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

Cai Liu
2014-01-23T01:38:43Z

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.

Thanks.

> debugfs_create_u64("pool_pages") will probably need to be changed to
> debugfs_create_file() with a read function that calls the function to
> sum up the total.
>
>
>>
>> Thanks.
>>
>>> >
>>> > Thanks.
>>> >
>>> > >
>>> > >>
>>> > >> Thanks.
>>> > >>
>>> > >> > Please say your usecase scenario and if it's really problem,
>>> > >> > we need more surgery.
>>> > >> >
>>> > >> > Thanks.
>>> > >> >
>>> > >> >> For *pool->pages_nr*, one way is to use pool->lock to protect. But I
>>> > >> >> think it is too heavy.
>>> > >> >> So does it ok to change pages_nr to atomic type too?
>>> > >> >>
>>> > >> >>
>>> > >> >> >
>>> > >> >> >> + }
>>> > >> >> >> +
>>> > >> >> >> + return page;
>>> > >> >> >> +}
>>> > >> >> >> +
>>> > >> >> >> +
>>> > >> >> >> /* Resets the struct page fields and frees the page */
>>> > >> >> >> -static void free_zbud_page(struct zbud_header *zhdr)
>>> > >> >> >> +static void free_zbud_page(struct zbud_pool *pool, struct zbud_header *zhdr)
>>> > >> >> >> {
>>> > >> >> >> __free_page(virt_to_page(zhdr));
>>> > >> >> >> +
>>> > >> >> >> + pool->pages_nr--;
>>> > >> >> >> + total_zbud_pages--;
>>> > >> >> >> }
>>> > >> >> >>
>>> > >> >> >> /*
>>> > >> >> >> @@ -279,11 +304,10 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
>>> > >> >> >>
>>> > >> >> >> /* Couldn't find unbuddied zbud page, create new one */
>>> > >> >> >> spin_unlock(&pool->lock);
>>> > >> >> >> - page = alloc_page(gfp);
>>> > >> >> >> + page = alloc_zbud_page(pool, gfp);
>>> > >> >> >> if (!page)
>>> > >> >> >> return -ENOMEM;
>>> > >> >> >> spin_lock(&pool->lock);
>>> > >> >> >> - pool->pages_nr++;
>>> > >> >> >> zhdr = init_zbud_page(page);
>>> > >> >> >> bud = FIRST;
>>> > >> >> >>
>>> > >> >> >> @@ -349,8 +373,7 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle)
>>> > >> >> >> 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--;
>>> > >> >> >> + free_zbud_page(pool, zhdr);
>>> > >> >> >> } else {
>>> > >> >> >> /* Add to unbuddied list */
>>> > >> >> >> freechunks = num_free_chunks(zhdr);
>>> > >> >> >> @@ -447,8 +470,7 @@ next:
>>> > >> >> >> * Both buddies are now free, free the zbud page and
>>> > >> >> >> * return success.
>>> > >> >> >> */
>>> > >> >> >> - free_zbud_page(zhdr);
>>> > >> >> >> - pool->pages_nr--;
>>> > >> >> >> + free_zbud_page(pool, zhdr);
>>> > >> >> >> spin_unlock(&pool->lock);
>>> > >> >> >> return 0;
>>> > >> >> >> } else if (zhdr->first_chunks == 0 ||
>>> > >> >> >> @@ -496,14 +518,12 @@ void zbud_unmap(struct zbud_pool *pool, unsigned long handle)
>>> > >> >> >>
>>> > >> >> >> /**
>>> > >> >> >> * zbud_get_pool_size() - gets the zbud pool size in pages
>>> > >> >> >> - * @pool: pool whose size is being queried
>>> > >> >> >> *
>>> > >> >> >> - * Returns: size in pages of the given pool. The pool lock need not be
>>> > >> >> >> - * taken to access pages_nr.
>>> > >> >> >> + * Returns: size in pages of all the zbud pools.
>>> > >> >> >> */
>>> > >> >> >> -u64 zbud_get_pool_size(struct zbud_pool *pool)
>>> > >> >> >> +u64 zbud_get_pool_size(void)
>>> > >> >> >> {
>>> > >> >> >> - return pool->pages_nr;
>>> > >> >> >> + return total_zbud_pages;
>>> > >> >> >> }
>>> > >> >> >>
>>> > >> >> >> static int __init init_zbud(void)
>>> > >> >> >> diff --git a/mm/zswap.c b/mm/zswap.c
>>> > >> >> >> index 5a63f78..ef44d9d 100644
>>> > >> >> >> --- a/mm/zswap.c
>>> > >> >> >> +++ b/mm/zswap.c
>>> > >> >> >> @@ -291,7 +291,7 @@ static void zswap_free_entry(struct zswap_tree *tree,
>>> > >> >> >> zbud_free(tree->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();
>>> > >> >> >> }
>>> > >> >> >>
>>> > >> >> >> /* caller must hold the tree lock */
>>> > >> >> >> @@ -716,7 +716,7 @@ 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();
>>> > >> >> >>
>>> > >> >> >> return 0;
>>> > >> >> >>
>>> > >> >> >> --
>>> > >> >> >> 1.7.10.4
>>> > >> >> >>
>>> > >> >> >> --
>>> > >> >> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>>> > >> >> >> the body to majordomo@kvack.org. For more info on Linux MM,
>>> > >> >> >> see: http://www.linux-mm.org/ .
>>> > >> >> >> Don't email: email@kvack.org
>>> > >> >> >
>>> > >> >> > --
>>> > >> >> > Kind regards,
>>> > >> >> > Minchan Kim
>>> > >> >>
>>> > >> >> --
>>> > >> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>>> > >> >> the body to majordomo@kvack.org. For more info on Linux MM,
>>> > >> >> see: http://www.linux-mm.org/ .
>>> > >> >> Don't email: email@kvack.org
>>> > >> >
>>> > >> > --
>>> > >> > Kind regards,
>>> > >> > Minchan Kim
>>> > >>
>>> > >> --
>>> > >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>>> > >> the body to majordomo@kvack.org. For more info on Linux MM,
>>> > >> see: http://www.linux-mm.org/ .
>>> > >> Don't email: email@kvack.org
>>> > >
>>> > > --
>>> > > Kind regards,
>>> > > Minchan Kim
>>> >
>>> > --
>>> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
>>> > the body to majordomo@kvack.org. For more info on Linux MM,
>>> > see: http://www.linux-mm.org/ .
>>> > Don't email: email@kvack.org
>>>
>>> --
>>> Kind regards,
>>> Minchan Kim
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@kvack.org. For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: email@kvack.org