The patch titled
Subject: mm: document how to use struct page
has been added to the -mm tree. Its filename is
mm-document-how-to-use-struct-page.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-document-how-to-use-struct-page.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-document-how-to-use-struct-page.patch
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------------------------------------------------------
From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Subject: mm: document how to use struct page
Be really explicit about what bits / bytes are reserved for users that
want to store extra information about the pages they allocate.
Link: http://lkml.kernel.org/r/20171220155552.15884-8-willy@xxxxxxxxxxxxx
Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm_types.h | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)
diff -puN include/linux/mm_types.h~mm-document-how-to-use-struct-page include/linux/mm_types.h
--- a/include/linux/mm_types.h~mm-document-how-to-use-struct-page
+++ a/include/linux/mm_types.h
@@ -31,7 +31,29 @@ 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.
+ * 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.
+ *
+ * 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 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.
*
* 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
provide-useful-debugging-information-for-vm_bug.patch
mm-align-struct-page-more-aesthetically.patch
mm-de-indent-struct-page.patch
mm-remove-misleading-alignment-claims.patch
mm-improve-comment-on-page-mapping.patch
mm-introduce-_slub_counter_t.patch
mm-store-compound_dtor-compound_order-as-bytes.patch
mm-document-how-to-use-struct-page.patch
mm-remove-reference-to-pg_buddy.patch
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