mm: rework memcg kernel stack accounting
Subsystems (7)
PATCH v2 1/3 mm: rework memcg kernel stack accounting
If CONFIG_VMAP_STACK is set, kernel stacks are allocated using __vmalloc_node_range() with __GFP_ACCOUNT. So kernel stack pages are charged against corresponding memory cgroups on allocation and uncharged on releasing them.
The problem is that we do cache kernel stacks in small per-cpu caches and do reuse them for new tasks, which can belong to different memory cgroups.
Each stack page still holds a reference to the original cgroup, so the cgroup can't be released until the vmap area is released.
To make this happen we need more than two subsequent exits without forks in between on the current cpu, which makes it very unlikely to happen. As a result, I saw a significant number of dying cgroups (in theory, up to 2 * number_of_cpu + number_of_tasks), which can't be released even by significant memory pressure.
As a cgroup structure can take a significant amount of memory (first of all, per-cpu data like memcg statistics), it leads to a noticeable waste of memory.
--- include/linux/memcontrol.h | 13 +++++++++- kernel/fork.c | 51 +++++++++++++++++++++++++++++++++----- 2 files changed, 57 insertions(+), 7 deletions(-)
include/linux/memcontrol.h +12 -1
@@ -1250,10 +1250,11 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep);void memcg_kmem_put_cache (struct kmem_cache *cachep);int memcg_kmem_charge_memcg (struct page *page,gfp_t gfp,int order,struct mem_cgroup *memcg); + +#ifdef CONFIG_MEMCG_KMEM int memcg_kmem_charge (struct page *page,gfp_t gfp,int order);void memcg_kmem_uncharge (struct page *page,int order); -#ifdef CONFIG_MEMCG_KMEM extern struct static_key_false memcg_kmem_enabled_key;extern struct workqueue_struct *memcg_kmem_cache_wq;
@@ -1289,6 +1290,16 @@ extern int memcg_expand_shrinker_maps(int new_id);extern void memcg_set_shrinker_bit (struct mem_cgroup *memcg,int nid,int shrinker_id);#else + +static inline int memcg_kmem_charge (struct page *page,gfp_t gfp,int order) +{ +return 0 ; +} + +static inline void memcg_kmem_uncharge (struct page *page,int order) +{ +} +#define for_each_memcg_cache_index (_idx) \ for (; NULL; )
kernel/fork.c +45 -6
85 lines not on this page · raw message
Signature · 1 line
2.17.1
PATCH v2 2/3 mm: drain memcg stocks on css offlining
Memcg charge is batched using per-cpu stocks, so an offline memcg can be pinned by a cached charge up to a moment, when a process belonging to some other cgroup will charge some memory on the same cpu. In other words, cached charges can prevent a memory cgroup from being reclaimed for some time, without any clear need.
Let's optimize it by explicit draining of all stocks on css offlining. As draining is performed asynchronously, and is skipped if any parallel draining is happening, it's cheap.
--- mm/memcontrol.c | 2 ++ 1 file changed, 2 insertions(+)
mm/memcontrol.c +2 -0
@@ -4518,6 +4518,8 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)memcg_offline_kmem (memcg);wb_memcg_offline (memcg); +drain_all_stock (memcg); +mem_cgroup_id_put (memcg); }
Signature · 1 line
2.17.1
PATCH v2 3/3 mm: don't miss the last page because of round-off error
In reply to Roman GushchinI've noticed, that dying memory cgroups are often pinned in memory by a single pagecache page. Even under moderate memory pressure they sometimes stayed in such state for a long time. That looked strange.
My investigation showed that the problem is caused by applying the LRU pressure balancing math:
scan = div64_u64(scan * fraction[lru], denominator),
where
denominator = fraction[anon] + fraction[file] + 1.
Because fraction[lru] is always less than denominator, if the initial scan size is 1, the result is always 0.
This means the last page is not scanned and has
no chances to be reclaimed.
Fix this by rounding up the result of the division.
In practice this change significantly improves the speed of dying cgroups reclaim.
