Re: [PATCH v2 01/18] mm: Make folios_put() the basis of release_pages()

[Matthew Wilcox <willy@infradead.org>]

On Mon, Feb 19, 2024 at 10:43:06AM +0100, David Hildenbrand wrote:
> On 17.02.24 03:25, Matthew Wilcox (Oracle) wrote:
> > By making release_pages() call folios_put(), we can get rid of the calls
> > to compound_head() for the callers that already know they have folios.
> > We can also get rid of the lock_batch tracking as we know the size
> > of the batch is limited by folio_batch.  This does reduce the maximum
> > number of pages for which the lruvec lock is held, from SWAP_CLUSTER_MAX
> > (32) to PAGEVEC_SIZE (15).  I do not expect this to make a significant
> > difference, but if it does, we can increase PAGEVEC_SIZE to 31.
> > 
> 
> I'm afraid that won't apply to current mm-unstable anymore, where we can now
> put multiple references to a single folio (as part of unmapping
> large PTE-mapped folios).

Argh.  I'm not a huge fan of that approach, but let's live with it for
now.  How about this as a replacement patch?  It compiles ...


diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1743bdeab506..42de41e469a1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -36,6 +36,7 @@ struct anon_vma;
 struct anon_vma_chain;
 struct user_struct;
 struct pt_regs;
+struct folio_batch;
 
 extern int sysctl_page_lock_unfairness;
 
@@ -1519,6 +1520,8 @@ static inline void folio_put_refs(struct folio *folio, int refs)
 		__folio_put(folio);
 }
 
+void folios_put_refs(struct folio_batch *folios, unsigned int *refs);
+
 /*
  * union release_pages_arg - an array of pages or folios
  *
@@ -1541,18 +1544,19 @@ void release_pages(release_pages_arg, int nr);
 /**
  * folios_put - Decrement the reference count on an array of folios.
  * @folios: The folios.
- * @nr: How many folios there are.
  *
- * Like folio_put(), but for an array of folios.  This is more efficient
- * than writing the loop yourself as it will optimise the locks which
- * need to be taken if the folios are freed.
+ * Like folio_put(), but for a batch of folios.  This is more efficient
+ * than writing the loop yourself as it will optimise the locks which need
+ * to be taken if the folios are freed.  The folios batch is returned
+ * empty and ready to be reused for another batch; there is no need to
+ * reinitialise it.
  *
  * Context: May be called in process or interrupt context, but not in NMI
  * context.  May be called while holding a spinlock.
  */
-static inline void folios_put(struct folio **folios, unsigned int nr)
+static inline void folios_put(struct folio_batch *folios)
 {
-	release_pages(folios, nr);
+	folios_put_refs(folios, NULL);
 }
 
 static inline void put_page(struct page *page)
diff --git a/mm/mlock.c b/mm/mlock.c
index 086546ac5766..1ed2f2ab37cd 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -206,8 +206,7 @@ static void mlock_folio_batch(struct folio_batch *fbatch)
 
 	if (lruvec)
 		unlock_page_lruvec_irq(lruvec);
-	folios_put(fbatch->folios, folio_batch_count(fbatch));
-	folio_batch_reinit(fbatch);
+	folios_put(fbatch);
 }
 
 void mlock_drain_local(void)
diff --git a/mm/swap.c b/mm/swap.c
index e5380d732c0d..6b736fceccfa 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -89,7 +89,7 @@ static void __page_cache_release(struct folio *folio)
 		__folio_clear_lru_flags(folio);
 		unlock_page_lruvec_irqrestore(lruvec, flags);
 	}
-	/* See comment on folio_test_mlocked in release_pages() */
+	/* See comment on folio_test_mlocked in folios_put() */
 	if (unlikely(folio_test_mlocked(folio))) {
 		long nr_pages = folio_nr_pages(folio);
 
@@ -175,7 +175,7 @@ static void lru_add_fn(struct lruvec *lruvec, struct folio *folio)
 	 * while the LRU lock is held.
 	 *
 	 * (That is not true of __page_cache_release(), and not necessarily
-	 * true of release_pages(): but those only clear the mlocked flag after
+	 * true of folios_put(): but those only clear the mlocked flag after
 	 * folio_put_testzero() has excluded any other users of the folio.)
 	 */
 	if (folio_evictable(folio)) {
@@ -221,8 +221,7 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
 
 	if (lruvec)
 		unlock_page_lruvec_irqrestore(lruvec, flags);
-	folios_put(fbatch->folios, folio_batch_count(fbatch));
-	folio_batch_reinit(fbatch);
+	folios_put(fbatch);
 }
 
 static void folio_batch_add_and_move(struct folio_batch *fbatch,
@@ -946,47 +945,30 @@ void lru_cache_disable(void)
 }
 
