Re: [PATCH v2] mm/zswap: Check all pool pages instead of one pool pages

Cai Liu
2014-01-23T06:30:19Z

Hello Minchan

2014/1/23 Minchan Kim <minchan@kernel.org>:
> Hello Cai,
>
> On Thu, Jan 23, 2014 at 09:38:41AM +0800, Cai Liu wrote:
>> Hello Dan
>>
>> 2014/1/22 Dan Streetman <ddstreet@ieee.org>:
>> > On Wed, Jan 22, 2014 at 7:16 AM, Cai Liu <liucai.lfn@gmail.com> wrote:
>> >> Hello Minchan
>> >>
>> >>
>> >> 2014/1/22 Minchan Kim <minchan@kernel.org>
>> >>>
>> >>> Hello Cai,
>> >>>
>> >>> On Tue, Jan 21, 2014 at 09:52:25PM +0800, Cai Liu wrote:
>> >>> > Hello Minchan
>> >>> >
>> >>> > 2014/1/21 Minchan Kim <minchan@kernel.org>:
>> >>> > > Hello,
>> >>> > >
>> >>> > > On Tue, Jan 21, 2014 at 02:35:07PM +0800, Cai Liu wrote:
>> >>> > >> 2014/1/21 Minchan Kim <minchan@kernel.org>:
>> >>> > >> > Please check your MUA and don't break thread.
>> >>> > >> >
>> >>> > >> > On Tue, Jan 21, 2014 at 11:07:42AM +0800, Cai Liu wrote:
>> >>> > >> >> Thanks for your review.
>> >>> > >> >>
>> >>> > >> >> 2014/1/21 Minchan Kim <minchan@kernel.org>:
>> >>> > >> >> > Hello Cai,
>> >>> > >> >> >
>> >>> > >> >> > On Mon, Jan 20, 2014 at 03:50:18PM +0800, Cai Liu wrote:
>> >>> > >> >> >> zswap can support multiple swapfiles. So we need to check
>> >>> > >> >> >> all zbud pool pages in zswap.
>> >>> > >> >> >>
>> >>> > >> >> >> Version 2:
>> >>> > >> >> >> * add *total_zbud_pages* in zbud to record all the pages in pools
>> >>> > >> >> >> * move the updating of pool pages statistics to
>> >>> > >> >> >> alloc_zbud_page/free_zbud_page to hide the details
>> >>> > >> >> >>
>> >>> > >> >> >> Signed-off-by: Cai Liu <cai.liu@samsung.com>
>> >>> > >> >> >> ---
>> >>> > >> >> >> include/linux/zbud.h | 2 +-
>> >>> > >> >> >> mm/zbud.c | 44 ++++++++++++++++++++++++++++++++------------
>> >>> > >> >> >> mm/zswap.c | 4 ++--
>> >>> > >> >> >> 3 files changed, 35 insertions(+), 15 deletions(-)
>> >>> > >> >> >>
>> >>> > >> >> >> diff --git a/include/linux/zbud.h b/include/linux/zbud.h
>> >>> > >> >> >> index 2571a5c..1dbc13e 100644
>> >>> > >> >> >> --- a/include/linux/zbud.h
>> >>> > >> >> >> +++ b/include/linux/zbud.h
>> >>> > >> >> >> @@ -17,6 +17,6 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle);
>> >>> > >> >> >> int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries);
>> >>> > >> >> >> void *zbud_map(struct zbud_pool *pool, unsigned long handle);
>> >>> > >> >> >> void zbud_unmap(struct zbud_pool *pool, unsigned long handle);
>> >>> > >> >> >> -u64 zbud_get_pool_size(struct zbud_pool *pool);
>> >>> > >> >> >> +u64 zbud_get_pool_size(void);
>> >>> > >> >> >>
>> >>> > >> >> >> #endif /* _ZBUD_H_ */
>> >>> > >> >> >> diff --git a/mm/zbud.c b/mm/zbud.c
>> >>> > >> >> >> index 9451361..711aaf4 100644
>> >>> > >> >> >> --- a/mm/zbud.c
>> >>> > >> >> >> +++ b/mm/zbud.c
>> >>> > >> >> >> @@ -52,6 +52,13 @@
>> >>> > >> >> >> #include <linux/spinlock.h>
>> >>> > >> >> >> #include <linux/zbud.h>
>> >>> > >> >> >>
>> >>> > >> >> >> +/*********************************
>> >>> > >> >> >> +* statistics
>> >>> > >> >> >> +**********************************/
>> >>> > >> >> >> +
>> >>> > >> >> >> +/* zbud pages in all pools */
>> >>> > >> >> >> +static u64 total_zbud_pages;
>> >>> > >> >> >> +
>> >>> > >> >> >> /*****************
>> >>> > >> >> >> * Structures
>> >>> > >> >> >> *****************/
>> >>> > >> >> >> @@ -142,10 +149,28 @@ static struct zbud_header *init_zbud_page(struct page *page)
>> >>> > >> >> >> return zhdr;
>> >>> > >> >> >> }
>> >>> > >> >> >>
>> >>> > >> >> >> +static struct page *alloc_zbud_page(struct zbud_pool *pool, gfp_t gfp)
>> >>> > >> >> >> +{
>> >>> > >> >> >> + struct page *page;
>> >>> > >> >> >> +
>> >>> > >> >> >> + page = alloc_page(gfp);
>> >>> > >> >> >> +
>> >>> > >> >> >> + if (page) {
>> >>> > >> >> >> + pool->pages_nr++;
>> >>> > >> >> >> + total_zbud_pages++;
>> >>> > >> >> >
>> >>> > >> >> > Who protect race?
>> >>> > >> >>
>> >>> > >> >> Yes, here the pool->pages_nr and also the total_zbud_pages are not protected.
>> >>> > >> >> I will re-do it.
>> >>> > >> >>
>> >>> > >> >> I will change *total_zbud_pages* to atomic type.
>> >>> > >> >
>> >>> > >> > Wait, it doesn't make sense. Now, you assume zbud allocator would be used
>> >>> > >> > for only zswap. It's true until now but we couldn't make sure it in future.
>> >>> > >> > If other user start to use zbud allocator, total_zbud_pages would be pointless.
>> >>> > >>
>> >>> > >> Yes, you are right. ZBUD is a common module. So in this patch calculate the
>> >>> > >> zswap pool size in zbud is not suitable.
