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

[David Hildenbrand <david@xxxxxxxxxx>]

On 09.08.24 10:13, Ryan Roberts wrote:
> On 08/08/2024 02:04, Barry Song wrote:
> > From: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > 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@xxxxxxxx>
> > ---
> >   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?

Doing it when we set/clear folio->mapping is straight forward.

Anon folios currently come to live when we first map them, and they stay 
that way until we free them.

In the future, we'll have to move that anon handling further out, when 
if have to allocate anon-specific memdesc ahead of time, then, it will 
be clued to that lifetime.

> 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?

Likely the deconstructor could work as well. Not sure if that is any 
better than the freeing path where folio->mapping currently gets cleared.

The generic constructor certainly won't work right now. That's not where 
the "anon" part comes to live.

Let's take a look how NR_FILE_THPS is handled:

__filemap_add_folio() increments it -- when we set folio->mapping
__filemap_remove_folio() (->filemap_unaccount_folio) decrements it -- 
after which we usually call page_cache_delete() to set folio->mapping = 
NULL;

--
Cheers,

David / dhildenb