[PATCH v14 002/138] mm: Introduce struct folio

Matthew Wilcox (Oracle)
2021-07-15T03:39:12Z

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. No change to generated code.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Howells <dhowells@redhat.com>
---
Documentation/core-api/mm-api.rst | 1 +
include/linux/mm.h | 74 +++++++++++++++++++++++++++++++
include/linux/mm_types.h | 60 +++++++++++++++++++++++++
include/linux/page-flags.h | 28 ++++++++++++
4 files changed, 163 insertions(+)

diff --git a/Documentation/core-api/mm-api.rst b/Documentation/core-api/mm-api.rst
index a42f9baddfbf..2a94e6164f80 100644
--- a/Documentation/core-api/mm-api.rst
+++ b/Documentation/core-api/mm-api.rst
@@ -95,6 +95,7 @@ More Memory Management Functions
.. kernel-doc:: mm/mempolicy.c
.. kernel-doc:: include/linux/mm_types.h
:internal:
+.. kernel-doc:: include/linux/page-flags.h
.. kernel-doc:: include/linux/mm.h
:internal:
.. kernel-doc:: include/linux/mmzone.h
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8dd65290bac0..5071084a71b9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -949,6 +949,20 @@ 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)
{
/*
@@ -1594,6 +1608,65 @@ 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)
+{
+ return (struct folio *)folio_page(folio, folio_nr_pages(folio));
+}
+
+/**
+ * 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()
*/
@@ -1699,6 +1772,7 @@ 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 52bbd2b7cb46..f023eaa866fe 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -231,6 +231,66 @@ 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 folio_attach_private()).
+ * Used for swp_entry_t if folio_test_swapcache().
+ * @_mapcount: Do not access this member directly. Use folio_mapcount() to
+ * find out how many times this folio is mapped by userspace.
+ * @_refcount: Do not access this member directly. Use folio_ref_count()
+ * to find how many references there are to this folio.
+ * @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. It may be mapped into userspace at an address which is
+ * at an arbitrary page offset, but its kernel virtual address is aligned
+ * to its size.
+ */
+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;
+ void *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;
+ };
+};
+
+static_assert(sizeof(struct page) == sizeof(struct folio));
+#define FOLIO_MATCH(pg, fl) \
+ static_assert(offsetof(struct page, pg) == offsetof(struct folio, fl))
+FOLIO_MATCH(flags, flags);
+FOLIO_MATCH(lru, lru);
+FOLIO_MATCH(compound_head, lru);
+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
+
static inline atomic_t *compound_mapcount_ptr(struct page *page)
{
return &page[1].compound_mapcount;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 5922031ffab6..70ede8345538 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -191,6 +191,34 @@ static inline unsigned long _compound_head(const struct page *page)

#define compound_head(page) ((typeof(page))_compound_head(page))

+/**
+ * page_folio - Converts from page to folio.
+ * @p: 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.
+ */
+#define page_folio(p) (_Generic((p), \
+ const struct page *: (const struct folio *)_compound_head(p), \
+ struct page *: (struct folio *)_compound_head(p)))
+
+/**
+ * folio_page - Return a page from a folio.
+ * @folio: The folio.
+ * @n: The page number to return.
+ *
+ * @n is relative to the start of the folio. This function does not
+ * check that the page number lies within @folio; the caller is presumed
+ * to have a reference to the page.
+ */
+#define folio_page(folio, n) nth_page(&(folio)->page, n)
+
static __always_inline int PageTail(struct page *page)
{
return READ_ONCE(page->compound_head) & 1;
--
2.30.2