>> >>> > >>
>> >>> > >> >
>> >>> > >> > Another concern is that what's your scenario for above two swap?
>> >>> > >> > How often we need to call zbud_get_pool_size?
>> >>> > >> > In previous your patch, you reduced the number of call so IIRC,
>> >>> > >> > we only called it in zswap_is_full and for debugfs.
>> >>> > >>
>> >>> > >> zbud_get_pool_size() is called frequently when adding/freeing zswap
>> >>> > >> entry happen in zswap . This is why in this patch I added a counter in zbud,
>> >>> > >> and then in zswap the iteration of zswap_list to calculate the pool size will
>> >>> > >> not be needed.
>> >>> > >
>> >>> > > We can remove updating zswap_pool_pages in zswap_frontswap_store and
>> >>> > > zswap_free_entry as I said. So zswap_is_full is only hot spot.
>> >>> > > Do you think it's still big overhead? Why? Maybe locking to prevent
>> >>> > > destroying? Then, we can use RCU to minimize the overhead as I mentioned.
>> >>> >
>> >>> > I get your point. Yes, In my previous patch, zswap_is_full() was the
>> >>> > only path to call
>> >>> > zbud_get_pool_size(). And your suggestion on patch v1 to remove the unnecessary
>> >>> > iteration will reduce the overhead further.
>> >>> >
>> >>> > So adding the calculating of all the pool size in zswap.c is better.
>> >>> >
>> >>> > >>
>> >>> > >> > Of course, it would need some lock or refcount to prevent destroy
>> >>> > >> > of zswap_tree in parallel with zswap_frontswap_invalidate_area but
>> >>> > >> > zswap_is_full doesn't need to be exact so RCU would be good fit.
>> >>> > >> >
>> >>> > >> > Most important point is that now zswap doesn't consider multiple swap.
>> >>> > >> > For example, Let's assume you uses 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%)i 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.
>> >>> > >> >
>> >>> > >>
>> >>> > >> The scenario is below:
>> >>> > >> There are 2 swap A, B in system. If pool size of A reach 19% of ram
>> >>> > >> size and swap A
>> >>> > >> is also full. Then swap B will be used. Pool size of B will be
>> >>> > >> increased until it hit
>> >>> > >> the 20% of the ram size. By now zswap pool size is about 39% of ram size.
>> >>> > >> If there are more than 2 swap file/device, zswap pool will expand out
>> >>> > >> of control
>> >>> > >> and there may be no swapout happened.
>> >>> > >
>> >>> > > I know.
>> >>> > >
>> >>> > >>
>> >>> > >> I think the original intention of zswap designer is to keep the total
>> >>> > >> zswap pools size below
>> >>> > >> 20% of RAM size.
>> >>> > >
>> >>> > > My point is your patch still doesn't solve the example I mentioned.
>> >>> >
>> >>> > Hmm. My patch only make sure all the zswap pools use maximum 20% of
>> >>> > RAM size. It is a new problem in your example. The zbud_reclaim_page would
>> >>> > not swap out the oldest zbud page when multiple swaps are used.
>> >>> >
>> >>> > Maybe the new problem can be resolved in another patch.
>> >>>
>> >>> It means current zswap has a problem in multiple swap but you want
>> >>> to fix a problem which happens only when it is used for multiple swap.
>> >>> So, I'm not sure we want a fix in this phase before discussing more
>> >>> fundamental thing.
>> >>>
>> >>
>> >> Yes, The bug which I want to fix only happens when multiple swap are used.
>> >>
>> >>> That's why I want to know why you want to use multiple swap with zswap
>> >>> but you are never saying it to us. :(
>> >>>
>> >>
>> >> If user uses more than one swap device/file, then this is an issue.
>> >> Zswap pool is created when a swap device/file is swapped on happens.
>> >> So there will be more than one zswap pool when user uses 2 or even
>> >> more swap devices/files.
>> >>
>> >> I am not sure whether multiple swap are popular. But if multiple swap
>> >> are swapped
>> >> on, then multiple zswap pool will be created. And the size of these pools may
>> >> out of control.
>> >
>> > Personally I don't think using multiple swap partitions/files has to
>> > be popular to need to solve this, it only needs to be possible, which
>> > it is.
>> >
>> > Why not just leave zbud unchanged, and sum up the total size using a
>> > list of active zswap_trees as Minchan suggested for the v1 patch? The
>>
>> Yes. This is what I want to do in the v3 patch after this bug is considered need
>> to be fixed.
>
> In my position, I'd like to fix zswap and multiple swap problem firstly
> and like the Weijie's suggestion.
>
> So, how about this?
> I didn't look at code in detail and want to show the concept.

