Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

[Jason A. Donenfeld]

On Thu, Jul 11, 2024 at 07:56:39PM +0200, Jason A. Donenfeld wrote:
> On Thu, Jul 11, 2024 at 07:54:34PM +0200, Jason A. Donenfeld wrote:
> > On Thu, Jul 11, 2024 at 07:27:27PM +0200, David Hildenbrand wrote:
> > > > PG_owner_priv_1 maps to PG_swapcache? :)
> > > 
> > > Maybe the combination !swapbacked && swapcache could be used to indicate 
> > > such folios. (we will never set swapbacked)
> > > 
> > > But likely we have to be a bit careful here. We don't want 
> > > folio_test_swapcache() to return for folios that ... are not in the 
> > > swapcache.
> > 
> > I was thinking that too, but I'm afraid it's going to be another
> > whack-a-mole nightmare. Even for things like task_mmu in procfs that
> > show stats, that's going to be wonky.
> > 
> > Any other flags we can overload that aren't going to be already used in
> > our case?
> 
> PG_error / folio_set_error seems unused in the non-IO case.

And PG_large_rmappable seems to only be used for hugetlb branches.

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b9e914e1face..7fdc03197438 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -190,6 +190,7 @@ enum pageflags {
 	/* At least one page in this folio has the hwpoison flag set */
 	PG_has_hwpoisoned = PG_error,
 	PG_large_rmappable = PG_workingset, /* anon or file-backed */
+	PG_droppable = PG_error, /* anon droppable, not hugetlb */
 };

 #define PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)
@@ -640,6 +641,8 @@ FOLIO_TEST_CLEAR_FLAG_FALSE(young)
 FOLIO_FLAG_FALSE(idle)
 #endif

+FOLIO_FLAG(droppable, FOLIO_SECOND_PAGE)
+
 /*
  * PageReported() is used to track reported free pages within the Buddy
  * allocator. We can use the non-atomic version of the test and set
diff --git a/mm/rmap.c b/mm/rmap.c
index 1f9b5a9cb121..73b4052b2f82 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1403,6 +1403,8 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 	 */
 	if (!(vma->vm_flags & VM_DROPPABLE))
 		__folio_set_swapbacked(folio);
+	else
+		folio_set_droppable(folio);
 	__folio_set_anon(folio, vma, address, true);

 	if (likely(!folio_test_large(folio))) {
@@ -1852,7 +1854,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				      * ones can be dropped even if they've
 				      * been dirtied.
 				      */
-				     (vma->vm_flags & VM_DROPPABLE))) {
+				      folio_test_droppable(folio))) {
 					dec_mm_counter(mm, MM_ANONPAGES);
 					goto discard;
 				}
@@ -1866,7 +1868,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				 * Unlike MADV_FREE mappings, VM_DROPPABLE ones
 				 * never get swap backed on failure to drop.
 				 */
-				if (!(vma->vm_flags & VM_DROPPABLE))
+				if (!folio_test_droppable(folio))
 					folio_set_swapbacked(folio);
 				ret = false;
 				page_vma_mapped_walk_done(&pvmw);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2e34de9cd0d4..41340f2a12c7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4266,7 +4266,8 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
 	}

 	/* dirty lazyfree */
-	if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio)) {
+	if (type == LRU_GEN_FILE && folio_test_anon(folio) &&
+	    folio_test_dirty(folio) && !folio_test_droppable(folio)) {
 		success = lru_gen_del_folio(lruvec, folio, true);
 		VM_WARN_ON_ONCE_FOLIO(!success, folio);
 		folio_set_swapbacked(folio);