--- include/linux/math64.h | 2 ++ mm/vmscan.c | 6 ++++-- 2 files changed, 6 insertions(+), 2 deletions(-)
include/linux/math64.h +2 -0
@@ -281,4 +281,6 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor) }#endif /* mul_u64_u32_div */ +#define DIV64_U64_ROUND_UP (ll, d) div64_u64((ll) + (d) - 1, (d)) +#endif /* _LINUX_MATH64_H */
mm/vmscan.c +4 -2
@@ -2445,9 +2445,11 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,/* * Scan types proportional to swappiness and * their relative recent reclaim efficiency. +* Make sure we don't miss the last page +* because of a round-off error. */ - scan =div64_u64 (scan * fraction[file], - denominator); + scan =DIV64_U64_ROUND_UP (scan * fraction[file], + denominator);break ;case SCAN_FILE :case SCAN_ANON :
Signature · 1 line
2.17.1
Quoted 20 lines · Roman Gushchin
If
CONFIG_VMAP_STACKis set, kernel stacks are allocated using__vmalloc_node_range()with__GFP_ACCOUNT. So kernel stack pages are charged against corresponding memory cgroups on allocation and uncharged on releasing them.The problem is that we do cache kernel stacks in small per-cpu caches and do reuse them for new tasks, which can belong to different memory cgroups.
Each stack page still holds a reference to the original cgroup, so the cgroup can't be released until the vmap area is released.
To make this happen we need more than two subsequent exits without forks in between on the current cpu, which makes it very unlikely to happen. As a result, I saw a significant number of dying cgroups (in theory, up to 2 *
number_of_cpu+number_of_tasks), which can't be released even by significant memory pressure.
Quoted 5 lines
As a cgroup structure can take a significant amount of memory (first of all, per-cpu data like memcg statistics), it leads to a noticeable waste of memory.
Signed-off-by Roman Gushchin <[email protected]>
BTW this makes a very good use-case for optimizing kmem uncharging similar to what you did for skmem uncharging.
Quoted 142 lines
Cc Johannes Weiner <[email protected]>Cc Michal Hocko <[email protected]>Cc Andy Lutomirski <[email protected]>Cc Konstantin Khlebnikov <[email protected]>Cc Tejun Heo <[email protected]>Cc Shakeel Butt <[email protected]>--- include/linux/memcontrol.h | 13 +++++++++- kernel/fork.c | 51 +++++++++++++++++++++++++++++++++----- 2 files changed, 57 insertions(+), 7 deletions(-)include/linux/memcontrol.h +12
-1@@ -1250,10 +1250,11 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep);void memcg_kmem_put_cache (struct kmem_cache *cachep);int memcg_kmem_charge_memcg (struct page *page,gfp_t gfp,int order,struct mem_cgroup *memcg); + +#ifdef CONFIG_MEMCG_KMEM int memcg_kmem_charge (struct page *page,gfp_t gfp,int order);void memcg_kmem_uncharge (struct page *page,int order); -#ifdef CONFIG_MEMCG_KMEM extern struct static_key_false memcg_kmem_enabled_key;extern struct workqueue_struct *memcg_kmem_cache_wq;@@ -1289,6 +1290,16 @@ extern int memcg_expand_shrinker_maps(int new_id);extern void memcg_set_shrinker_bit (struct mem_cgroup *memcg,int nid,int shrinker_id);#else + +static inline int memcg_kmem_charge (struct page *page,gfp_t gfp,int order) +{ +return 0 ; +} + +static inline void memcg_kmem_uncharge (struct page *page,int order) +{ +} +#define for_each_memcg_cache_index (_idx) \ for (; NULL; )kernel/fork.c +45
