The patch titled
Subject: mm: update _mapcount and page_type documentation
has been added to the -mm mm-unstable branch. Its filename is
mm-update-_mapcount-and-page_type-documentation.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-update-_mapcount-and-page_type-documentation.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm: update _mapcount and page_type documentation
Date: Wed, 29 May 2024 13:18:59 +0200
Patch series "mm: page_type, zsmalloc and page_mapcount_reset()", v2.
Wanting to remove the remaining abuser of _mapcount/page_type along with
page_mapcount_reset(), I stumbled over zsmalloc, which is yet to be
converted away from "struct page" [1].
Unfortunately, we cannot stop using the page_type field in zsmalloc code
completely for its own purposes. All other fields in "struct page" are
used one way or the other. Could we simply store a 2-byte offset value at
the beginning of each page? Likely, but that will require a bit more
work; and once we have memdesc we might want to move the offset in there
(struct zsalloc?) again.
.. but we can limit the abuse to 16 bit, glue it to a page type that
must be set, and document it. page_has_type() will always successfully
indicate such zsmalloc pages, and such zsmalloc pages only.
We lose zsmalloc support for PAGE_SIZE > 64KB, which should be tolerable.
We could use more bits from the page type, but 16 bit sounds like a good
idea for now.
So clarify the _mapcount/page_type documentation, use a proper page_type
for zsmalloc, and remove page_mapcount_reset().
[1] https://lore.kernel.org/all/20231130101242.2590384-1-42.hyeyoo@xxxxxxxxx/
This patch (of 6):
Let's make it clearer that _mapcount must no longer be used for own
purposes, and how _mapcount and page_type behaves nowadays (also in the
context of hugetlb folios, which are typed folios that will be mapped to
user space).
Move the documentation regarding "-1" over from page_mapcount_reset(),
which we will remove next. Move "page_type" before "mapcount", to make it
clearer what typed folios are.
Link: https://lkml.kernel.org/r/20240529111904.2069608-1-david@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20240529111904.2069608-2-david@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 5 -----
include/linux/mm_types.h | 28 +++++++++++++++++-----------
2 files changed, 17 insertions(+), 16 deletions(-)
--- a/include/linux/mm.h~mm-update-_mapcount-and-page_type-documentation
+++ a/include/linux/mm.h
@@ -1206,11 +1206,6 @@ static inline int folio_entire_mapcount(
return atomic_read(&folio->_entire_mapcount) + 1;
}
-/*
- * The atomic page->_mapcount, starts from -1: so that transitions
- * both from it and to it can be tracked, using atomic_inc_and_test
- * and atomic_add_negative(-1).
- */
static inline void page_mapcount_reset(struct page *page)
{
atomic_set(&(page)->_mapcount, -1);
--- a/include/linux/mm_types.h~mm-update-_mapcount-and-page_type-documentation
+++ a/include/linux/mm_types.h
@@ -46,9 +46,7 @@ struct mem_cgroup;
* which is guaranteed to be aligned. If you use the same storage as
* page->mapping, you must restore it to NULL before freeing the page.
*
- * If your page will not be mapped to userspace, you can also use the four
- * bytes in the mapcount union, but you must call page_mapcount_reset()
- * before freeing it.
+ * The mapcount field must not be used for own purposes.
*
* If you want to use the refcount field, it must be used in such a way
* that other CPUs temporarily incrementing and then decrementing the
@@ -152,18 +150,26 @@ struct page {
union { /* This union is 4 bytes in size. */
/*
- * If the page can be mapped to userspace, encodes the number
- * of times this page is referenced by a page table.
+ * For head pages of typed folios, the value stored here
+ * allows for determining what this page is used for. The
+ * tail pages of typed folios will not store a type
+ * (page_type == _mapcount == -1).
+ *
+ * See page-flags.h for a list of page types which are currently
+ * stored here.
*/
- atomic_t _mapcount;
+ unsigned int page_type;
/*
- * If the page is neither PageSlab nor mappable to userspace,
- * the value stored here may help determine what this page
- * is used for. See page-flags.h for a list of page types
- * which are currently stored here.
+ * For pages that are part of non-typed folios for which mappings
+ * are tracked via the RMAP, encodes the number of times this page
+ * is directly referenced by a page table.
+ *
+ * Note that the mapcount is always initialized to -1, so that
+ * transitions both from it and to it can be tracked, using
+ * atomic_inc_and_test() and atomic_add_negative(-1).
*/
- unsigned int page_type;
+ atomic_t _mapcount;
};
/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-memory-move-page_count-check-into-validate_page_before_insert.patch
mm-memory-cleanly-support-zeropage-in-vm_insert_page-vm_map_pages-and-vmf_insert_mixed.patch
mm-rmap-sanity-check-that-zeropages-are-not-passed-to-rmap.patch
mm-update-_mapcount-and-page_type-documentation.patch
mm-allow-reuse-of-the-lower-16-bit-of-the-page-type-with-an-actual-type.patch
mm-zsmalloc-use-a-proper-page-type.patch
mm-page_alloc-clear-pagebuddy-using-__clearpagebuddy-for-bad-pages.patch
mm-filemap-reinitialize-folio-_mapcount-directly.patch
mm-mm_init-initialize-page-_mapcount-directly-in-__init_single_page.patch
