On Fri, Jan 24, 2025 at 10:01:40AM +0800, kernel test robot wrote: > tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master > head: e0b1f59142746f74476a03040f745329c8355a7e > commit: e6fd5564c07c3c749ff3d1b2aa35540b4047e395 mm/gup: cache p4d in follow_p4d_mask() > date: 9 months ago > config: alpha-randconfig-r112-20250124 (https://download.01.org/0day-ci/archive/20250124/202501240922.a4YzQD7u-lkp@xxxxxxxxx/config) > compiler: alpha-linux-gcc (GCC) 14.2.0 > reproduce: (https://download.01.org/0day-ci/archive/20250124/202501240922.a4YzQD7u-lkp@xxxxxxxxx/reproduce) > > If you fix the issue in a separate patch/commit (i.e. not just a new version of > the same patch/commit), kindly add following tags > | Reported-by: kernel test robot <lkp@xxxxxxxxx> > | Closes: https://lore.kernel.org/oe-kbuild-all/202501240922.a4YzQD7u-lkp@xxxxxxxxx/ > > sparse warnings: (new ones prefixed by >>) > >> mm/gup.c:778:15: sparse: sparse: cast to non-scalar > >> mm/gup.c:778:15: sparse: sparse: cast from non-scalar > mm/gup.c: note: in included file (through include/linux/mm.h): > include/linux/pgtable.h:315:16: sparse: sparse: cast to non-scalar > include/linux/pgtable.h:315:16: sparse: sparse: cast from non-scalar > include/linux/pgtable.h:315:16: sparse: sparse: cast to non-scalar > include/linux/pgtable.h:315:16: sparse: sparse: cast from non-scalar > mm/gup.c: note: in included file (through include/linux/mmzone.h, include/linux/gfp.h, include/linux/mm.h): > include/linux/page-flags.h:241:46: sparse: sparse: self-comparison always evaluates to false > mm/gup.c:682:9: sparse: sparse: context imbalance in 'follow_page_pte' - unexpected unlock > mm/gup.c: note: in included file (through include/linux/mm.h): > include/linux/pgtable.h:322:16: sparse: sparse: cast to non-scalar > include/linux/pgtable.h:322:16: sparse: sparse: cast from non-scalar > include/linux/pgtable.h:315:16: sparse: sparse: cast to non-scalar > include/linux/pgtable.h:315:16: sparse: sparse: cast from non-scalar > mm/gup.c:911:18: sparse: sparse: context imbalance in 'get_gate_page' - unexpected unlock > > vim +778 mm/gup.c > > 769 > 770 static struct page *follow_p4d_mask(struct vm_area_struct *vma, > 771 unsigned long address, pgd_t *pgdp, > 772 unsigned int flags, > 773 struct follow_page_context *ctx) > 774 { > 775 p4d_t *p4dp, p4d; > 776 > 777 p4dp = p4d_offset(pgdp, address); > > 778 p4d = READ_ONCE(*p4dp); > 779 if (p4d_none(p4d)) > 780 return no_page_table(vma, flags); > 781 BUILD_BUG_ON(p4d_huge(p4d)); > 782 if (unlikely(p4d_bad(p4d))) > 783 return no_page_table(vma, flags); > 784 > 785 return follow_pud_mask(vma, address, p4dp, flags, ctx); > 786 } > 787 There's one more similar sparse warning here, both reported today: https://lore.kernel.org/all/202501241212.q9AAshQc-lkp@xxxxxxxxx/ Firstly, I reproduced this sparse warning locally. And yes it yells about that line, and even one more similar case of READ_ONCE(). The question is I thought READ_ONCE() should be ok to be used on things like p4d_t, even if they can be structs / non-scalar types. At least from sparse POV I cannot trigger similar sparse on non-alpha. So looks like Alpha has something special. Then I noticed that Alpha indeed has a custom definition of __READ_ONCE() which is probably needed due to the mb(): #define __READ_ONCE(x) \ ({ \ __unqual_scalar_typeof(x) __x = \ (*(volatile typeof(__x) *)(&(x))); \ mb(); \ (typeof(x))__x; \ }) I didn't obviously see yet on how that could affect sparse, if READ_ONCE() works for non-scalar somehow on most of the rest archs. I tried to switch to the default __READ_ONCE for alpha build (it misses mb() so it isn't correct, but trying to figure out whether it'll trigger similar sparse warning), then the warning is gone. Then I found that this change can also work for alpha: diff --git a/arch/alpha/include/asm/rwonce.h b/arch/alpha/include/asm/rwonce.h index 35542bcf92b3..0d61183ff764 100644 --- a/arch/alpha/include/asm/rwonce.h +++ b/arch/alpha/include/asm/rwonce.h @@ -25,7 +25,7 @@ __unqual_scalar_typeof(x) __x = \ (*(volatile typeof(__x) *)(&(x))); \ mb(); \ - (typeof(x))__x; \ + __x; \ }) I had a feeling that sparse somehow thinks the "typeof(x)" isn't the same struct v.s. "__unqual_scalar_typeof(x)" above, even if IIUC the macro was only trying to tear down type qualifiers. Especially in Alpha's case IIUC p4d_t is a struct (in my case, STRICT_MM_TYPECHECKS=n): typedef struct { pgd_t pgd; } p4d_t; typedef unsigned long pgd_t; So I suppose __unqual_scalar_typeof() should do nothing on a non-scalar type. So even if above oneliner seems to fix this specific sparse warning, I don't really know how it worked. I copied Will (who introduced both the alpha's __READ_ONCE, and the unqual macro for generic code) in case he has some thoughts.. Thanks, -- Peter Xu
