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SubjectRe: Review request: draft userfaultfd(2) manual page
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Hello Mike,

On 04/21/2017 01:06 PM, Mike Rapoport wrote:
> On Fri, Apr 21, 2017 at 08:30:55AM +0200, Michael Kerrisk (man-pages) wrote:
>> Hello Mike,
>>
>> On 03/21/2017 03:01 PM, Mike Rapoport wrote:
>>> Hello Michael,
>>>
>>> On Mon, Mar 20, 2017 at 09:08:05PM +0100, Michael Kerrisk (man-pages) wrote:
>>>> Hello Andrea, Mike, and all,
>>>>
>>>> Mike: thanks for the page that you sent. I've reworked it
>>>> a bit, and also added a lot of further information,
>>>> and an example program. In the process, I split the page
>>>> into two pieces, with one piece describing the userfaultfd()
>>>> system call and the other describing the ioctl() operations.
>>>>
>>>> I'd like to get review input, especially from you and
>>>> Andrea, but also anyone else, for the current version
>>>> of this page, which includes a few FIXMEs to be sorted.
>>>
>>> Thanks for the update. I'm adressing the FIXME points you've mentioned
>>> below.
>>
>> Thanks!
>>
>>> Otherwise, everything seems the right description of the current upstream.
>>> 4.11 will have quite a few updates to userfault and we'll need to udpate
>>> this page and ioctl_userfaultfd(2) to address those updates. I am planning
>>> to work on the man update in the next few weeks.
>>>
>>>> I've shown the rendered version of the page below.
>>>> The groff source is attached, and can also be found
>>>> at the branch here:
>>>
>>>> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=draft_userfaultfd
>>>>
>>>> The new ioctl_userfaultfd(2) page follows this mail.
>>>>
>>>> Cheers,
>>>>
>>>> Michael
>>>
>>> --
>>> Sincerely yours,
>>> Mike.
>>>
>>>
>>>> USERFAULTFD(2) Linux Programmer's Manual USERFAULTFD(2)
>>>>
>>>> ┌─────────────────────────────────────────────────────┐
>>>> │FIXME │
>>>> ├─────────────────────────────────────────────────────┤
>>>> │Need to describe close(2) semantics for userfaulfd │
>>>> │file descriptor: what happens when the userfaultfd │
>>>> │FD is closed? │
>>>> │ │
>>>> └─────────────────────────────────────────────────────┘
>>>
>>> When userfaultfd is closed, it unregisters all memory ranges that were
>>> previously registered with it and flushes the outstanding page fault
>>> events.
>>
>> Presumably, this is more precisely stated as, "when the last
>> file descriptor referring to a userfaultfd object is closed..."?
>
> You are right.

Thanks for the confirmation.

>> I've made the text:
>>
>> When the last file descriptor referring to a userfaultfd object
>> is closed, all memory ranges that were registered with the
>> object are unregistered and unread page-fault events are
>> flushed.
>>
>> [...]
>
> Perfect.
>

[...]

>>>> Each read(2) from the userfaultfd file descriptor returns one
>>>> or more uffd_msg structures, each of which describes a page-
>>>> fault event:
>>>>
>>>> struct uffd_msg {
>>>> __u8 event; /* Type of event */
>>>> ...
>>>> union {
>>>> struct {
>>>> __u64 flags; /* Flags describing fault */
>>>> __u64 address; /* Faulting address */
>>>> } pagefault;
>>>> ...
>>>> } arg;
>>>>
>>>> /* Padding fields omitted */
>>>> } __packed;
>>>>
>>>> If multiple events are available and the supplied buffer is
>>>> large enough, read(2) returns as many events as will fit in the
>>>> supplied buffer. If the buffer supplied to read(2) is smaller
>>>> than the size of the uffd_msg structure, the read(2) fails with
>>>> the error EINVAL.
>>>>
>>>> The fields set in the uffd_msg structure are as follows:
>>>>
>>>> event The type of event. Currently, only one value can appear
>>>> in this field: UFFD_EVENT_PAGEFAULT, which indicates a
>>>> page-fault event.
>>>>
>>>> address
>>>> The address that triggered the page fault.
>>>>
>>>> flags A bit mask of flags that describe the event. For
>>>> UFFD_EVENT_PAGEFAULT, the following flag may appear:
>>>>
>>>> UFFD_PAGEFAULT_FLAG_WRITE
>>>> If the address is in a range that was registered
>>>> with the UFFDIO_REGISTER_MODE_MISSING flag (see
>>>> ioctl_userfaultfd(2)) and this flag is set, this
>>>> a write fault; otherwise it is a read fault.
>>>>
>>>> A read(2) on a userfaultfd file descriptor can fail with the
>>>> following errors:
>>>>
>>>> EINVAL The userfaultfd object has not yet been enabled using
>>>> the UFFDIO_API ioctl(2) operation
>>>>
>>>> The userfaultfd file descriptor can be monitored with poll(2),
>>>> select(2), and epoll(7). When events are available, the file
>>>> descriptor indicates as readable.
>>>>
>>>>
>>>> ┌─────────────────────────────────────────────────────┐
>>>> │FIXME │
>>>> ├─────────────────────────────────────────────────────┤
>>>> │But, it seems, the object must be created with │
>>>> │O_NONBLOCK. What is the rationale for this require‐ │
>>>> │ment? Something needs to be said in this manual │
>>>> │page. │
>>>> └─────────────────────────────────────────────────────┘
>>>
>>> The object can be created without O_NONBLOCK, so probably the above
>>> sentence can be rephrased as:
>>>
>>> When the userfaultfd file descriptor is opened in non-blocking mode, it can
>>> be monitored with ...
>>
>> Yes, but why is there this requirement for poll() etc. with the
>> O_NONBLOCK flag? I think something about that needs to be said in the
>> man page. Sorry, my FIXME was not clear enough. I've reworded the text
>> and the FIXME:
>>
>> If the O_NONBLOCK flag is enabled in the associated open file
>> description, the userfaultfd file descriptor can be monitored
>> with poll(2), select(2), and epoll(7). When events are avail‐
>> able, the file descriptor indicates as readable. If the O_NON‐
>> BLOCK flag is not enabled, then poll(2) (always) indicates the
>> file as having a POLLERR condition, and select(2) indicates the
>> file descriptor as both readable and writable.
>>
>> ┌─────────────────────────────────────────────────────┐
>> │FIXME │
>> ├─────────────────────────────────────────────────────┤
>> │What is the reason for this seemingly odd behavior │
>> │with respect to the O_NONBLOCK flag? (see user‐ │
>> │faultfd_poll() in fs/userfaultfd.c). Something │
>> │needs to be said about this. │
>> └─────────────────────────────────────────────────────┘
>
> Andrea, can you please help with this one as well?

Let's see what Andrea has to say.

Cheers,

Michael



--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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