Re: [PATCH RFC 1/2] mm: collect the number of anon large folios

[Ryan Roberts <ryan.roberts@arm.com>]

On 08/08/2024 02:04, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> When a new anonymous mTHP is added to the rmap, we increase the count.
> We reduce the count whenever an mTHP is completely unmapped.
> 
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst |  5 +++++
>  include/linux/huge_mm.h                    | 15 +++++++++++++--
>  mm/huge_memory.c                           |  2 ++
>  mm/rmap.c                                  |  3 +++
>  4 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 058485daf186..715f181543f6 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -527,6 +527,11 @@ split_deferred
>          it would free up some memory. Pages on split queue are going to
>          be split under memory pressure, if splitting is possible.
>  
> +anon_num
> +       the number of anon huge pages we have in the whole system.
> +       These huge pages could be still entirely mapped and have partially
> +       unmapped and unused subpages.

nit: "entirely mapped and have partially unmapped and unused subpages" ->
"entirely mapped or have partially unmapped/unused subpages"

> +
>  As the system ages, allocating huge pages may be expensive as the
>  system uses memory compaction to copy data around memory to free a
>  huge page for use. There are some counters in ``/proc/vmstat`` to help
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e25d9ebfdf89..294c348fe3cc 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -281,6 +281,7 @@ enum mthp_stat_item {
>  	MTHP_STAT_SPLIT,
>  	MTHP_STAT_SPLIT_FAILED,
>  	MTHP_STAT_SPLIT_DEFERRED,
> +	MTHP_STAT_NR_ANON,
>  	__MTHP_STAT_COUNT
>  };
>  
> @@ -291,14 +292,24 @@ struct mthp_stat {
>  #ifdef CONFIG_SYSFS
>  DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
>  
> -static inline void count_mthp_stat(int order, enum mthp_stat_item item)
> +static inline void mod_mthp_stat(int order, enum mthp_stat_item item, int delta)
>  {
>  	if (order <= 0 || order > PMD_ORDER)
>  		return;
>  
> -	this_cpu_inc(mthp_stats.stats[order][item]);
> +	this_cpu_add(mthp_stats.stats[order][item], delta);
> +}
> +
> +static inline void count_mthp_stat(int order, enum mthp_stat_item item)
> +{
> +	mod_mthp_stat(order, item, 1);
>  }
> +
>  #else
> +static inline void mod_mthp_stat(int order, enum mthp_stat_item item, int delta)
> +{
> +}
> +
>  static inline void count_mthp_stat(int order, enum mthp_stat_item item)
>  {
>  }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 697fcf89f975..b6bc2a3791e3 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -578,6 +578,7 @@ DEFINE_MTHP_STAT_ATTR(shmem_fallback_charge, MTHP_STAT_SHMEM_FALLBACK_CHARGE);
>  DEFINE_MTHP_STAT_ATTR(split, MTHP_STAT_SPLIT);
>  DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
>  DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
> +DEFINE_MTHP_STAT_ATTR(anon_num, MTHP_STAT_NR_ANON);
>  
>  static struct attribute *stats_attrs[] = {
>  	&anon_fault_alloc_attr.attr,
> @@ -591,6 +592,7 @@ static struct attribute *stats_attrs[] = {
>  	&split_attr.attr,
>  	&split_failed_attr.attr,
>  	&split_deferred_attr.attr,
> +	&anon_num_attr.attr,
>  	NULL,
>  };
>  
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 901950200957..2b722f26224c 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1467,6 +1467,7 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>  	}
>  
>  	__folio_mod_stat(folio, nr, nr_pmdmapped);
> +	mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
>  }
>  
>  static __always_inline void __folio_add_file_rmap(struct folio *folio,
> @@ -1582,6 +1583,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>  	    list_empty(&folio->_deferred_list))
>  		deferred_split_folio(folio);
>  	__folio_mod_stat(folio, -nr, -nr_pmdmapped);
> +	if (folio_test_anon(folio) && !atomic_read(mapped))

Agree that atomic_read() is dodgy here.

Not sure I fully understand why David prefers to do the unaccounting at
free-time though? It feels unbalanced to me to increment when first mapped but
decrement when freed. Surely its safer to either use alloc/free or use first
map/last map?

If using alloc/free isn't there a THP constructor/destructor that prepares the
deferred list? (My memory may be failing me). Could we use that?

> +		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, -1);
>  
>  	/*
>  	 * It would be tidy to reset folio_test_anon mapping when fully