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authorMatthew Wilcox <mawilcox@microsoft.com>2018-06-07 17:08:53 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2018-06-07 17:34:37 -0700
commit97b4a671987123009724dfbdf188ec6e122573d4 (patch)
treea1612351baa09582b98f6cfa41bd1232d995c289 /include
parent4da1984edbbed0ffc961006d265d78a7b0095ea4 (diff)
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@infradead.org Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com> Reviewed-by: Randy Dunlap <rdunlap@infradead.org> Acked-by: Vlastimil Babka <vbabka@suse.cz> Cc: Christoph Lameter <cl@linux.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Jérôme Glisse <jglisse@redhat.com> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com> Cc: Lai Jiangshan <jiangshanlai@gmail.com> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/mm_types.h40
1 files changed, 19 insertions, 21 deletions
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index dfe6d648e055..9b45760cc797 100644
--- a/include/linux/mm_types.h
+++ b/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