Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

[Minchan Kim <minchan@kernel.org>]

On Wed, May 11, 2022 at 04:15:17PM -0700, Minchan Kim wrote:
> On Wed, May 11, 2022 at 04:13:10PM -0700, John Hubbard wrote:
> > On 5/11/22 16:08, Minchan Kim wrote:
> > > > OK, so the code checks the wrong item each time. But the code really
> > > > only needs to know "is either _CMA or _ISOLATE set?". And so you
> > > 
> > > Yes.
> > > 
> > > > can just sidestep the entire question by writing it like this:
> > > > 
> > > > int mt = get_pageblock_migratetype(page);
> > > > 
> > > > if (mt & (MIGRATE_ISOLATE | MIGRATE_CMA))
> > > > 	return false;
> > > 
> > > I am confused. Isn't it same question?
> > > 
> > >                                                      set_pageblock_migratetype(MIGRATE_ISOLATE)
> > > if (get_pageblock_migrate(page) & MIGRATE_CMA)
> > > 
> > >                                                      set_pageblock_migratetype(MIGRATE_CMA)
> > > 
> > > if (get_pageblock_migrate(page) & MIGRATE_ISOLATE)
> > 
> > Well no, because the "&" operation is a single operation on the CPU, and
> > isn't going to get split up like that.
> 
> Oh, if that's true, yeah, I could live with it.
> 
> Thanks, let me post next revision with commenting about that.

This is delta to confirm before posting next revision.

Are you okay with this one?

diff --git a/include/linux/mm.h b/include/linux/mm.h
index cbf79eb790e0..7b2df6780552 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1626,14 +1626,14 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
 static inline bool is_pinnable_page(struct page *page)
 {
 #ifdef CONFIG_CMA
+       int mt = get_pageblock_migratetype(page);
+
        /*
-        * use volatile to use local variable mt instead of
-        * refetching mt value.
+        * "&" operation would prevent compiler split up
+        * get_pageblock_migratetype two times for each
+        * condition check: refetching mt value two times.
         */
-       int __mt = get_pageblock_migratetype(page);
-       int mt = __READ_ONCE(__mt);
-
-       if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
+       if (mt & (MIGRATE_ISOLATE | MIGRATE_CMA))
                return false;
 #endif