On Thu, Jan 24, 2013 at 10:43:11AM +0800, Shaohua Li wrote:
> On Thu, Jan 24, 2013 at 11:22:41AM +0900, Minchan Kim wrote:
> > Hi Simon,
> >
> > On Wed, Jan 23, 2013 at 07:39:56PM -0600, Simon Jeons wrote:
> > > On Wed, 2013-01-23 at 17:04 +0900, Minchan Kim wrote:
> > > > On Wed, Jan 23, 2013 at 03:36:55PM +0800, Shaohua Li wrote:
> > > > > On Wed, Jan 23, 2013 at 03:16:45PM +0900, Minchan Kim wrote:
> > > > > > Looks good to me. Below just nitpicks.
> > > > > > I saw Andrew already took this into mmotm so I'm not sure he or you will do
> > > > > > next spin but anyway, review goes. Just nitpicks and a question.
> > > > > >
> > > > > > On Tue, Jan 22, 2013 at 10:29:51AM +0800, Shaohua Li wrote:
> > > > > > >
> > > > > > > When I use several fast SSD to do swap, swapper_space.tree_lock is heavily
> > > > > > > contended. This makes each swap partition have one address_space to reduce the
> > > > > > > lock contention. There is an array of address_space for swap. The swap entry
> > > > > > > type is the index to the array.
> > > > > > >
> > > > > > > In my test with 3 SSD, this increases the swapout throughput 20%.
> > > > > > >
> > > > > > > V1->V2: simplify code
> > > > > > >
> > > > > > > Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> > > > > >
> > > > > > Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> > > > > >
> > > > > > > ---
> > > > > > > fs/proc/meminfo.c | 4 +--
> > > > > > > include/linux/swap.h | 9 ++++----
> > > > > > > mm/memcontrol.c | 4 +--
> > > > > > > mm/mincore.c | 5 ++--
> > > > > > > mm/swap.c | 9 ++++++--
> > > > > > > mm/swap_state.c | 57 ++++++++++++++++++++++++++++++++++-----------------
> > > > > > > mm/swapfile.c | 5 ++--
> > > > > > > mm/util.c | 10 ++++++--
> > > > > > > 8 files changed, 68 insertions(+), 35 deletions(-)
> > > > > > >
> > > > > > > Index: linux/include/linux/swap.h
> > > > > > > ===================================================================
> > > > > > > --- linux.orig/include/linux/swap.h 2013-01-22 09:13:14.000000000 +0800
> > > > > > > +++ linux/include/linux/swap.h 2013-01-22 09:34:44.923011706 +0800
> > > > > > > @@ -8,7 +8,7 @@
> > > > > > > #include <linux/memcontrol.h>
> > > > > > > #include <linux/sched.h>
> > > > > > > #include <linux/node.h>
> > > > > > > -
> > > > > > > +#include <linux/fs.h>
> > > > > > > #include <linux/atomic.h>
> > > > > > > #include <asm/page.h>
> > > > > > >
> > > > > > > @@ -330,8 +330,9 @@ int generic_swapfile_activate(struct swa
> > > > > > > sector_t *);
> > > > > > >
> > > > > > > /* linux/mm/swap_state.c */
> > > > > > > -extern struct address_space swapper_space;
> > > > > > > -#define total_swapcache_pages swapper_space.nrpages
> > > > > > > +extern struct address_space swapper_spaces[];
> > > > > > > +#define swap_address_space(entry) (&swapper_spaces[swp_type(entry)])
> > > > > >
> > > > > > How about this naming?
