Re: [PATCH 1/1] mm,swapops: Fix compile error on set_pmd_migration_entry.

[wale zhang <wale.zhang.ftd@gmail.com>]

On Sat, Dec 27, 2025 at 2:43 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Sat, Dec 27, 2025 at 12:46 AM wale zhang <wale.zhang.ftd@gmail.com> wrote:
> >
> > On Thu, Dec 25, 2025 at 6:22 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Thu, Dec 25, 2025 at 10:41 PM Wale Zhang <wale.zhang.ftd@gmail.com> wrote:
> > > >
> > > > ../include/linux/swapops.h: In function ‘set_pmd_migration_entry’:
> > > > ../include/linux/swapops.h:346:1: error: no return statement in function returning non-void [-Werror=return-type]
> > > >   346 | }
> > > >       | ^
> > > > cc1: some warnings being treated as errors
> > > >
> > > > Signed-off-by: Wale Zhang <wale.zhang.ftd@gmail.com>
> > >
> > > Sorry. NAK.
> > >
> > > This looks odd. We already have a BUILD_BUG(), so no fix is needed.
> > >
> > > we should already have:
> > > ././include/linux/compiler_types.h:631:45: error: call to
> > > ‘__compiletime_assert_907’ declared with attribute error: BUILD_BUG
> > > failed
> > >   631 |         _compiletime_assert(condition, msg,
> > > __compiletime_assert_, __COUNTER__)
> > >       |                                             ^
> > > ././include/linux/compiler_types.h:612:25: note: in definition of
> > > macro ‘__compiletime_assert’
> > >   612 |                         prefix ## suffix();
> > >          \
> > >       |                         ^~~~~~
> > >
> > > Do you actually see this being built? If so, please help identify why it
> > > gets built.
> >
> > Hello Barry,
> >
> > I compile the 6.19-rc2 for arm64 with -O0, this kind of error log will appear.
> > In file included from /home/wale/repo/linux/master/include/linux/leafops.h:11,
> >                  from
> > /home/wale/repo/linux/master/include/linux/userfaultfd_k.h:19,
> >                  from /home/wale/repo/linux/master/include/linux/mm_inline.h:10,
> >                  from /home/wale/repo/linux/master/mm/internal.h:13,
> >                  from /home/wale/repo/linux/master/mm/fadvise.c:24:
> > /home/wale/repo/linux/master/include/linux/swapops.h: In function
> > ‘set_pmd_migration_entry’:
> > /home/wale/repo/linux/master/include/linux/swapops.h:346:1: error: no
> > return statement in function returning non-void [-Werror=return-type]
> >   346 | }
> >       | ^
> > cc1: some warnings being treated as errors
> > make[4]: *** [/home/wale/repo/linux/master/scripts/Makefile.build:287:
> > mm/fadvise.o] Error 1
> > make[4]: *** Waiting for unfinished jobs....
> >
> > There is an example on my
> > github(https://github.com/wale-ftd/linux/commits/linux-6.12/) that
> > used -O0 to compile the 6.12 kernel. If you're interested, you can
> > check them out.
>
> This is not my question. The BUILD_BUG here means this code
> should never be built. If it is built, then that indicates a bug.
> So my question is: under what kernel configuration would this
> code actually get built?

Hello Barry,

I'm very sorry for misunderstanding your meaning.

For -O0, __OPTIMIZE__ isn't defined, so line 615 is hit.

595 #ifdef __OPTIMIZE__
602 # define __compiletime_assert(condition, msg, prefix, suffix)           \
612                         prefix ## suffix();
614 #else
615 # define __compiletime_assert(condition, msg, prefix, suffix)
((void)(condition))
616 #endif

Finally, set_pmd_migration_entry is defined as follow:
static inline int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
                struct page *page)
{
-        BUILD_BUG();
+        ((void)(0));
}

> Thanks
> Barry

--
Thanks
Wale