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Date: Mon, 24 Feb 2014 10:22:49 -0600
From: Seth Jennings <sjennings@...iantweb.net>
To: Minchan Kim <minchan@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
cai.liu@...sung.com, weijie.yang.kh@...il.com,
Bob Liu <bob.liu@...cle.com>
Subject: Re: [PATCH] mm/zswap: support multiple swap devices
On Mon, Feb 24, 2014 at 02:13:25PM +0900, Minchan Kim wrote:
> Cai Liu reporeted that now zbud pool pages counting has a problem
> when multiple swap is used because it just counts only one swap
> intead of all of swap so zswap cannot control writeback properly.
> The result is unnecessary writeback or no writeback when we should
> really writeback.
>
> IOW, it made zswap crazy.
>
> Another problem in zswap is following as.
> For example, let's assume we use two swap A and B with different
> priority and A already has charged 19% long time ago and let's assume
> that A swap is full now so VM start to use B so that B has charged 1%
> recently. It menas zswap charged (19% + 1%) is full by default.
> Then, if VM want to swap out more pages into B, zbud_reclaim_page
> would be evict one of pages in B's pool and it would be repeated
> continuously. It's totally LRU reverse problem and swap thrashing
> in B would happen.
>
> This patch makes zswap consider mutliple swap by creating *a* zbud
> pool which will be shared by multiple swap so all of zswap pages
> in multiple swap keep order by LRU so it can prevent above two
> problems.
I don't have any issues with it at first glance. Then again
I can't remember the rationale for using a pool per swaptype...
>
> Cc: Bob Liu <bob.liu@...cle.com>
> Cc: Seth Jennings <sjennings@...iantweb.net>
> Reported-by: Cai Liu <cai.liu@...sung.com>
> Suggested-by: Weijie Yang <weijie.yang.kh@...il.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
> mm/zswap.c | 64 ++++++++++++++++++++++++++++++++------------------------------
> 1 file changed, 33 insertions(+), 31 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 25312eb373a0..7ab2b36e3340 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -89,6 +89,9 @@ static unsigned int zswap_max_pool_percent = 20;
> module_param_named(max_pool_percent,
> zswap_max_pool_percent, uint, 0644);
>
> +/* zbud_pool is shared by all of zswap backend */
> +static struct zbud_pool *shared_mem_pool;
nit, maybe "zswap_pool" or "zswap_zbud_pool" just to keep with
the zswap_* convention.
> +
> /*********************************
> * compression functions
> **********************************/
> @@ -189,7 +192,6 @@ struct zswap_header {
> struct zswap_tree {
> struct rb_root rbroot;
> spinlock_t lock;
> - struct zbud_pool *pool;
> };
>
> static struct zswap_tree *zswap_trees[MAX_SWAPFILES];
> @@ -285,13 +287,12 @@ static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
> * Carries out the common pattern of freeing and entry's zbud allocation,
> * freeing the entry itself, and decrementing the number of stored pages.
> */
> -static void zswap_free_entry(struct zswap_tree *tree,
> - struct zswap_entry *entry)
> +static void zswap_free_entry(struct zswap_entry *entry)
> {
> - zbud_free(tree->pool, entry->handle);
> + zbud_free(shared_mem_pool, entry->handle);
> zswap_entry_cache_free(entry);
> atomic_dec(&zswap_stored_pages);
> - zswap_pool_pages = zbud_get_pool_size(tree->pool);
> + zswap_pool_pages = zbud_get_pool_size(shared_mem_pool);
> }
>
> /* caller must hold the tree lock */
> @@ -311,7 +312,7 @@ static void zswap_entry_put(struct zswap_tree *tree,
> BUG_ON(refcount < 0);
> if (refcount == 0) {
> zswap_rb_erase(&tree->rbroot, entry);
> - zswap_free_entry(tree, entry);
> + zswap_free_entry(entry);
> }
> }
>
> @@ -545,7 +546,7 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
we can probably take the zbud_pool arg out right?
> zbud_unmap(pool, handle);
> tree = zswap_trees[swp_type(swpentry)];
> offset = swp_offset(swpentry);
> - BUG_ON(pool != tree->pool);
> + BUG_ON(pool != shared_mem_pool);
in which case, this isn't needed.