> > > > > >
> > > > > > #define swapper_space(entry) (&swapper_spaces[swp_type(entry)])
> > > > > >
> > > > > > > +extern unsigned long total_swapcache_pages(void);
> > > > > > > extern void show_swap_cache_info(void);
> > > > > > > extern int add_to_swap(struct page *);
> > > > > > > extern int add_to_swap_cache(struct page *, swp_entry_t, gfp_t);
> > > > > > > @@ -382,7 +383,7 @@ mem_cgroup_uncharge_swapcache(struct pag
> > > > > > >
> > > > > > > #define nr_swap_pages 0L
> > > > > > > #define total_swap_pages 0L
> > > > > > > -#define total_swapcache_pages 0UL
> > > > > > > +#define total_swapcache_pages() 0UL
> > > > > > >
> > > > > > > #define si_swapinfo(val) \
> > > > > > > do { (val)->freeswap = (val)->totalswap = 0; } while (0)
> > > > > > > Index: linux/mm/memcontrol.c
> > > > > > > ===================================================================
> > > > > > > --- linux.orig/mm/memcontrol.c 2013-01-22 09:13:14.000000000 +0800
> > > > > > Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> > > > > >
> > > > > > > +++ linux/mm/memcontrol.c 2013-01-22 09:29:29.374977700 +0800
> > > > > > > @@ -6279,7 +6279,7 @@ static struct page *mc_handle_swap_pte(s
> > > > > > > * Because lookup_swap_cache() updates some statistics counter,
> > > > > > > * we call find_get_page() with swapper_space directly.
> > > > > > > */
> > > > > > > - page = find_get_page(&swapper_space, ent.val);
> > > > > > > + page = find_get_page(swap_address_space(ent), ent.val);
> > > > > > > if (do_swap_account)
> > > > > > > entry->val = ent.val;
> > > > > > >
> > > > > > > @@ -6320,7 +6320,7 @@ static struct page *mc_handle_file_pte(s
> > > > > > > swp_entry_t swap = radix_to_swp_entry(page);
> > > > > > > if (do_swap_account)
> > > > > > > *entry = swap;
> > > > > > > - page = find_get_page(&swapper_space, swap.val);
> > > > > > > + page = find_get_page(swap_address_space(swap), swap.val);
> > > > > > > }
> > > > > > > #endif
> > > > > > > return page;
> > > > > > > Index: linux/mm/mincore.c
> > > > > > > ===================================================================
> > > > > > > --- linux.orig/mm/mincore.c 2013-01-22 09:13:14.000000000 +0800
> > > > > > > +++ linux/mm/mincore.c 2013-01-22 09:29:29.378977649 +0800
> > > > > > > @@ -75,7 +75,7 @@ static unsigned char mincore_page(struct
> > > > > > > /* shmem/tmpfs may return swap: account for swapcache page too. */
> > > > > > > if (radix_tree_exceptional_entry(page)) {
> > > > > > > swp_entry_t swap = radix_to_swp_entry(page);
> > > > > > > - page = find_get_page(&swapper_space, swap.val);
> > > > > > > + page = find_get_page(swap_address_space(swap), swap.val);
> > > > > > > }
> > > > > > > #endif
> > > > > > > if (page) {
> > > > > > > @@ -135,7 +135,8 @@ static void mincore_pte_range(struct vm_
> > > > > > > } else {
> > > > > > > #ifdef CONFIG_SWAP
> > > > > > > pgoff = entry.val;
> > > > > > > - *vec = mincore_page(&swapper_space, pgoff);
> > > > > > > + *vec = mincore_page(swap_address_space(entry),
> > > > > > > + pgoff);
> > > > > > > #else
> > > > > > > WARN_ON(1);
> > > > > > > *vec = 1;
> > > > > > > Index: linux/mm/swap.c
> > > > > > > ===================================================================
> > > > > > > --- linux.orig/mm/swap.c 2013-01-22 09:13:14.000000000 +0800
> > > > > > > +++ linux/mm/swap.c 2013-01-22 09:29:29.378977649 +0800
> > > > > > > @@ -855,9 +855,14 @@ EXPORT_SYMBOL(pagevec_lookup_tag);
> > > > > > > void __init swap_setup(void)
> > > > > > > {
> > > > > > > unsigned long megs = totalram_pages >> (20 - PAGE_SHIFT);
> > > > > > > -
> > > > > > > #ifdef CONFIG_SWAP
> > > > > > > - bdi_init(swapper_space.backing_dev_info);
> > > > > > > + int i;
> > > > > > > +
> > > > > > > + for (i = 0; i < MAX_SWAPFILES; i++) {
> > > > > > > + bdi_init(swapper_spaces[i].backing_dev_info);
> > > > > > > + spin_lock_init(&swapper_spaces[i].tree_lock);
