The patch titled
Subject: mm: improve struct page documentation
has been added to the -mm tree. Its filename is
mm-improve-struct-page-documentation.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-improve-struct-page-documentation.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-improve-struct-page-documentation.patch
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------------------------------------------------------
From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Subject: mm: improve struct page documentation
Rewrite the documentation to describe what you can use in struct page
rather than what you can't.
Link: http://lkml.kernel.org/r/20180518194519.3820-12-willy@xxxxxxxxxxxxx
Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Reviewed-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm_types.h | 40 +++++++++++++++++--------------------
1 file changed, 19 insertions(+), 21 deletions(-)
diff -puN include/linux/mm_types.h~mm-improve-struct-page-documentation include/linux/mm_types.h
--- a/include/linux/mm_types.h~mm-improve-struct-page-documentation
+++ a/include/linux/mm_types.h
@@ -33,29 +33,27 @@ struct hmm;
* it to keep track of whatever it is we are using the page for at the
* moment. Note that we have no way to track which tasks are using
* a page, though if it is a pagecache page, rmap structures can tell us
- * who is mapping it. If you allocate the page using alloc_pages(), you
- * can use some of the space in struct page for your own purposes.
+ * who is mapping it.
*
- * Pages that were once in the page cache may be found under the RCU lock
- * even after they have been recycled to a different purpose. The page
- * cache reads and writes some of the fields in struct page to pin the
- * page before checking that it's still in the page cache. It is vital
- * that all users of struct page:
- * 1. Use the first word as PageFlags.
- * 2. Clear or preserve bit 0 of page->compound_head. It is used as
- * PageTail for compound pages, and the page cache must not see false
- * positives. Some users put a pointer here (guaranteed to be at least
- * 4-byte aligned), other users avoid using the field altogether.
- * 3. page->_refcount must either not be used, or must be used in such a
- * way that other CPUs temporarily incrementing and then decrementing the
- * refcount does not cause problems. On receiving the page from
- * alloc_pages(), the refcount will be positive.
- * 4. Either preserve page->_mapcount or restore it to -1 before freeing it.
+ * If you allocate the page using alloc_pages(), you can use some of the
+ * space in struct page for your own purposes. The five words in the main
+ * union are available, except for bit 0 of the first word which must be
+ * kept clear. Many users use this word to store a pointer to an object
+ * 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 you allocate pages of order > 0, you can use the fields in the struct
- * page associated with each page, but bear in mind that the pages may have
- * been inserted individually into the page cache, so you must use the above
- * four fields in a compatible way for each struct 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.
+ *
+ * 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
+ * refcount does not cause problems. On receiving the page from
+ * alloc_pages(), the refcount will be positive.
+ *
+ * If you allocate pages of order > 0, you can use some of the fields
+ * in each subpage, but you may need to restore some of their values
+ * afterwards.
*
* SLUB uses cmpxchg_double() to atomically update its freelist and
* counters. That requires that freelist & counters be adjacent and
_
Patches currently in -mm which might be from mawilcox@xxxxxxxxxxxxx are
lib-test_bitmapc-fix-bitmap-optimisation-tests-to-report-errors-correctly.patch
slab-__gfp_zero-is-incompatible-with-a-constructor.patch
s390-use-_refcount-for-pgtables.patch
mm-split-page_type-out-from-_mapcount.patch
mm-mark-pages-in-use-for-page-tables.patch
mm-switch-s_mem-and-slab_cache-in-struct-page.patch
mm-move-private-union-within-struct-page.patch
mm-move-_refcount-out-of-struct-page-union.patch
mm-combine-first-three-unions-in-struct-page.patch
mm-use-page-deferred_list.patch
mm-move-lru-union-within-struct-page.patch
mm-combine-lru-and-main-union-in-struct-page.patch
mm-improve-struct-page-documentation.patch
mm-add-pt_mm-to-struct-page.patch
mm-add-hmm_data-to-struct-page.patch
slabslub-remove-rcu_head-size-checks.patch
slub-remove-kmem_cache-reserved.patch
slub-remove-reserved-file-from-sysfs.patch
mm-distinguish-vmalloc-pages.patch
ida-remove-simple_ida_lock.patch
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