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Date: Thu, 21 Oct 2010 11:52:17 +0200 From: Peter Zijlstra <a.p.zijlstra@...llo.nl> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Li Zefan <lizf@...fujitsu.com>, linux-kernel@...r.kernel.org Subject: Re: mmotm 2010-10-20-15-01 uploaded On Thu, 2010-10-21 at 00:30 -0700, Andrew Morton wrote: > On Thu, 21 Oct 2010 09:17:26 +0200 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote: > > > > It would have been clearer to do > > > > > > WARN_ON_ONCE(vaddr != __fix_to_virt(FIX_KMAP_BEGIN + idx)) > > > > > > but I don't see what's gone wrong here. Peter? > > > > Right, so that's the warning that the unmapped address isn't actually > > the top-of-stack one. > > > > Your version is much nicer, although I haven't looked too hard at it, I > > probably got my head in a twist. > > > > But like you, I cannot directly see what's going wrong here, > > zero_user_segments() seems a simple enough function without any > > kmap_atomic nesting, so I'm not at all seeing how that's going wrong. > > > > /me goes find where you hide your mmotm patch-queue and stare at the > > actual code. OK, so found it, and its really rather embarrassing.. I really blotched those x86 WARN_ON_ONCE()s, for some reason I got my brain in a twist and messed those up big-time. All other architectures have the form you suggested earlier. The problem is that fixmaps are top-down, so the original warning ended up trying to compare x with (-x) >> PAGE_SHIFT, which clearly won't work. The sad part is that I apparently only compile tested the debug code :/ With the warning fixed (and CONFIG_DEBUG_HIGHMEM=y) an otherwise i386-defconfig seems to fully complete boot without warnings on a qemu with 2048 MB of memory. --- Subject: mm, x86: Fix stack based kmap_atomic debug warnings From: Peter Zijlstra <a.p.zijlstra@...llo.nl> Date: Thu Oct 21 11:45:08 CEST 2010 Due to a massive brainfart I got the x86 kunmap_atomic debug code that is supposed to avoid stack violations wrong. Use the form all other architectures already use (which was also independently suggested by Andrew). Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl> --- arch/x86/mm/highmem_32.c | 3 +-- arch/x86/mm/iomap_32.c | 3 +-- 2 files changed, 2 insertions(+), 4 deletions(-) Index: linux-2.6/arch/x86/mm/highmem_32.c =================================================================== --- linux-2.6.orig/arch/x86/mm/highmem_32.c +++ linux-2.6/arch/x86/mm/highmem_32.c @@ -78,8 +78,7 @@ void __kunmap_atomic(void *kvaddr) idx = type + KM_TYPE_NR * smp_processor_id(); #ifdef CONFIG_DEBUG_HIGHMEM - WARN_ON_ONCE(idx != - ((vaddr - __fix_to_virt(FIX_KMAP_BEGIN)) >> PAGE_SHIFT)); + WARN_ON_ONCE(vaddr != __fix_to_virt(FIX_KMAP_BEGIN + idx)); #endif /* * Force other mappings to Oops if they'll try to access this Index: linux-2.6/arch/x86/mm/iomap_32.c =================================================================== --- linux-2.6.orig/arch/x86/mm/iomap_32.c +++ linux-2.6/arch/x86/mm/iomap_32.c @@ -102,8 +102,7 @@ iounmap_atomic(void __iomem *kvaddr) idx = type + KM_TYPE_NR * smp_processor_id(); #ifdef CONFIG_DEBUG_HIGHMEM - WARN_ON_ONCE(idx != - ((vaddr - __fix_to_virt(FIX_KMAP_BEGIN)) >> PAGE_SHIFT)); + WARN_ON_ONCE(vaddr != __fix_to_virt(FIX_KMAP_BEGIN + idx)); #endif /* * Force other mappings to Oops if they'll try to access this -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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