-7@@ -223,9 +223,14 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)return s->addr; } +/* +* Allocated stacks are cached and later reused by new threads, +* so memcg accounting is performed manually on assigning/releasing +* stacks to tasks. Drop __GFP_ACCOUNT. +*/ stack =__vmalloc_node_range (THREAD_SIZE ,THREAD_ALIGN ,VMALLOC_START ,VMALLOC_END , -THREADINFO_GFP , +THREADINFO_GFP & ~__GFP_ACCOUNT,PAGE_KERNEL ,0 , node,__builtin_return_address (0 ));@@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)static inline void free_thread_stack (struct task_struct *tsk) {#ifdef CONFIG_VMAP_STACK -if (task_stack_vm_area (tsk)) { +struct vm_struct *vm =task_stack_vm_area (tsk); + +if (vm) {int i; +for (i =0 ; i <THREAD_SIZE /PAGE_SIZE ; i++) { +mod_memcg_page_state (vm->pages[i], +MEMCG_KERNEL_STACK_KB , + -(int )(PAGE_SIZE /1024 )); + +memcg_kmem_uncharge (vm->pages[i], +compound_order (vm->pages[i])); + } +for (i =0 ; i <NR_CACHED_STACKS ; i++) {if (this_cpu_cmpxchg (cached_stacks[i],NULL , tsk->stack_vm_area) !=NULL )@@ -350,10 +366,6 @@ static void account_kernel_stack(struct task_struct *tsk, int account)NR_KERNEL_STACK_KB , PAGE_SIZE / 1024 * account); } - -/* All stack pages belong to the same memcg. */ -mod_memcg_page_state (vm->pages[0 ],MEMCG_KERNEL_STACK_KB , - account * (THREAD_SIZE /1024 )); }else {/* * All stack pages are in the same zone and belong to the @@ -369,6 +381,30 @@ static void account_kernel_stack(struct task_struct *tsk, int account) } } +static int memcg_charge_kernel_stack (struct task_struct *tsk) +{ +#ifdef CONFIG_VMAP_STACK +struct vm_struct *vm =task_stack_vm_area (tsk); +int ret; + +if (vm) { +int i; + +for (i =0 ; i <THREAD_SIZE /PAGE_SIZE ; i++) { + ret =memcg_kmem_charge (vm->pages[i],GFP_KERNEL , +compound_order (vm->pages[i])); +if (ret) +return ret; + +mod_memcg_page_state (vm->pages[i], +MEMCG_KERNEL_STACK_KB , +PAGE_SIZE /1024 ); + } + } +#endif +return 0 ; +} +static void release_task_stack (struct task_struct *tsk) {if (WARN_ON (tsk->state !=TASK_DEAD ))@@ -807,6 +843,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)if (!stack) goto free_tsk; +if (memcg_charge_kernel_stack (tsk)) + goto free_stack; + stack_vm_area =task_stack_vm_area (tsk); err =arch_dup_task_struct (tsk, orig); --2.17.1
Quoted 21 lines · Shakeel Butt
Quoted 20 lines · Roman Gushchin
If
CONFIG_VMAP_STACKis set, kernel stacks are allocated using__vmalloc_node_range()with__GFP_ACCOUNT. So kernel stack pages are charged against corresponding memory cgroups on allocation and uncharged on releasing them.The problem is that we do cache kernel stacks in small per-cpu caches and do reuse them for new tasks, which can belong to different memory cgroups.
Each stack page still holds a reference to the original cgroup, so the cgroup can't be released until the vmap area is released.
To make this happen we need more than two subsequent exits without forks in between on the current cpu, which makes it very unlikely to happen. As a result, I saw a significant number of dying cgroups (in theory, up to 2 *
number_of_cpu+number_of_tasks), which can't be released even by significant memory pressure.
Quoted 7 lines
Quoted 5 lines
As a cgroup structure can take a significant amount of memory (first of all, per-cpu data like memcg statistics), it leads to a noticeable waste of memory.
Signed-off-by Roman Gushchin <[email protected]>Reviewed-by Shakeel Butt <[email protected]>
Thanks!
Quoted 3 lines
BTW this makes a very good use-case for optimizing kmem uncharging similar to what you did for skmem uncharging.
Good point!
Let me prepare the patch.
