Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios

From: Yin, Fengwei
Date: Fri Apr 19 2024 - 09:48:09 EST




On 4/19/2024 5:19 PM, David Hildenbrand wrote:
On 19.04.24 04:29, Yin, Fengwei wrote:


On 4/10/2024 3:22 AM, David Hildenbrand wrote:
@@ -2200,7 +2200,22 @@ static inline size_t folio_size(struct folio *folio)
    */
   static inline bool folio_likely_mapped_shared(struct folio *folio)
   {
-    return page_mapcount(folio_page(folio, 0)) > 1;
+    int mapcount = folio_mapcount(folio);
+
+    /* Only partially-mappable folios require more care. */
+    if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
+        return mapcount > 1;
My understanding is that mapcount > folio_nr_pages(folio) can cover
order 0 folio. And also folio_entire_mapcount() can cover hugetlb (I am
not 100% sure for this one).  I am wondering whether we can drop above
two lines? Thanks.

folio_entire_mapcount() does not apply to small folios, so we must not call that for small folios.
Right. I missed this part. Thanks for clarification.


Regards
Yin, Fengwei


Regarding hugetlb, subpage mapcounts are completely unused, except subpage 0 mapcount, which is now *always* negative (storing a page type) -- so there is no trusting on that value at all.

So in the end, it all looked cleanest when only special-casing on partially-mappable folios where we know the entire mapcount exists and we know that subapge mapcount 0 actually stores something reasonable (not a type).

Thanks!