Re: [PATCH v2 1/2] mm: collect the number of anon large folios

[David Hildenbrand]

On 12.08.24 00:49, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> Anon large folios come from three places:
> 1. new allocated large folios in PF, they will call folio_add_new_anon_rmap()
> for rmap;
> 2. a large folio is split into multiple lower-order large folios;
> 3. a large folio is migrated to a new large folio.
>
> In all above three counts, we increase nr_anon by 1;
>
> Anon large folios might go either because of be split or be put
> to free, in these cases, we reduce the count by 1.
>
> Folios that have been added to the swap cache but have not yet received
> an anon mapping won't be counted. This is consistent with the AnonPages
> statistics in /proc/meminfo.

Thinking out loud, I wonder if we want to have something like that for 
any anon folios (including small ones).

Assume we longterm-pinned an anon folio and unmapped/zapped it. It would 
be quite interesting to see that these are actually anon pages still 
consuming memory. Same with memory leaks, when an anon folio doesn't get 
freed for some reason.

The whole "AnonPages" counter thingy is just confusing, it only counts 
what's currently mapped ... so we'd want something different.

But it's okay to start with large folios only, there we have a new 
interface without that legacy stuff :)

> 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                           | 13 ++++++++++---
>   mm/migrate.c                               |  4 ++++
>   mm/page_alloc.c                            |  5 ++++-
>   mm/rmap.c                                  |  1 +
>   6 files changed, 37 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 058485daf186..9fdfb46e4560 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.
>   
> +nr_anon
> +       the number of anon huge pages we have in the whole system.

"transparent ..." otherwise people might confuse it with anon hugetlb 
"huge pages" ... :)

I briefly tried coming up with a better name than "nr_anon" but failed.


[...]

> @@ -447,6 +449,8 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>   	 */
>   	newfolio->index = folio->index;
>   	newfolio->mapping = folio->mapping;
> +	if (folio_test_anon(folio) && folio_test_large(folio))
> +		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
>   	folio_ref_add(newfolio, nr); /* add cache reference */
>   	if (folio_test_swapbacked(folio)) {
>   		__folio_set_swapbacked(newfolio);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 84a7154fde93..382c364d3efa 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1084,8 +1084,11 @@ __always_inline bool free_pages_prepare(struct page *page,
>   			(page + i)->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
>   		}
>   	}
> -	if (PageMappingFlags(page))
> +	if (PageMappingFlags(page)) {
> +		if (PageAnon(page) && compound)
> +			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);

I wonder if you could even drop the "compound" check. mod_mthp_stat 
would handle order == 0 just fine. Not that I think it makes much 
difference.


Nothing else jumped at me.

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb