Messages in this thread | | | Subject | Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode | From | Mike Kravetz <> | Date | Thu, 25 Feb 2021 10:48:04 -0800 |
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On 2/25/21 9:49 AM, Axel Rasmussen wrote: > On Wed, Feb 24, 2021 at 4:26 PM Mike Kravetz <mike.kravetz@oracle.com> wrote: >> >> On 2/18/21 4:48 PM, Axel Rasmussen wrote: >> <snip> >>> @@ -401,8 +398,10 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) >>> >>> BUG_ON(ctx->mm != mm); >>> >>> - VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP)); >>> - VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP)); >>> + /* Any unrecognized flag is a bug. */ >>> + VM_BUG_ON(reason & ~__VM_UFFD_FLAGS); >>> + /* 0 or > 1 flags set is a bug; we expect exactly 1. */ >>> + VM_BUG_ON(!reason || !!(reason & (reason - 1))); >> >> I may be confused, but that seems to be checking for a flag value of 1 >> as opposed to one flag being set? > > (Assuming I implemented it correctly!) It's the logical negation of > this trick: https://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2 > So, it's "VM_BUG_ON(reason is *not* a power of 2)". > > Maybe the double negation makes it overly confusing? It ought to be > equivalent if we remove it and just say: > VM_BUG_ON(!reason || (reason & (reason - 1)));
Sorry, my bad. In my mind I was thinking,
VM_BUG_ON(!reason || !!(reason && (reason - 1)));
-- Mike Kravetz
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