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

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