linux-kernel - Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode

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Date:   Thu, 25 Feb 2021 10:48:04 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Koutný <mkoutny@...e.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Xu <peterx@...hat.com>, Shaohua Li <shli@...com>,
        Shawn Anastasio <shawn@...stas.io>,
        Steven Rostedt <rostedt@...dmis.org>,
        Steven Price <steven.price@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Adam Ruprecht <ruprecht@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode

On 2/25/21 9:49 AM, Axel Rasmussen wrote:
> On Wed, Feb 24, 2021 at 4:26 PM Mike Kravetz <mike.kravetz@...cle.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