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

[Minchan Kim]

Hello Cai,

On Thu, Jan 23, 2014 at 09:38:41AM +0800, Cai Liu wrote:
> Hello Dan
> 
> 2014/1/22 Dan Streetman <ddstreet@xxxxxxxx>:
> > On Wed, Jan 22, 2014 at 7:16 AM, Cai Liu <liucai.lfn@xxxxxxxxx> wrote:
> >> Hello Minchan
> >>
> >>
> >> 2014/1/22 Minchan Kim <minchan@xxxxxxxxxx>
> >>>
> >>> Hello Cai,
> >>>
> >>> On Tue, Jan 21, 2014 at 09:52:25PM +0800, Cai Liu wrote:
> >>> > Hello Minchan
> >>> >
> >>> > 2014/1/21 Minchan Kim <minchan@xxxxxxxxxx>:
> >>> > > Hello,
> >>> > >
> >>> > > On Tue, Jan 21, 2014 at 02:35:07PM +0800, Cai Liu wrote:
> >>> > >> 2014/1/21 Minchan Kim <minchan@xxxxxxxxxx>:
> >>> > >> > 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@xxxxxxxxxx>:
> >>> > >> >> > 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@xxxxxxxxxxx>
> >>> > >> >> >> ---
> >>> > >> >> >>  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.
That's why I added RFC tag.