Date:	Wed, 22 Jan 2014 17:02:38 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Cai Liu <liucai.lfn@...il.com>
Cc:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Bob Liu <bob.liu@...cle.com>, Cai Liu <cai.liu@...sung.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm/zswap: Check all pool pages instead of one pool
 pages

Hello Cai,

On Tue, Jan 21, 2014 at 09:52:25PM +0800, Cai Liu wrote:
> Hello Minchan
> 
> 2014/1/21 Minchan Kim <minchan@...nel.org>:
> > Hello,
> >
> > On Tue, Jan 21, 2014 at 02:35:07PM +0800, Cai Liu wrote:
> >> 2014/1/21 Minchan Kim <minchan@...nel.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@...nel.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@...sung.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.

That's why I want to know why you want to use multiple swap with zswap
but you are never saying it to us. :(

> 
> 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@...ck.org.  For more info on Linux MM,
> >> >> >> see: http://www.linux-mm.org/ .
> >> >> >> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> >> >> >
> >> >> > --
> >> >> > Kind regards,
> >> >> > Minchan Kim
> >> >>
> >> >> --
> >> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> >> the body to majordomo@...ck.org.  For more info on Linux MM,
> >> >> see: http://www.linux-mm.org/ .
> >> >> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> >> >
> >> > --
> >> > Kind regards,
> >> > Minchan Kim
> >>
> >> --
> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> the body to majordomo@...ck.org.  For more info on Linux MM,
> >> see: http://www.linux-mm.org/ .
> >> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> >
> > --
> > Kind regards,
> > Minchan Kim
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@...ck.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>

-- 
Kind regards,
Minchan Kim
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