defined(CONFIG_ZPOOL) || defined(CONFIG_COMPACTION) */
If I recall correctly, the defined(CONFIG_ZPOOL) condition is
only needed for the lock_zspage() call in zs_reclaim_page().
You might be able to get away with just
#ifdef CONFIG_COMPACTION if you're removing writeback.
Do fact-check me of course - try to build with these CONFIGs turned
off and see. I'm surprised kernel test robot has not complained about
unused static function lock_zspage() in the case
CONFIG_ZPOOL && !CONFIG_COMPACTION
>
> -#ifdef CONFIG_ZPOOL
> -/*
> - * Unlocks all the pages of the zspage.
> - *
> - * pool->lock must be held before this function is called
> - * to prevent the underlying pages from migrating.
> - */
> -static void unlock_zspage(struct zspage *zspage)
> -{
> - struct page *page = get_first_page(zspage);
> -
> - do {
> - unlock_page(page);
> - } while ((page = get_next_page(page)) != NULL);
> -}
> -#endif /* CONFIG_ZPOOL */
> -
> static void migrate_lock_init(struct zspage *zspage)
> {
> rwlock_init(&zspage->lock);
> @@ -2126,9 +2030,6 @@ static void async_free_zspage(struct work_struct *work)
> VM_BUG_ON(fullness != ZS_INUSE_RATIO_0);
> class = pool->size_class[class_idx];
> spin_lock(&pool->lock);
> -#ifdef CONFIG_ZPOOL
> - list_del(&zspage->lru);
> -#endif
> __free_zspage(pool, class, zspage);
> spin_unlock(&pool->lock);
> }
> @@ -2474,10 +2375,6 @@ struct zs_pool *zs_create_pool(const char *name)
> */
> zs_register_shrinker(pool);
>
> -#ifdef CONFIG_ZPOOL
> - INIT_LIST_HEAD(&pool->lru);
> -#endif
> -
> return pool;
>
> err:
> @@ -2520,190 +2417,6 @@ void zs_destroy_pool(struct zs_pool *pool)
> }
> EXPORT_SYMBOL_GPL(zs_destroy_pool);
>
> -#ifdef CONFIG_ZPOOL
> -static void restore_freelist(struct zs_pool *pool, struct size_class *class,
> - struct zspage *zspage)
> -{
> - unsigned int obj_idx = 0;
> - unsigned long handle, off = 0; /* off is within-page offset */
> - struct page *page = get_first_page(zspage);
> - struct link_free *prev_free = NULL;
> - void *prev_page_vaddr = NULL;
> -
> - /* in case no free object found */
> - set_freeobj(zspage, (unsigned int)(-1UL));
> -
> - while (page) {
> - void *vaddr = kmap_atomic(page);
> - struct page *next_page;
> -
> - while (off < PAGE_SIZE) {
> - void *obj_addr = vaddr + off;
> -
> - /* skip allocated object */
> - if (obj_allocated(page, obj_addr, &handle)) {
> - obj_idx++;
> - off += class->size;
> - continue;
> - }
> -
> - /* free deferred handle from reclaim attempt */
> - if (obj_stores_deferred_handle(page, obj_addr, &handle))
> - cache_free_handle(pool, handle);
> -
> - if (prev_free)
> - prev_free->next = obj_idx << OBJ_TAG_BITS;
> - else /* first free object found */
> - set_freeobj(zspage, obj_idx);
> -
> - prev_free = (struct link_free *)vaddr + off / sizeof(*prev_free);
> - /* if last free object in a previous page, need to unmap */
> - if (prev_page_vaddr) {
> - kunmap_atomic(prev_page_vaddr);
> - prev_page_vaddr = NULL;
> - }
> -
> - obj_idx++;
> - off += class->size;
> - }
> -
> - /*
> - * Handle the last (full or partial) object on this page.
> - */
> - next_page = get_next_page(page);
> - if (next_page) {
> - if (!prev_free || prev_page_vaddr) {
> - /*
> - * There is no free object in this page, so we can safely
> - * unmap it.