I read the RFC patch. I think it's perfect.

> That's why I added RFC tag.
>
> From 67c64746e977a091ee30ca37bbc034990adf5ca5 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@kernel.org>
> Date: Thu, 23 Jan 2014 11:41:44 +0900
> Subject: [RFC] zswap: support multiple swap
>
> Cai Liu reporeted that now zbud pool pages counting has a problem
> when multiple swap is used because it just counts one of swap
> among mutliple 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.
>
> Reported-by: Cai Liu <cai.liu@samsung.com>
> Suggested-by: Weijie Yang <weijie.yang.kh@gmail.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> mm/zswap.c | 56 +++++++++++++++++++++++++++++---------------------------
> 1 file changed, 29 insertions(+), 27 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 5a63f78a5601..96039e86db79 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -89,6 +89,8 @@ static unsigned int zswap_max_pool_percent = 20;
> module_param_named(max_pool_percent,
> zswap_max_pool_percent, uint, 0644);
>
> +static struct zbud_pool *mem_pool;
> +
> /*********************************
> * compression functions
> **********************************/
> @@ -189,7 +191,6 @@ 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];
> @@ -288,10 +289,10 @@ static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
> static void zswap_free_entry(struct zswap_tree *tree,
> struct zswap_entry *entry)
> {
> - zbud_free(tree->pool, entry->handle);
> + zbud_free(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(mem_pool);
> }
>
> /* caller must hold the tree lock */
> @@ -545,7 +546,7 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
> zbud_unmap(pool, handle);
> tree = zswap_trees[swp_type(swpentry)];
> offset = swp_offset(swpentry);
> - BUG_ON(pool != tree->pool);
> + BUG_ON(pool != mem_pool);
>
> /* 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(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(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(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(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(mem_pool, handle);
> zhdr->swpentry = swp_entry(type, offset);
> buf = (u8 *)(zhdr + 1);
> memcpy(buf, dst, dlen);
> - zbud_unmap(tree->pool, handle);
> + zbud_unmap(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(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(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(mem_pool, entry->handle);
> BUG_ON(ret);
>
> spin_lock(&tree->lock);
> @@ -807,8 +808,6 @@ static void zswap_frontswap_invalidate_area(unsigned type)
> zswap_free_entry(tree, 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");
> +
> + mem_pool = zbud_create_pool(GFP_KERNEL, &zswap_zbud_ops);
> + if (!mem_pool)
> + 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;
> }
> +
> +
> 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(mem_pool);
> error:
> return -ENOMEM;
> }
> --
> 1.8.5.2
>
>
> --
> Kind regards,
> Minchan Kim