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

[David Hildenbrand <david@redhat.com>]

On 09.08.24 09:04, Barry Song wrote:
>>>> 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.
> 
> Hi David,
> 
> The conceptual code is shown below. Does this make more
> sense to you? we have a line "mod_mthp_stat(new_order,
> MTHP_STAT_NR_ANON, 1 << (order - new_order));"
> 
> @@ -3270,8 +3272,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>   	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>   	/* reset xarray order to new order after split */
>   	XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
> -	struct anon_vma *anon_vma = NULL;
> +	bool is_anon = folio_test_anon(folio);
>   	struct address_space *mapping = NULL;
> +	struct anon_vma *anon_vma = NULL;
>   	int order = folio_order(folio);
>   	int extra_pins, ret;
>   	pgoff_t end;
> @@ -3283,7 +3286,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>   	if (new_order >= folio_order(folio))
>   		return -EINVAL;
>   
> -	if (folio_test_anon(folio)) {
> +	if (is_anon) {
>   		/* order-1 is not supported for anonymous THP. */
>   		if (new_order == 1) {
>   			VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> @@ -3323,7 +3326,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>   	if (folio_test_writeback(folio))
>   		return -EBUSY;
>   
> -	if (folio_test_anon(folio)) {
> +	if (is_anon) {
>   		/*
>   		 * The caller does not necessarily hold an mmap_lock that would
>   		 * prevent the anon_vma disappearing so we first we take a
> @@ -3437,6 +3440,10 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>   			}
>   		}
>   
> +		if (is_anon) {
> +			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
> +			mod_mthp_stat(new_order, MTHP_STAT_NR_ANON, 1 << (order - new_order));
> +		}
>   		__split_huge_page(page, list, end, new_order);
>   		ret = 0;
>   	} else {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 408ef3d25cf5..c869d0601614 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1039,6 +1039,7 @@ __always_inline bool free_pages_prepare(struct page *page,
>   	bool skip_kasan_poison = should_skip_kasan_poison(page);
>   	bool init = want_init_on_free();
>   	bool compound = PageCompound(page);
> +	bool anon = PageAnon(page);
>   
>   	VM_BUG_ON_PAGE(PageTail(page), page);
>   
> @@ -1130,6 +1131,9 @@ __always_inline bool free_pages_prepare(struct page *page,
>   
>   	debug_pagealloc_unmap_pages(page, 1 << order);
>   
> +	if (anon && compound)
> +		mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
> +
>   	return true;

I'd have placed it here, when we are already passed the "PageMappingFlags" check and
shouldn't have any added overhead for most !anon pages IIRC (mostly only anon/ksm pages should
run into that path).

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 408ef3d25cf5..a11b9dd62964 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1079,8 +1079,11 @@ __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);
                 page->mapping = NULL;
+       }
         if (is_check_pages_enabled()) {
                 if (free_page_is_bad(page))
                         bad++;

Conceptually LGTM. We account an anon folio as long as it's anon,
even when still GUP-pinned after unmapping it or when temporarily
unmapping+remapping it during migration.

-- 
Cheers,

David / dhildenb