Quoted 25 lines · Shakeel Butt
Quoted 24 lines · Roman Gushchin
If
CONFIG_VMAP_STACKis set, kernel stacks are allocated using__vmalloc_node_range()with__GFP_ACCOUNT. So kernel stack pages are charged against corresponding memory cgroups on allocation and uncharged on releasing them.The problem is that we do cache kernel stacks in small per-cpu caches and do reuse them for new tasks, which can belong to different memory cgroups.
Each stack page still holds a reference to the original cgroup, so the cgroup can't be released until the vmap area is released.
To make this happen we need more than two subsequent exits without forks in between on the current cpu, which makes it very unlikely to happen. As a result, I saw a significant number of dying cgroups (in theory, up to 2 *
number_of_cpu+number_of_tasks), which can't be released even by significant memory pressure.As a cgroup structure can take a significant amount of memory (first of all, per-cpu data like memcg statistics), it leads to a noticeable waste of memory.
Quoted 6 lines
Quoted 1 line
Signed-off-by Roman Gushchin <[email protected]>Reviewed-by Shakeel Butt <[email protected]>BTW this makes a very good use-case for optimizing kmem uncharging similar to what you did for skmem uncharging.
The only thing I'm slightly worried here is that it can make reclaiming of memory cgroups harder. Probably, it's still ok, but let me first finish the work I'm doing on optimizing the whole memcg reclaim process, and then return to this case.
Thanks!
Quoted 31 lines · Roman Gushchin
Quoted 29 lines · Shakeel Butt
Quoted 20 lines · Roman Gushchin
If
CONFIG_VMAP_STACKis set, kernel stacks are allocated using__vmalloc_node_range()with__GFP_ACCOUNT. So kernel stack pages are charged against corresponding memory cgroups on allocation and uncharged on releasing them.The problem is that we do cache kernel stacks in small per-cpu caches and do reuse them for new tasks, which can belong to different memory cgroups.
Each stack page still holds a reference to the original cgroup, so the cgroup can't be released until the vmap area is released.
To make this happen we need more than two subsequent exits without forks in between on the current cpu, which makes it very unlikely to happen. As a result, I saw a significant number of dying cgroups (in theory, up to 2 *
number_of_cpu+number_of_tasks), which can't be released even by significant memory pressure.Quoted 5 lines
As a cgroup structure can take a significant amount of memory (first of all, per-cpu data like memcg statistics), it leads to a noticeable waste of memory.
Signed-off-by Roman Gushchin <[email protected]>Reviewed-by Shakeel Butt <[email protected]>
Quoted 8 lines
Quoted 2 lines
BTW this makes a very good use-case for optimizing kmem uncharging similar to what you did for skmem uncharging.
The only thing I'm slightly worried here is that it can make reclaiming of memory cgroups harder. Probably, it's still ok, but let me first finish the work I'm doing on optimizing the whole memcg reclaim process, and then return to this case.
Yes, maybe we can disable that optimization for offlined memcgs. Anyways, we can discuss this later as you have suggested.
Shakeel
Quoted 23 lines · Roman Gushchin
If
CONFIG_VMAP_STACKis set, kernel stacks are allocated using__vmalloc_node_range()with__GFP_ACCOUNT. So kernel stack pages are charged against corresponding memory cgroups on allocation and uncharged on releasing them.The problem is that we do cache kernel stacks in small per-cpu caches and do reuse them for new tasks, which can belong to different memory cgroups.
Each stack page still holds a reference to the original cgroup, so the cgroup can't be released until the vmap area is released.
To make this happen we need more than two subsequent exits without forks in between on the current cpu, which makes it very unlikely to happen. As a result, I saw a significant number of dying cgroups (in theory, up to 2 *
number_of_cpu+number_of_tasks), which can't be released even by significant memory pressure.As a cgroup structure can take a significant amount of memory (first of all, per-cpu data like memcg statistics), it leads to a noticeable waste of memory.
Quoted 7 lines
Signed-off-by Roman Gushchin <[email protected]>Cc Johannes Weiner <[email protected]>Cc Michal Hocko <[email protected]>Cc Andy Lutomirski <[email protected]>Cc Konstantin Khlebnikov <[email protected]>Cc Tejun Heo <[email protected]>Cc Shakeel Butt <[email protected]>
Looks good to me. Two nits below.
I am not sure stable tree backport is really needed but it would be nice to put
Quoted 7 lines
kernel/fork.c +2
-1@@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)static inline void free_thread_stack (struct task_struct *tsk) {#ifdef CONFIG_VMAP_STACK -if (task_stack_vm_area (tsk)) { +struct vm_struct *vm =task_stack_vm_area (tsk); +
kernel/fork.c static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node) · line 267
Quoted 10 lines
kernel/fork.c +8
-0+if (vm) {int i; +for (i =0 ; i <THREAD_SIZE /PAGE_SIZE ; i++) { +mod_memcg_page_state (vm->pages[i], +MEMCG_KERNEL_STACK_KB , + -(int )(PAGE_SIZE /1024 )); + +memcg_kmem_uncharge (vm->pages[i], +compound_order (vm->pages[i]));
when do we have order > 0 here? Also I was wondering how come this doesn't blow up on partially charged stacks but both mod_memcg_page_state and memcg_kmem_uncharge check for page->mem_cgroup so this is safe. Maybe a comment would save people from scratching their heads.
Signature · 2 lines
Michal Hocko SUSE Labs
Quoted 32 lines · Michal Hocko
Quoted 23 lines · Roman Gushchin
If
CONFIG_VMAP_STACKis set, kernel stacks are allocated using__vmalloc_node_range()with__GFP_ACCOUNT. So kernel stack pages are charged against corresponding memory cgroups on allocation and uncharged on releasing them.The problem is that we do cache kernel stacks in small per-cpu caches and do reuse them for new tasks, which can belong to different memory cgroups.
Each stack page still holds a reference to the original cgroup, so the cgroup can't be released until the vmap area is released.
To make this happen we need more than two subsequent exits without forks in between on the current cpu, which makes it very unlikely to happen. As a result, I saw a significant number of dying cgroups (in theory, up to 2 *
number_of_cpu+number_of_tasks), which can't be released even by significant memory pressure.As a cgroup structure can take a significant amount of memory (first of all, per-cpu data like memcg statistics), it leads to a noticeable waste of memory.
Quoted 7 lines
Signed-off-by Roman Gushchin <[email protected]>Cc Johannes Weiner <[email protected]>Cc Michal Hocko <[email protected]>Cc Andy Lutomirski <[email protected]>Cc Konstantin Khlebnikov <[email protected]>Cc Tejun Heo <[email protected]>Cc Shakeel Butt <[email protected]>
Quoted 7 lines
Looks good to me. Two nits below.
I am not sure stable tree backport is really needed but it would be nice to put
Fixes ac496bf48d97 ("fork: Optimize task creation by caching two thread stacks per CPU if CONFIG_VMAP_STACK=y")Acked-by Michal Hocko <[email protected]>
Will add, thanks!
Quoted 11 lines
Quoted 11 lines
kernel/fork.c +3
-1@@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)static inline void free_thread_stack (struct task_struct *tsk) {#ifdef CONFIG_VMAP_STACK -if (task_stack_vm_area (tsk)) { +struct vm_struct *vm =task_stack_vm_area (tsk); + +if (vm) {int i;
Quoted 9 lines
kernel/fork.c static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node) · line 267
Quoted 7 lines
kernel/fork.c +7
-0+for (i =0 ; i <THREAD_SIZE /PAGE_SIZE ; i++) { +mod_memcg_page_state (vm->pages[i], +MEMCG_KERNEL_STACK_KB , + -(int )(PAGE_SIZE /1024 )); + +memcg_kmem_uncharge (vm->pages[i], +compound_order (vm->pages[i]));when do we have order > 0 here?
I guess, it's not possible, but hard-coded 1 looked a bit crappy. Do you think it's better?
Quoted 5 lines
Also I was wondering how come this
doesn't blow up on partially charged stacks but bothmod_memcg_page_stateandmemcg_kmem_unchargecheck forpage->mem_cgroupso this is safe. Maybe a comment would save people from scratching their heads.
Ok, will add.
Thank you!
