Re: [RFC PATCH v2 2/4] mm/__free_one_page: skip merge for order-0 page unless compaction failed

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 20, 2018 at 10:11:01PM +0800, Aaron Lu wrote:
> > > A new document file called "struct_page_filed" is added to explain
> > > the newly reused field in "struct page".
> > 
> > Sounds rather ad-hoc for a single field, I'd rather document it via
> > comments.
> 
> Dave would like to have a document to explain all those "struct page"
> fields that are repurposed under different scenarios and this is the
> very start of the document :-)
> 
> I probably should have explained the intent of the document more.

Dave and I are in agreement on "Shouldn't struct page be better documented".
I came up with this a few weeks ago; never quite got round to turning it
into a patch:

+---+-----------+-----------+--------------+----------+--------+--------------+
| B | slab      | pagecache | tail 1       | anon     | tail 1 | hugetlb      |
+===+===========+===========+==============+==========+========+==============+
| 0 | flags                                                                   |
+---+                                                                         |
| 4 |                                                                         |
+---+-----------+-----------+--------------+----------+--------+--------------+
| 8 | s_mem     | mapping   | cmp_mapcount | anon_vma | defer  | mapping      |
+---+           |           +--------------+          | list   |              |
|12 |           |           |              |          |        |              |
+---+-----------+-----------+--------------+----------+        +--------------+
|16 | freelist  | index                               |        | index        |
+---+           |                                     |        | (shifted)    |
|20 |           |                                     |        |              |
+---+-----------+-------------------------------------+--------+--------------+
|24 | counters  | mapcount                                                    |
+---+           +-----------+--------------+----------+--------+--------------+
|28 |           | refcount  |              |          |        | refcount     |
+---+-----------+-----------+--------------+----------+--------+--------------+
|32 | next      | lru       | cmpd_head    |                   | lru          |
+---+           |           |              +-------------------+              +
|36 |           |           |              |                   |              |
+---+-----------+           +--------------+-------------------+              +
|40 | pages     |           | dtor / order |                   |              |
+---+-----------+           +--------------+-------------------+              +
|44 | pobjects  |           |              |                   |              |
+---+-----------+-----------+--------------+----------------------------------+
|48 | slb_cache | private   |              |                                  |
+---+           |           +--------------+----------------------------------+
|52 |           |           |              |                                  |
+---+-----------+-----------+--------------+----------------------------------+