i386 mm: Is the BUILD_BUG() macro broken?

[Delien, Robert]

Hi,

I am building Kernel 4.9.4 i386_defconfig and I notice a lot of (unconditional) use of the macro HPAGE_PMD_NR:

In mm/huge_mm.h:
...
#define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
#define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
...
#else /* CONFIG_TRANSPARENT_HUGEPAGE */
#define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
#define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
#define HPAGE_PMD_SIZE ({ BUILD_BUG(); 0; })
...

Using HPAGE_PMD_SHIFT with CONFIG_TRANSPARENT_HUGEPAGE  out-configured should trip the BUILD_BUG() macro, but somehow doesn't. Or, more likely, I am missing something.

However, the BUILD_BUG() macro _does_ trip as expected when trying to compile without inlines (-fno-inline), at the locations on the list below. If I replace the offending macros in the list below with '0', no other compiler errors occur. The Kernel still won't link, but that was expected.

When building with -fno-inline and CONFIG_TRANSPARENT_HUGEPAGE  configured-in, no BUILD_BUG() (or other compiler errors) occur, as expected. So I do think this is a valid test case for the BUILD_BUG() macro.

Am I missing something obvious here? Any hint is appreciated!

Thanks,

        Robert.


mm/filemap.c:338: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/truncate.c:161: 'HPAGE_PMD_SIZE' -> BUILD_BUG()
mm/truncate.c:502: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/truncate.c:503: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/truncate.c:505: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:557: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:562: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:567: 'THP_FILE_ALLOC' -> BUILD_BUG()
mm/shmem.c:791: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:800: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:801: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:892: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:896: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:905: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:906: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:1728: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:1764: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/shmem.c:2197: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/gup.c:311: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/memory.c:1240: 'HPAGE_PMD_SIZE' -> BUILD_BUG()
mm/mprotect.c:165: 'HPAGE_PMD_SIZE' -> BUILD_BUG()
mm/mprotect.c:174: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/mremap.c:216: 'HPAGE_PMD_SIZE' -> BUILD_BUG()
mm/rmap.c:1319: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()
mm/rmap.c:1278: 'HPAGE_PMD_NR' - > 'HPAGE_PMD_ORDER' -> 'HPAGE_PMD_SHIFT' -> BUILD_BUG()

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