Re: [PATCH -mm -v8 1/3] mm, THP, swap: Delay splitting THP during swap out

[Johannes Weiner <hannes@cmpxchg.org>]

On Sat, Apr 15, 2017 at 09:17:04AM +0800, Huang, Ying wrote:
> Hi, Johannes,
> 
> Johannes Weiner <hannes@cmpxchg.org> writes:
> 
> > Hi Huang,
> >
> > I reviewed this patch based on the feedback I already provided, but
> > eventually gave up and rewrote it. Please take review feedback more
> > seriously in the future.
> 
> Thanks a lot for your help!  I do respect all your review and effort.
> The -v8 patch doesn't take all your comments, just because I thought we
> have not reach consensus for some points and I want to use -v8 patch to
> discuss them.
> 
> One concern I have before is whether to split THP firstly when swap
> space or memcg swap is used up.  Now I think your solution is
> acceptable. And if we receive any regression report for that in the
> future, it's not very hard to deal with.

If you look at get_scan_count(), we'll stop scanning anonymous pages
altogether when swap space runs out. So it might happen to a few THP,
but it shouldn't be a big deal.

And yes, in that case I'd really rather wait for any real problems to
materialize before we complicate things.

> > Attached below is the reworked patch. Most changes are to the layering
> > (page functions, cluster functions, range functions) so that we don't
> > make the lowest swap range code require a notion of huge pages, or
> > make the memcg page functions take size information that can be
> > gathered from the page itself. I turned the config symbol into a
> > generic THP_SWAP that can later be extended when we add 2MB IO. The
> > rest is function naming, #ifdef removal etc.
> 
> For some #ifdef in swapfile.c, it is to avoid unnecessary code size
> increase for !CONFIG_TRANSPARENT_HUGEPAGE or platform with THP swap
> optimization disabled.  Is it an issue?

It saves some code size, but it looks like the biggest cost comes from
bloating PageSwapCache(). This is mm/builtin.o with !CONFIG_THP:

add/remove: 1/0 grow/shrink: 34/5 up/down: 920/-311 (609)
function                                     old     new   delta
__free_cluster                                 -     106    +106
get_swap_pages                               465     555     +90
migrate_page_move_mapping                   1404    1479     +75
shrink_page_list                            3573    3632     +59
__delete_from_swap_cache                     235     293     +58
__test_set_page_writeback                    626     678     +52
__swap_writepage                             766     812     +46
try_to_unuse                                1763    1795     +32
madvise_free_pte_range                       882     912     +30
__set_page_dirty_nobuffers                   245     268     +23
migrate_page_copy                            565     587     +22
swap_slot_free_notify                        133     151     +18
shmem_replace_page                           616     633     +17
try_to_free_swap                             135     151     +16
test_clear_page_writeback                    512     528     +16
swap_set_page_dirty                          109     125     +16
swap_readpage                                384     400     +16
shmem_unuse                                 1535    1551     +16
reuse_swap_page                              340     356     +16
page_mapping                                 144     160     +16
migrate_huge_page_move_mapping               483     499     +16
free_swap_and_cache                          409     425     +16
free_pages_and_swap_cache                    161     177     +16
free_page_and_swap_cache                     145     161     +16
do_swap_page                                1216    1232     +16
__remove_mapping                             408     424     +16
__page_file_mapping                           82      98     +16
__page_file_index                             70      85     +15
try_to_unmap_one                            1324    1337     +13
shmem_getpage_gfp.isra                      2358    2371     +13
add_to_swap_cache                             47      60     +13
inc_cluster_info_page                        204     210      +6
get_swap_page                                411     415      +4
shmem_writepage                              922     925      +3
sys_swapon                                  4210    4211      +1
swapcache_free_entries                       786     768     -18
__add_to_swap_cache                          445     406     -39
delete_from_swap_cache                       149     104     -45
scan_swap_map_slots                         1953    1889     -64
swap_do_scheduled_discard                    713     568    -145
Total: Before=454535, After=455144, chg +0.13%

