On Thu, Aug 8, 2024 at 1:05 PM Barry Song <21cnbao@xxxxxxxxx> 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.
> +
> 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))
could have a risk here two processes unmap at the same time, so
they both get zero on atomic_read(mapped)? should read the value
of atomic_dec_return() instead to confirm we are the last one
doing unmap?
diff --git a/mm/rmap.c b/mm/rmap.c
index 2b722f26224c..f4a0f343f995 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1530,6 +1530,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
atomic_t *mapped = &folio->_nr_pages_mapped;
int last, nr = 0, nr_pmdmapped = 0;
bool partially_mapped = false;
+ bool last_unmapped = false;
__folio_rmap_sanity_checks(folio, page, nr_pages, level);
@@ -1545,6 +1546,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
last = atomic_add_negative(-1, &page->_mapcount);
if (last) {
last = atomic_dec_return_relaxed(mapped);
+ if (!last)
+ last_unmapped = true;
if (last < ENTIRELY_MAPPED)
nr++;
}
@@ -1557,6 +1560,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
last = atomic_add_negative(-1, &folio->_entire_mapcount);
if (last) {
nr = atomic_sub_return_relaxed(ENTIRELY_MAPPED, mapped);
+ if (nr == 0)
+ last_unmapped = true;
if (likely(nr < ENTIRELY_MAPPED)) {
nr_pmdmapped = folio_nr_pages(folio);
nr = nr_pmdmapped - (nr & FOLIO_PAGES_MAPPED);
@@ -1583,7 +1588,7 @@ 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))
+ if (folio_test_anon(folio) && last_unmapped)
mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, -1);
/*
> + mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, -1);
>
> /*
> * It would be tidy to reset folio_test_anon mapping when fully
> --
> 2.34.1
>
