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

Cai Liu
2014-01-22T12:16:31Z

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.

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