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

[Barry Song <21cnbao@gmail.com>]

On Sat, Dec 27, 2025 at 8:51 PM wale zhang <wale.zhang.ftd@gmail.com> wrote:
>
> 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.

No. You still don’t understand my point. The BUILD_BUG means this
code will never have a chance to be compiled under any possible
kernel configuration. That makes it dead code. If there is any
kernel .config in which it can be built, then we should fix it.

This set_pmd_migration_entry() has no chance of ever being built.

let me give you an example:
#if 0
aaa;
#endif

main()
{

}

Imagine this set_pmd_migration_entry() as aaa.

>
> 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