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

David Hildenbrand <david@xxxxxxxxxx> · Thu, 8 Aug 2024 10:17:10 +0200

On 08.08.24 10:08, David Hildenbrand wrote:
On 08.08.24 10:03, David Hildenbrand wrote:
On 08.08.24 09:08, Barry Song wrote:
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?

I would appreciate if we leave the rmap out here.

Can't we handle that when actually freeing the folio? folio_test_anon()
is sticky until freed.

To be clearer: we increment the counter when we set a folio anon, which
should indeed only happen in folio_add_new_anon_rmap(). We'll have to
ignore hugetlb here where we do it in hugetlb_add_new_anon_rmap().

Then, when we free an anon folio we decrement the counter. (hugetlb
should clear the anon flag when an anon folio gets freed back to its
allocator -- likely that is already done).


Sorry that I am talking to myself: I'm wondering if we also have to 
adjust the counter when splitting a large folio to multiple 
smaller-but-still-large folios.

--
Cheers,

David / dhildenb