Michal Hocko
maintainerQuoted 1 line · Roman Gushchin
On Wed, Aug 22, 2018 at 04:12:13PM +0200, Michal Hocko wrote:
[...]
Quoted 20 lines
Quoted 10 lines
Quoted 10 lines
kernel/fork.c +3
-1@@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)static inline void free_thread_stack (struct task_struct *tsk) {#ifdef CONFIG_VMAP_STACK -if (task_stack_vm_area (tsk)) { +struct vm_struct *vm =task_stack_vm_area (tsk); + +if (vm) {int i;Quoted 9 lines
kernel/fork.c static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node) · line 267
Quoted 7 lines
kernel/fork.c +7
-0+for (i =0 ; i <THREAD_SIZE /PAGE_SIZE ; i++) { +mod_memcg_page_state (vm->pages[i], +MEMCG_KERNEL_STACK_KB , + -(int )(PAGE_SIZE /1024 )); + +memcg_kmem_uncharge (vm->pages[i], +compound_order (vm->pages[i]));when do we have order > 0 here?
Quoted 2 lines
I guess, it's not possible, but hard-coded 1 looked a bit crappy. Do you think it's better?
I guess you meant 0 here. Well, I do not mind, I was just wondering whether I am missing something.
Signature · 2 lines
Michal Hocko SUSE Labs
Johannes Weiner
maintainerQuoted 22 lines · Roman Gushchin
Quoted 11 lines · Michal Hocko
Quoted 10 lines · Roman Gushchin
kernel/fork.c +3
-1@@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)static inline void free_thread_stack (struct task_struct *tsk) {#ifdef CONFIG_VMAP_STACK -if (task_stack_vm_area (tsk)) { +struct vm_struct *vm =task_stack_vm_area (tsk); + +if (vm) {int i;Quoted 9 lines
kernel/fork.c static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node) · line 267
Quoted 7 lines
kernel/fork.c +7
-0+for (i =0 ; i <THREAD_SIZE /PAGE_SIZE ; i++) { +mod_memcg_page_state (vm->pages[i], +MEMCG_KERNEL_STACK_KB , + -(int )(PAGE_SIZE /1024 )); + +memcg_kmem_uncharge (vm->pages[i], +compound_order (vm->pages[i]));when do we have order > 0 here?
Quoted 2 lines
I guess, it's not possible, but hard-coded 1 looked a bit crappy. Do you think it's better?
Yes, specifying the known value (order 0) is much better. I asked myself the same question as Michal: we're walking through THREAD_SIZE in PAGE_SIZE steps, how could it possibly be a higher order page?
It adds an unnecessary branch to the code and the reader's brain.
Quoted 23 lines · Johannes Weiner
Quoted 22 lines · Roman Gushchin
Quoted 20 lines · Michal Hocko
Quoted 7 lines · Roman Gushchin
kernel/fork.c +2
-1@@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)static inline void free_thread_stack (struct task_struct *tsk) {#ifdef CONFIG_VMAP_STACK -if (task_stack_vm_area (tsk)) { +struct vm_struct *vm =task_stack_vm_area (tsk); +kernel/fork.c static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node) · line 267
Quoted 10 lines
kernel/fork.c +8
-0+if (vm) {int i; +for (i =0 ; i <THREAD_SIZE /PAGE_SIZE ; i++) { +mod_memcg_page_state (vm->pages[i], +MEMCG_KERNEL_STACK_KB , + -(int )(PAGE_SIZE /1024 )); + +memcg_kmem_uncharge (vm->pages[i], +compound_order (vm->pages[i]));when do we have order > 0 here?
Quoted 8 lines
Quoted 2 lines
I guess, it's not possible, but hard-coded 1 looked a bit crappy. Do you think it's better?
Yes, specifying the known value (order 0) is much better. I asked myself the same question as Michal: we're walking through
THREAD_SIZEinPAGE_SIZEsteps, how could it possibly be a higher order page?It adds an unnecessary branch to the code and the reader's brain.
Fair enough. Will switch over hard-coded order 0 in v3.
Thanks!