Messages in this thread | | | From | Randy Dunlap <> | Subject | Re: i386 mm: Is the BUILD_BUG() macro broken? | Date | Tue, 17 Jan 2017 12:08:18 -0800 |
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Please wrap long text lines around column 70 or so. Thanks.
I'll take a shot at answering this. Maybe someone will correct me if I am mistaken.
On 01/17/17 08:32, Delien, Robert wrote: > 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()
Use of HPAGE_PMD_NR is guarded by if (PageTransHuge(page) && !PageHuge(page)) { which is always false, so the block contents are not compiled.
Look at the object code generated for delete_from_page_cache() [the usage above] and you won't see code for that block.
> 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()
above uses are guarded by PageTransHuge() [false]
OTOH, (still in truncate.c) if I use HPAGE_PMD_NR at the beginning of invalidate_mapping_pages(), like this: ///pagevec_init(&pvec, 0); pagevec_init(&pvec, HPAGE_PMD_NR); it does cause a build-time error.
Or in delete_from_page_cache(), if I change the call to put_page() like this: } else { ///put_page(page); put_page((struct page *)(HPAGE_PMD_NR)); } this also causes a build-time error.
Note that if I make a typo (e.g. HPAGE_PMD_RN) inside a [false] block, that will still cause a syntax error.
> 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()
above are guarded by PageTransHuge() [false] or by !IS_ENABLED(CONFIG_some_HUGE_whatever) [false]
> 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()
above are guarded by pmd_trans_huge(), which is always false.
> 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()
above are guarded by PageTransHuge(), which is always false. or by: if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) return;
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Not!
-- ~Randy
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