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authorMatthew Wilcox <mawilcox@microsoft.com>2018-01-31 16:19:06 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2018-01-31 17:18:38 -0800
commitbe50015d7eec0e96b312468291d8209c1cc49908 (patch)
treed2f83adc7cc05ad041793d3e883b457d4894ee50
parent036e7aa49fb29e0b49b99a56fa5611d4a5b99fb1 (diff)
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@infradead.org Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Reviewed-by: Randy Dunlap <rdunlap@infradead.org> Acked-by: Michal Hocko <mhocko@suse.com> Cc: Christoph Lameter <cl@linux.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/mm_types.h24
1 files changed, 23 insertions, 1 deletions
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3e7e99784656..3f1fae8fb140 100644
--- a/include/linux/mm_types.h
+++ b/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