> - */
> - kunmap_atomic(vaddr);
> - } else {
> - /* update prev_page_vaddr since prev_free is on this page */
> - prev_page_vaddr = vaddr;
> - }
> - } else { /* this is the last page */
> - if (prev_free) {
> - /*
> - * Reset OBJ_TAG_BITS bit to last link to tell
> - * whether it's allocated object or not.
> - */
> - prev_free->next = -1UL << OBJ_TAG_BITS;
> - }
> -
> - /* unmap previous page (if not done yet) */
> - if (prev_page_vaddr) {
> - kunmap_atomic(prev_page_vaddr);
> - prev_page_vaddr = NULL;
> - }
> -
> - kunmap_atomic(vaddr);
> - }
> -
> - page = next_page;
> - off %= PAGE_SIZE;
> - }
> -}
> -
> -static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries)
> -{
> - int i, obj_idx, ret = 0;
> - unsigned long handle;
> - struct zspage *zspage;
> - struct page *page;
> - int fullness;
> -
> - /* Lock LRU and fullness list */
> - spin_lock(&pool->lock);
> - if (list_empty(&pool->lru)) {
> - spin_unlock(&pool->lock);
> - return -EINVAL;
> - }
> -
> - for (i = 0; i < retries; i++) {
> - struct size_class *class;
> -
> - zspage = list_last_entry(&pool->lru, struct zspage, lru);
> - list_del(&zspage->lru);
> -
> - /* zs_free may free objects, but not the zspage and handles */
> - zspage->under_reclaim = true;
> -
> - class = zspage_class(pool, zspage);
> - fullness = get_fullness_group(class, zspage);
> -
> - /* Lock out object allocations and object compaction */
> - remove_zspage(class, zspage, fullness);
> -
> - spin_unlock(&pool->lock);
> - cond_resched();
> -
> - /* Lock backing pages into place */
> - lock_zspage(zspage);
> -
> - obj_idx = 0;
> - page = get_first_page(zspage);
> - while (1) {
> - handle = find_alloced_obj(class, page, &obj_idx);
> - if (!handle) {
> - page = get_next_page(page);
> - if (!page)
> - break;
> - obj_idx = 0;
> - continue;
> - }
> -
> - /*
> - * This will write the object and call zs_free.
> - *
> - * zs_free will free the object, but the
> - * under_reclaim flag prevents it from freeing
> - * the zspage altogether. This is necessary so
> - * that we can continue working with the
> - * zspage potentially after the last object
> - * has been freed.
> - */
> - ret = pool->zpool_ops->evict(pool->zpool, handle);
> - if (ret)
> - goto next;
> -
> - obj_idx++;
> - }
> -
> -next:
> - /* For freeing the zspage, or putting it back in the pool and LRU list. */
> - spin_lock(&pool->lock);
> - zspage->under_reclaim = false;
> -
> - if (!get_zspage_inuse(zspage)) {
> - /*
> - * Fullness went stale as zs_free() won't touch it
> - * while the page is removed from the pool. Fix it
> - * up for the check in __free_zspage().
> - */
> - zspage->fullness = ZS_INUSE_RATIO_0;
> -
> - __free_zspage(pool, class, zspage);
> - spin_unlock(&pool->lock);
> - return 0;
> - }
> -
> - /*
> - * Eviction fails on one of the handles, so we need to restore zspage.
> - * We need to rebuild its freelist (and free stored deferred handles),
> - * put it back to the correct size class, and add it to the LRU list.
> - */
> - restore_freelist(pool, class, zspage);
> - putback_zspage(class, zspage);
> - list_add(&zspage->lru, &pool->lru);
> - unlock_zspage(zspage);
> - }
> -
> - spin_unlock(&pool->lock);
> - return -EAGAIN;
> -}
> -#endif /* CONFIG_ZPOOL */
> -
> static int __init zs_init(void)
> {
> int ret;
> --
> 2.34.1
>
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