> > > > > > > + INIT_LIST_HEAD(&swapper_spaces[i].i_mmap_nonlinear);
> > > > > > > + }
> > > > > > > #endif
> > > > > > >
> > > > > > > /* Use a smaller cluster for small-memory machines */
> > > > > > > Index: linux/mm/swap_state.c
> > > > > > > ===================================================================
> > > > > > > --- linux.orig/mm/swap_state.c 2013-01-22 09:13:14.000000000 +0800
> > > > > > > +++ linux/mm/swap_state.c 2013-01-22 09:29:29.378977649 +0800
> > > > > > > @@ -36,12 +36,12 @@ static struct backing_dev_info swap_back
> > > > > > > .capabilities = BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
> > > > > > > };
> > > > > > >
> > > > > > > -struct address_space swapper_space = {
> > > > > > > - .page_tree = RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
> > > > > > > - .tree_lock = __SPIN_LOCK_UNLOCKED(swapper_space.tree_lock),
> > > > > > > - .a_ops = &swap_aops,
> > > > > > > - .i_mmap_nonlinear = LIST_HEAD_INIT(swapper_space.i_mmap_nonlinear),
> > > > > > > - .backing_dev_info = &swap_backing_dev_info,
> > > > > > > +struct address_space swapper_spaces[MAX_SWAPFILES] = {
> > > > > > > + [0 ... MAX_SWAPFILES - 1] = {
> > > > > > > + .page_tree = RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
> > > > > > > + .a_ops = &swap_aops,
> > > > > > > + .backing_dev_info = &swap_backing_dev_info,
> > > > > > > + }
> > > > > > > };
> > > > > > >
> > > > > > > #define INC_CACHE_INFO(x) do { swap_cache_info.x++; } while (0)
> > > > > > > @@ -53,9 +53,19 @@ static struct {
> > > > > > > unsigned long find_total;
> > > > > > > } swap_cache_info;
> > > > > > >
> > > > > > > +unsigned long total_swapcache_pages(void)
> > > > > > > +{
> > > > > > > + int i;
> > > > > > > + unsigned long ret = 0;
> > > > > > > +
> > > > > > > + for (i = 0; i < MAX_SWAPFILES; i++)
> > > > > > > + ret += swapper_spaces[i].nrpages;
> > > > > > > + return ret;
> > > > > > > +}
> > > > > > > +
> > > > > > > void show_swap_cache_info(void)
> > > > > > > {
> > > > > > > - printk("%lu pages in swap cache\n", total_swapcache_pages);
> > > > > > > + printk("%lu pages in swap cache\n", total_swapcache_pages());
> > > > > > > printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu\n",
> > > > > > > swap_cache_info.add_total, swap_cache_info.del_total,
> > > > > > > swap_cache_info.find_success, swap_cache_info.find_total);
> > > > > > > @@ -70,23 +80,26 @@ void show_swap_cache_info(void)
> > > > > > > static int __add_to_swap_cache(struct page *page, swp_entry_t entry)
> > > > > > > {
> > > > > > > int error;
> > > > > > > + struct address_space *address_space;
> > > > > > >
> > > > > > > VM_BUG_ON(!PageLocked(page));
> > > > > > > VM_BUG_ON(PageSwapCache(page));
> > > > > > > VM_BUG_ON(!PageSwapBacked(page));
> > > > > > >
> > > > > > > page_cache_get(page);
> > > > > > > - SetPageSwapCache(page);
> > > > > > > set_page_private(page, entry.val);
> > > > > > > + SetPageSwapCache(page);
> > > > > >
> > > > > > Why did you move this line? Is there any special reason?
> > > > >
> > > > > Originally I'm afraid page_mapping() gets invalid page_private(), but I then
> > > > > realized we hold page lock. There are some places we don't hold page lock.
> > > > > either such page isn't swap page or the caller can tolerate race. I forgot
> > > > > removing this change in the patch. But I certainly can be wrong. We can add
> > > > > memory barrier if required.
> > > >
> > > > Yeb. While I reviewed the patch, I was concern about that but I fail to find
> > > > a problem, too. But maybe it's valuable to add comment about that race
> > > > (!PageSwapCache(page) but page_mapping could return swapper_space) in page_mapping.
> > >
> > > Hi Minchan,
> > >
> > > If the race Shaohua mentioned should be PageSwapCache(page) but
> > > page_mapping couldn't return swapper_space since page_mapping() gets
> > > invalid page_private().
