[PATCH v6 01/27] mm: Introduce struct folio

Matthew Wilcox (Oracle)
2021-03-31T18:48:44Z

A struct folio is a new abstraction to replace the venerable struct page.
A function which takes a struct folio argument declares that it will
operate on the entire (possibly compound) page, not just PAGE_SIZE bytes.
In return, the caller guarantees that the pointer it is passing does
not point to a tail page.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
include/linux/mm.h | 78 ++++++++++++++++++++++++++++++++++++++++
include/linux/mm_types.h | 65 +++++++++++++++++++++++++++++++++
mm/util.c | 19 ++++++++++
3 files changed, 162 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3e4dc6678eb2..761063e733bf 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -936,6 +936,20 @@ static inline unsigned int compound_order(struct page *page)
return page[1].compound_order;
}

+/**
+ * folio_order - The allocation order of a folio.
+ * @folio: The folio.
+ *
+ * A folio is composed of 2^order pages. See get_order() for the definition
+ * of order.
+ *
+ * Return: The order of the folio.
+ */
+static inline unsigned int folio_order(struct folio *folio)
+{
+ return compound_order(&folio->page);
+}
+
static inline bool hpage_pincount_available(struct page *page)
{
/*
@@ -1581,6 +1595,69 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
#endif
}

+/**
+ * folio_nr_pages - The number of pages in the folio.
+ * @folio: The folio.
+ *
+ * Return: A number which is a power of two.
+ */
+static inline unsigned long folio_nr_pages(struct folio *folio)
+{
+ return compound_nr(&folio->page);
+}
+
+/**
+ * folio_next - Move to the next physical folio.
+ * @folio: The folio we're currently operating on.
+ *
+ * If you have physically contiguous memory which may span more than
+ * one folio (eg a &struct bio_vec), use this function to move from one
+ * folio to the next. Do not use it if the memory is only virtually
+ * contiguous as the folios are almost certainly not adjacent to each
+ * other. This is the folio equivalent to writing ``page++``.
+ *
+ * Context: We assume that the folios are refcounted and/or locked at a
+ * higher level and do not adjust the reference counts.
+ * Return: The next struct folio.
+ */
+static inline struct folio *folio_next(struct folio *folio)
+{
+#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
+ return (struct folio *)nth_page(&folio->page, folio_nr_pages(folio));
+#else
+ return folio + folio_nr_pages(folio);
+#endif
+}
+
+/**
+ * folio_shift - The number of bits covered by this folio.
+ * @folio: The folio.
+ *
+ * A folio contains a number of bytes which is a power-of-two in size.
+ * This function tells you which power-of-two the folio is.
+ *
+ * Context: The caller should have a reference on the folio to prevent
+ * it from being split. It is not necessary for the folio to be locked.
+ * Return: The base-2 logarithm of the size of this folio.
+ */
+static inline unsigned int folio_shift(struct folio *folio)
+{
+ return PAGE_SHIFT + folio_order(folio);
+}
+
+/**
+ * folio_size - The number of bytes in a folio.
+ * @folio: The folio.
+ *
+ * Context: The caller should have a reference on the folio to prevent
+ * it from being split. It is not necessary for the folio to be locked.
+ * Return: The number of bytes in this folio.
+ */
+static inline size_t folio_size(struct folio *folio)
+{
+ return PAGE_SIZE << folio_order(folio);
+}
+
/*
* Some inline functions in vmstat.h depend on page_zone()
*/
@@ -1685,6 +1762,7 @@ extern void pagefault_out_of_memory(void);

#define offset_in_page(p) ((unsigned long)(p) & ~PAGE_MASK)
#define offset_in_thp(page, p) ((unsigned long)(p) & (thp_size(page) - 1))
+#define offset_in_folio(folio, p) ((unsigned long)(p) & (folio_size(folio) - 1))

/*
* Flags passed to show_mem() and show_free_areas() to suppress output in
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6613b26a8894..a0c7894fad1d 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -224,6 +224,71 @@ struct page {
#endif
} _struct_page_alignment;

+/**
+ * struct folio - Represents a contiguous set of bytes.
+ * @flags: Identical to the page flags.
+ * @lru: Least Recently Used list; tracks how recently this folio was used.
+ * @mapping: The file this page belongs to, or refers to the anon_vma for
+ * anonymous pages.
+ * @index: Offset within the file, in units of pages. For anonymous pages,
+ * this is the index from the beginning of the mmap.
+ * @private: Filesystem per-folio data (see attach_folio_private()).
+ * Used for swp_entry_t if FolioSwapCache().
+ * @_mapcount: How many times this folio is mapped to userspace. Use
+ * folio_mapcount() to access it.
+ * @_refcount: Number of references to this folio. Use folio_ref_count()
+ * to read it.
+ * @memcg_data: Memory Control Group data.
+ *
+ * A folio is a physically, virtually and logically contiguous set
+ * of bytes. It is a power-of-two in size, and it is aligned to that
+ * same power-of-two. It is at least as large as %PAGE_SIZE. If it is
+ * in the page cache, it is at a file offset which is a multiple of that
+ * power-of-two.
+ */
+struct folio {
+ /* private: don't document the anon union */
+ union {
+ struct {
+ /* public: */
+ unsigned long flags;
+ struct list_head lru;
+ struct address_space *mapping;
+ pgoff_t index;
+ unsigned long private;
+ atomic_t _mapcount;
+ atomic_t _refcount;
+#ifdef CONFIG_MEMCG
+ unsigned long memcg_data;
+#endif
+ /* private: the union with struct page is transitional */
+ };
+ struct page page;
+ };
+};
+
+/**
+ * page_folio - Converts from page to folio.
+ * @page: The page.
+ *
+ * Every page is part of a folio. This function cannot be called on a
+ * NULL pointer.
+ *
+ * Context: No reference, nor lock is required on @page. If the caller
+ * does not hold a reference, this call may race with a folio split, so
+ * it should re-check the folio still contains this page after gaining
+ * a reference on the folio.
+ * Return: The folio which contains this page.
+ */
+static inline struct folio *page_folio(struct page *page)
+{
+ unsigned long head = READ_ONCE(page->compound_head);
+
+ if (unlikely(head & 1))
+ return (struct folio *)(head - 1);
+ return (struct folio *)page;
+}
+
static inline atomic_t *compound_mapcount_ptr(struct page *page)
{
return &page[1].compound_mapcount;
diff --git a/mm/util.c b/mm/util.c
index 0b6dd9d81da7..521a772f06eb 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -686,6 +686,25 @@ struct anon_vma *page_anon_vma(struct page *page)
return __page_rmapping(page);
}

+static inline void folio_build_bug(void)
+{
+#define FOLIO_MATCH(pg, fl) \
+BUILD_BUG_ON(offsetof(struct page, pg) != offsetof(struct folio, fl));
+
+ FOLIO_MATCH(flags, flags);
+ FOLIO_MATCH(lru, lru);
+ FOLIO_MATCH(mapping, mapping);
+ FOLIO_MATCH(index, index);
+ FOLIO_MATCH(private, private);
+ FOLIO_MATCH(_mapcount, _mapcount);
+ FOLIO_MATCH(_refcount, _refcount);
+#ifdef CONFIG_MEMCG
+ FOLIO_MATCH(memcg_data, memcg_data);
+#endif
+#undef FOLIO_MATCH
+ BUILD_BUG_ON(sizeof(struct page) != sizeof(struct folio));
+}
+
struct address_space *page_mapping(struct page *page)
{
struct address_space *mapping;
--
2.30.2