Re: [PATCH 2/2] userfaultfd.2: fix userfaultfd_demo output

From: Nadav Amit <namit@vmware.com>
To: Alejandro Colomar <alx.manpages@gmail.com>
Cc: "linux-man@vger.kernel.org" <linux-man@vger.kernel.org>,
	"Michael Kerrisk )" <mtk.manpages@gmail.com>,
	Peter Xu <peterx@redhat.com>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH 2/2] userfaultfd.2: fix userfaultfd_demo output
Date: Mon, 10 Apr 2023 19:57:11 +0000	[thread overview]
Message-ID: <2C89D6DA-8B73-493F-95D7-456507D00EEE@vmware.com> (raw)
In-Reply-To: <5bce07b9-d4b7-8cb4-273d-72c3867682ba@gmail.com>

> On Apr 8, 2023, at 6:34 AM, Alejandro Colomar <alx.manpages@gmail.com> wrote:
> 
> Hi Nadav,
> 
> On 3/7/22 19:48, Nadav Amit wrote:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> A bug in the kernel caused in recent version a different output (masked
>> offset). Update the output of the demo program accordingly.
> 
> I'd like to have some more information about this in the commit message.
> I don't really understand it.  When was the bug introduced?  When was it
> fixed?  Was the example in the page created in a buggy version?

Hi Alex,

Sorry for the late response.

The bug was introduced in 1a29d85eb0f1 ("mm: use vmf->address instead of of
vmf->virtual_address”) which is circa 2016. It was introduced shortly after
userfaultfd, so it was determined that no users are likely to rely on the
intended correct behavior that was documented in the man page, but there
might be users who relied on the wrong behavior, which was in the code
for several years.

More information is available in the patch of the fix:

https://lore.kernel.org/all/20220226022655.350562-1-namit@vmware.com/

Let me know if it answers you questions, and whether you want me to send
an updated patch for the man page.