If I make the compound_head() in there conditional, this patch
actually ends up shrinking the code due to the refactoring of the
cluster functions:

add/remove: 1/0 grow/shrink: 10/5 up/down: 302/-327 (-25)
function                                     old     new   delta
__free_cluster                                 -     106    +106
get_swap_pages                               465     555     +90
__delete_from_swap_cache                     235     277     +42
shmem_replace_page                           616     633     +17
migrate_page_move_mapping                   1404    1418     +14
add_to_swap_cache                             47      60     +13
migrate_page_copy                            565     571      +6
inc_cluster_info_page                        204     210      +6
get_swap_page                                411     415      +4
shmem_writepage                              922     925      +3
sys_swapon                                  4210    4211      +1
swapcache_free_entries                       786     768     -18
delete_from_swap_cache                       149     104     -45
__add_to_swap_cache                          445     390     -55
scan_swap_map_slots                         1953    1889     -64
swap_do_scheduled_discard                    713     568    -145
Total: Before=454535, After=454510, chg -0.01%

But PageSwapCache() is somewhat ugly either way. Even with THP_SWAP
compiled in, it seems like most callsites wouldn't test tailpages?
Can we get rid of the compound_head() and annotate any callsites
working on potential tail pages?

> > Please review whether this is an acceptable version for you.
> 
> Yes.  It is good for me.  I will give it more test on next Monday.

Thanks

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index f4acd6c4f808..d33e3280c8ad 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -326,7 +326,9 @@ PAGEFLAG_FALSE(HighMem)
 #ifdef CONFIG_SWAP
 static __always_inline int PageSwapCache(struct page *page)
 {
+#ifdef CONFIG_THP_SWAP
 	page = compound_head(page);
+#endif
 	return PageSwapBacked(page) && test_bit(PG_swapcache, &page->flags);
 
 }
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 34adac6e9457..a4dba6975e7b 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -401,7 +401,6 @@ extern int swap_duplicate(swp_entry_t);
 extern int swapcache_prepare(swp_entry_t);
 extern void swap_free(swp_entry_t);
 extern void swapcache_free(swp_entry_t);
-extern void swapcache_free_cluster(swp_entry_t);
 extern void swapcache_free_entries(swp_entry_t *entries, int n);
 extern int free_swap_and_cache(swp_entry_t);
 extern int swap_type_of(dev_t, sector_t, struct block_device **);
@@ -467,10 +466,6 @@ static inline void swapcache_free(swp_entry_t swp)
 {
 }
 
-static inline void swapcache_free_cluster(swp_entry_t swp)
-{
-}
-
 static inline struct page *swapin_readahead(swp_entry_t swp, gfp_t gfp_mask,
 			struct vm_area_struct *vma, unsigned long addr)
 {
@@ -592,5 +587,13 @@ static inline bool mem_cgroup_swap_full(struct page *page)
 }
 #endif
 
+#ifdef CONFIG_THP_SWAP
+extern void swapcache_free_cluster(swp_entry_t);
+#else
+static inline void swapcache_free_cluster(swp_entry_t swp)
+{
+}
+#endif
+
 #endif /* __KERNEL__*/
 #endif /* _LINUX_SWAP_H */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index f597cabcaab7..eeaf145b2a20 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -825,6 +825,7 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 	return n_ret;
 }
 
+#ifdef CONFIG_THP_SWAP
 static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
 {
 	unsigned long idx;
@@ -862,6 +863,13 @@ static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
 	unlock_cluster(ci);
 	swap_range_free(si, offset, SWAPFILE_CLUSTER);
 }
+#else
+static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
+{
+	VM_WARN_ON_ONCE(1);
+	return 0;
+}
+#endif /* CONFIG_THP_SWAP */
 
 static unsigned long scan_swap_map(struct swap_info_struct *si,
 				   unsigned char usage)
@@ -1145,6 +1153,7 @@ void swapcache_free(swp_entry_t entry)
 	}
 }
 
+#ifdef CONFIG_THP_SWAP
 void swapcache_free_cluster(swp_entry_t entry)
 {
 	unsigned long offset = swp_offset(entry);
@@ -1170,6 +1179,7 @@ void swapcache_free_cluster(swp_entry_t entry)
 	swap_free_cluster(si, idx);
 	spin_unlock(&si->lock);
 }
+#endif /* CONFIG_THP_SWAP */
 
 void swapcache_free_entries(swp_entry_t *entries, int n)
 {