Re: [PATCH] mm/zswap: support multiple swap devices

[Minchan Kim]

Hello, Seth

On Mon, Feb 24, 2014 at 10:22:49AM -0600, Seth Jennings wrote:
> 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@xxxxxxxxxx>
> > Cc: Seth Jennings <sjennings@xxxxxxxxxxxxxx>
> > Reported-by: Cai Liu <cai.liu@xxxxxxxxxxx>
> > Suggested-by: Weijie Yang <weijie.yang.kh@xxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> >  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.

No problem. 

> 
> > +
> >  /*********************************
> >  * 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.

Right.

> 
> >  
> >  	/* 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.

Yeb.

> 
> > +		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!

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