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 5:58 PM wale zhang <wale.zhang.ftd@gmail.com> wrote:
>
> On Sat, Dec 27, 2025 at 4:16 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > 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.
>
> Hello Barry,
>
> set_pmd_migration_entry() is defined in include/linux/swapops.h.
> For not defining CONFIG_ARCH_ENABLE_THP_MIGRATION, line 342~346 will
> be built as long as *.c file includes swapops.h.
> There are many .c files including the header file directly or
> indirectly, such as fadvise.c, shm.c ...

As far as I understand, set_pmd_migration_entry is never
called unless CONFIG_ARCH_ENABLE_THP_MIGRATION is enabled.
If that is correct, no fix is required.