 /**
- * release_pages - batched put_page()
- * @arg: array of pages to release
- * @nr: number of pages
+ * folios_put_refs - Reduce the reference count on a batch of folios.
+ * @folios: The folios.
+ * @refs: The number of refs to subtract from each folio.
  *
- * Decrement the reference count on all the pages in @arg.  If it
- * fell to zero, remove the page from the LRU and free it.
+ * Like folio_put(), but for a batch of folios.  This is more efficient
+ * than writing the loop yourself as it will optimise the locks which need
+ * to be taken if the folios are freed.  The folios batch is returned
+ * empty and ready to be reused for another batch; there is no need
+ * to reinitialise it.  If @refs is NULL, we subtract one from each
+ * folio refcount.
  *
- * Note that the argument can be an array of pages, encoded pages,
- * or folio pointers. We ignore any encoded bits, and turn any of
- * them into just a folio that gets free'd.
+ * Context: May be called in process or interrupt context, but not in NMI
+ * context.  May be called while holding a spinlock.
  */
-void release_pages(release_pages_arg arg, int nr)
+void folios_put_refs(struct folio_batch *folios, unsigned int *refs)
 {
 	int i;
-	struct encoded_page **encoded = arg.encoded_pages;
 	LIST_HEAD(pages_to_free);
 	struct lruvec *lruvec = NULL;
 	unsigned long flags = 0;
-	unsigned int lock_batch;
 
-	for (i = 0; i < nr; i++) {
-		unsigned int nr_refs = 1;
-		struct folio *folio;
-
-		/* Turn any of the argument types into a folio */
-		folio = page_folio(encoded_page_ptr(encoded[i]));
-
-		/* Is our next entry actually "nr_pages" -> "nr_refs" ? */
-		if (unlikely(encoded_page_flags(encoded[i]) &
-			     ENCODED_PAGE_BIT_NR_PAGES_NEXT))
-			nr_refs = encoded_nr_pages(encoded[++i]);
-
-		/*
-		 * Make sure the IRQ-safe lock-holding time does not get
-		 * excessive with a continuous string of pages from the
-		 * same lruvec. The lock is held only if lruvec != NULL.
-		 */
-		if (lruvec && ++lock_batch == SWAP_CLUSTER_MAX) {
-			unlock_page_lruvec_irqrestore(lruvec, flags);
-			lruvec = NULL;
-		}
+	for (i = 0; i < folios->nr; i++) {
+		struct folio *folio = folios->folios[i];
+		unsigned int nr_refs = refs ? refs[i] : 1;
 
 		if (is_huge_zero_page(&folio->page))
 			continue;
@@ -1016,13 +998,8 @@ void release_pages(release_pages_arg arg, int nr)
 		}
 
 		if (folio_test_lru(folio)) {
-			struct lruvec *prev_lruvec = lruvec;
-
 			lruvec = folio_lruvec_relock_irqsave(folio, lruvec,
 									&flags);
-			if (prev_lruvec != lruvec)
-				lock_batch = 0;
-
 			lruvec_del_folio(lruvec, folio);
 			__folio_clear_lru_flags(folio);
 		}
@@ -1046,6 +1023,47 @@ void release_pages(release_pages_arg arg, int nr)
 
 	mem_cgroup_uncharge_list(&pages_to_free);
 	free_unref_page_list(&pages_to_free);
+	folios->nr = 0;
+}
+EXPORT_SYMBOL(folios_put);
+
+/**
+ * release_pages - batched put_page()
+ * @arg: array of pages to release
+ * @nr: number of pages
+ *
+ * Decrement the reference count on all the pages in @arg.  If it
+ * fell to zero, remove the page from the LRU and free it.
+ *
+ * Note that the argument can be an array of pages, encoded pages,
+ * or folio pointers. We ignore any encoded bits, and turn any of
+ * them into just a folio that gets free'd.
+ */
+void release_pages(release_pages_arg arg, int nr)
+{
+	struct folio_batch fbatch;
+	int refs[PAGEVEC_SIZE];
+	struct encoded_page **encoded = arg.encoded_pages;
+	int i;
+
+	folio_batch_init(&fbatch);
+	for (i = 0; i < nr; i++) {
+		/* Turn any of the argument types into a folio */
+		struct folio *folio = page_folio(encoded_page_ptr(encoded[i]));
+
+		/* Is our next entry actually "nr_pages" -> "nr_refs" ? */
+		refs[fbatch.nr] = 1;
+		if (unlikely(encoded_page_flags(encoded[i]) &
+			     ENCODED_PAGE_BIT_NR_PAGES_NEXT))
+			refs[fbatch.nr] = encoded_nr_pages(encoded[++i]);
+
+		if (folio_batch_add(&fbatch, folio) > 0)
+			continue;
+		folios_put_refs(&fbatch, refs);
+	}
+
+	if (fbatch.nr)
+		folios_put_refs(&fbatch, refs);
 }
 EXPORT_SYMBOL(release_pages);