From: Mike Kravetz <mike.kravetz@oracle.com>
To: Axel Rasmussen <axelrasmussen@google.com>
Cc: "Alexander Viro" <viro@zeniv.linux.org.uk>,
"Alexey Dobriyan" <adobriyan@gmail.com>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Anshuman Khandual" <anshuman.khandual@arm.com>,
"Catalin Marinas" <catalin.marinas@arm.com>,
"Chinwen Chang" <chinwen.chang@mediatek.com>,
"Huang Ying" <ying.huang@intel.com>,
"Ingo Molnar" <mingo@redhat.com>, "Jann Horn" <jannh@google.com>,
"Jerome Glisse" <jglisse@redhat.com>,
"Lokesh Gidra" <lokeshgidra@google.com>,
"Matthew Wilcox (Oracle)" <willy@infradead.org>,
"Michael Ellerman" <mpe@ellerman.id.au>,
"Michal Koutný" <mkoutny@suse.com>,
"Michel Lespinasse" <walken@google.com>,
"Mike Rapoport" <rppt@linux.vnet.ibm.com>,
"Nicholas Piggin" <npiggin@gmail.com>,
"Peter Xu" <peterx@redhat.com>, "Shaohua Li" <shli@fb.com>,
"Shawn Anastasio" <shawn@anastas.io>,
"Steven Rostedt" <rostedt@goodmis.org>,
"Steven Price" <steven.price@arm.com>,
"Vlastimil Babka" <vbabka@suse.cz>,
LKML <linux-kernel@vger.kernel.org>,
linux-fsdevel@vger.kernel.org, "Linux MM" <linux-mm@kvack.org>,
"Adam Ruprecht" <ruprecht@google.com>,
"Cannon Matthews" <cannonmatthews@google.com>,
"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
"David Rientjes" <rientjes@google.com>,
"Mina Almasry" <almasrymina@google.com>,
"Oliver Upton" <oupton@google.com>
Subject: Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode
Date: Thu, 25 Feb 2021 10:48:04 -0800 [thread overview]
Message-ID: <0c0c8e86-cf60-12d1-2260-2a7215493fd5@oracle.com> (raw)
In-Reply-To: <CAJHvVch4iweAW274Ub5Q_oKgZaTHvEkGnE4=jo6SfxOs1qCf6A@mail.gmail.com>
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
next prev parent reply other threads:[~2021-02-25 18:50 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-19 0:48 [PATCH v7 0/6] userfaultfd: add minor fault handling Axel Rasmussen
2021-02-19 0:48 ` [PATCH v7 1/6] userfaultfd: add minor fault registration mode Axel Rasmussen
2021-02-23 15:20 ` Peter Xu
2021-02-25 0:24 ` Mike Kravetz
2021-02-25 17:49 ` Axel Rasmussen
2021-02-25 18:48 ` Mike Kravetz [this message]
2021-02-19 0:48 ` [PATCH v7 2/6] userfaultfd: disable huge PMD sharing for MINOR registered VMAs Axel Rasmussen
2021-02-25 0:33 ` Mike Kravetz
2021-02-19 0:48 ` [PATCH v7 3/6] userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled Axel Rasmussen
2021-02-19 0:48 ` [PATCH v7 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl Axel Rasmussen
2021-02-23 15:38 ` Peter Xu
2021-02-23 18:05 ` Axel Rasmussen
2021-02-23 18:28 ` Peter Xu
2021-02-19 0:48 ` [PATCH v7 5/6] userfaultfd: update documentation to describe minor fault handling Axel Rasmussen
2021-02-19 0:48 ` [PATCH v7 6/6] userfaultfd/selftests: add test exercising " Axel Rasmussen
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