On Tue, Feb 23, 2016 at 03:32:44PM -0800, Andrew Morton wrote:

On Tue, 23 Feb 2016 16:21:17 +0900 js1304@gmail.com wrote:

quoted

From: Joonsoo Kim <redacted>

Success of CMA allocation largely depends on success of migration
and key factor of it is page reference count. Until now, page reference
is manipulated by direct calling atomic functions so we cannot follow up
who and where manipulate it. Then, it is hard to find actual reason
of CMA allocation failure. CMA allocation should be guaranteed to succeed
so finding offending place is really important.

In this patch, call sites where page reference is manipulated are converted
to introduced wrapper function. This is preparation step to add tracepoint
to each page reference manipulation function. With this facility, we can
easily find reason of CMA allocation failure. There is no functional change
in this patch.

...

--- a/arch/mips/mm/gup.c
+++ b/arch/mips/mm/gup.c

@@ -64,7 +64,7 @@ static inline void get_head_page_multiple(struct page *page, int nr)
 {
 	VM_BUG_ON(page != compound_head(page));
 	VM_BUG_ON(page_count(page) == 0);
-	atomic_add(nr, &page->_count);
+	page_ref_add(page, nr);

Seems reasonable.  Those open-coded refcount manipulations have always
bugged me.

I think so.

The patches will be a bit of a pain to maintain but surprisingly they
apply OK at present.  It's possible that by the time they hit upstream,
some direct ->_count references will still be present and it will
require a second pass to complete the conversion.

In fact, the patch doesn't change direct ->_count reference for
*read*. That's the reason that it is surprisingly OK at present.

It's a good idea to change direct ->_count reference even for read.
How about changing it in rc2 after mering this patch in rc1?

After that pass is completed I suggest we rename page._count to
something else (page.ref_count_dont_use_this_directly_you_dope?).  That
way, any attempts to later add direct page._count references will
hopefully break, alerting the programmer to the new regime.

Agreed.

Thanks.

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