>
> /* find and ref zswap entry */
> spin_lock(&tree->lock);
> @@ -573,13 +574,13 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
> case ZSWAP_SWAPCACHE_NEW: /* page is locked */
> /* decompress */
> dlen = PAGE_SIZE;
> - src = (u8 *)zbud_map(tree->pool, entry->handle) +
> + src = (u8 *)zbud_map(shared_mem_pool, entry->handle) +
> sizeof(struct zswap_header);
> dst = kmap_atomic(page);
> ret = zswap_comp_op(ZSWAP_COMPOP_DECOMPRESS, src,
> entry->length, dst, &dlen);
> kunmap_atomic(dst);
> - zbud_unmap(tree->pool, entry->handle);
> + zbud_unmap(shared_mem_pool, entry->handle);
> BUG_ON(ret);
> BUG_ON(dlen != PAGE_SIZE);
>
> @@ -652,7 +653,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> /* reclaim space if needed */
> if (zswap_is_full()) {
> zswap_pool_limit_hit++;
> - if (zbud_reclaim_page(tree->pool, 8)) {
> + if (zbud_reclaim_page(shared_mem_pool, 8)) {
> zswap_reject_reclaim_fail++;
> ret = -ENOMEM;
> goto reject;
> @@ -679,7 +680,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>
> /* store */
> len = dlen + sizeof(struct zswap_header);
> - ret = zbud_alloc(tree->pool, len, __GFP_NORETRY | __GFP_NOWARN,
> + ret = zbud_alloc(shared_mem_pool, len, __GFP_NORETRY | __GFP_NOWARN,
> &handle);
> if (ret == -ENOSPC) {
> zswap_reject_compress_poor++;
> @@ -689,11 +690,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> zswap_reject_alloc_fail++;
> goto freepage;
> }
> - zhdr = zbud_map(tree->pool, handle);
> + zhdr = zbud_map(shared_mem_pool, handle);
> zhdr->swpentry = swp_entry(type, offset);
> buf = (u8 *)(zhdr + 1);
> memcpy(buf, dst, dlen);
> - zbud_unmap(tree->pool, handle);
> + zbud_unmap(shared_mem_pool, handle);
> put_cpu_var(zswap_dstmem);
>
> /* populate entry */
> @@ -716,7 +717,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>
> /* update stats */
> atomic_inc(&zswap_stored_pages);
> - zswap_pool_pages = zbud_get_pool_size(tree->pool);
> + zswap_pool_pages = zbud_get_pool_size(shared_mem_pool);
>
> return 0;
>
> @@ -752,13 +753,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>
> /* decompress */
> dlen = PAGE_SIZE;
> - src = (u8 *)zbud_map(tree->pool, entry->handle) +
> + src = (u8 *)zbud_map(shared_mem_pool, entry->handle) +
> sizeof(struct zswap_header);
> dst = kmap_atomic(page);
> ret = zswap_comp_op(ZSWAP_COMPOP_DECOMPRESS, src, entry->length,
> dst, &dlen);
> kunmap_atomic(dst);
> - zbud_unmap(tree->pool, entry->handle);
> + zbud_unmap(shared_mem_pool, entry->handle);
> BUG_ON(ret);
>
> spin_lock(&tree->lock);
> @@ -804,11 +805,9 @@ static void zswap_frontswap_invalidate_area(unsigned type)
> /* walk the tree and free everything */
> spin_lock(&tree->lock);
> rbtree_postorder_for_each_entry_safe(entry, n, &tree->rbroot, rbnode)
> - zswap_free_entry(tree, entry);
> + zswap_free_entry(entry);
> tree->rbroot = RB_ROOT;
> spin_unlock(&tree->lock);
> -
> - zbud_destroy_pool(tree->pool);
> kfree(tree);
> zswap_trees[type] = NULL;
> }
> @@ -822,20 +821,14 @@ static void zswap_frontswap_init(unsigned type)
> struct zswap_tree *tree;
>
> tree = kzalloc(sizeof(struct zswap_tree), GFP_KERNEL);
> - if (!tree)
> - goto err;
> - tree->pool = zbud_create_pool(GFP_KERNEL, &zswap_zbud_ops);
> - if (!tree->pool)
> - goto freetree;
> + if (!tree) {
> + pr_err("alloc failed, zswap disabled for swap type %d\n", type);
> + return;
> + }
> +
> tree->rbroot = RB_ROOT;
> spin_lock_init(&tree->lock);
> zswap_trees[type] = tree;
> - return;
> -
> -freetree:
> - kfree(tree);
> -err:
> - pr_err("alloc failed, zswap disabled for swap type %d\n", type);
> }
>
> static struct frontswap_ops zswap_frontswap_ops = {
> @@ -907,9 +900,14 @@ static int __init init_zswap(void)
> return 0;
>
> pr_info("loading zswap\n");
> +
> + shared_mem_pool = zbud_create_pool(GFP_KERNEL, &zswap_zbud_ops);
> + if (!shared_mem_pool)
need an error message here.
> + goto error;
> +
> if (zswap_entry_cache_create()) {
> pr_err("entry cache creation failed\n");
> - goto error;
> + goto cachefail;
> }
> if (zswap_comp_init()) {
> pr_err("compressor initialization failed\n");
> @@ -919,6 +917,8 @@ static int __init init_zswap(void)
> pr_err("per-cpu initialization failed\n");
> goto pcpufail;
> }
> +
> +
new whitesapce?
Thanks for your work on this!
Seth
> frontswap_register_ops(&zswap_frontswap_ops);
> if (zswap_debugfs_init())
> pr_warn("debugfs initialization failed\n");
> @@ -927,6 +927,8 @@ pcpufail:
> zswap_comp_exit();
> compfail:
> zswap_entry_cache_destory();
> +cachefail:
> + zbud_destroy_pool(shared_mem_pool);
> error:
> return -ENOMEM;
> }
> --
> 1.8.5.3
>
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