> >
> > Right you are. In such case, it could be a problem.
> > Let's see following case.
> >
> > isolate_migratepages_range
> > __isolate_lru_page
> > __add_to_swap_cache
> > set_page_private(page, entry.val)
> > SetPageSwapCache(page)
> > PageSwapCache
> > entry.val = page_private(page);
> > mapping = swap_address_space(entry);
> >
> > In this case, if memory ordering happens, mapping would be dangling pointer,
> > One of the problem by dangling mapping is the page could be passed into
> > shrink_page_list and try to pageout with dangling mapping. Of course,
> > we have many locks until reaching the page_mapping in shrink_page_list so
> > the problem will not happen but we shouldn't depends on such implicit locks
> > instead of explicit memory barrier because we could remove all locks in reclaim
> > path if super hero comes in or new user of page_mapping would do new something
> > to make a problem. :)
> >
> > I will send a patch if anyone doesn't oppose.
>
> That't fine in the case, because page private 0 still return a valide mapping,
> and we only check mapping here. But I agree this is too subtle. Adding memory
It could try to do many things with wrong address_space so it is very error-prone.
> fence is safer. I had this yesterday if you don't mind:
Of course. Below just some nitpicks.
>
>
> Subject: mm: add memory to prevent SwapCache bit and page private out of order
mm: Get rid of memory reordering of SwapCache and PagePrivate
>
> page_mapping() checks SwapCache bit first and then read page private. Adding
> memory barrier so page private has correct value before SwapCache bit set.
Please write down the problem if we don't apply the patch.
>
> Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
> mm/swap_state.c | 7 +++++--
> mm/util.c | 6 ++++++
> 2 files changed, 11 insertions(+), 2 deletions(-)
>
> Index: linux/mm/swap_state.c
> ===================================================================
> --- linux.orig/mm/swap_state.c 2013-01-22 10:12:33.514490665 +0800
> +++ linux/mm/swap_state.c 2013-01-23 15:42:00.737459987 +0800
> @@ -89,6 +89,7 @@ static int __add_to_swap_cache(struct pa
>
> page_cache_get(page);
> set_page_private(page, entry.val);
> + smp_wmb();
> SetPageSwapCache(page);
>
> address_space = swap_address_space(entry);
> @@ -109,8 +110,9 @@ static int __add_to_swap_cache(struct pa
> * So add_to_swap_cache() doesn't returns -EEXIST.
> */
> VM_BUG_ON(error == -EEXIST);
> - set_page_private(page, 0UL);
> ClearPageSwapCache(page);
> + smp_mb__after_clear_bit();
Could you use smp_wmb() instead of smp_mb__after_clear_bit?
Yes. ClearPageSwapCache does clear_bit so it's okay now but we might change it
in future so let's not make unnecessary dependency. It's not really hot path.
> + set_page_private(page, 0UL);
> page_cache_release(page);
> }
>
> @@ -146,8 +148,9 @@ void __delete_from_swap_cache(struct pag
> entry.val = page_private(page);
> address_space = swap_address_space(entry);
> radix_tree_delete(&address_space->page_tree, page_private(page));
> - set_page_private(page, 0);
> ClearPageSwapCache(page);
> + smp_mb__after_clear_bit();
> + set_page_private(page, 0);
> address_space->nrpages--;
> __dec_zone_page_state(page, NR_FILE_PAGES);
> INC_CACHE_INFO(del_total);
> Index: linux/mm/util.c
> ===================================================================
> --- linux.orig/mm/util.c 2013-01-22 10:11:58.310933234 +0800
> +++ linux/mm/util.c 2013-01-23 15:50:22.147155111 +0800
> @@ -389,6 +389,12 @@ struct address_space *page_mapping(struc
> if (unlikely(PageSwapCache(page))) {
> swp_entry_t entry;
>
> + /*
> + * set page_private() first then set SwapCache bit. clearing
> + * the bit first then zero page private. This memory barrier
> + * matches the rule.
> + */
> + smp_rmb();
> entry.val = page_private(page);
> mapping = swap_address_space(entry);
> } else
>
> --
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--
Kind regards,
Minchan Kim
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