Re: [PATCH man-pages 1/2] userfaultfd.2: start documenting non-cooperative events

[Mike Rapoprt <rppt@linux.vnet.ibm.com>]

On April 27, 2017 8:26:16 PM GMT+03:00, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> wrote:
>Hi Mike,
>
>I've applied this, but have some questions/points I think 
>further clarification.
>
>On 04/27/2017 04:14 PM, Mike Rapoport wrote:
>> Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
>> ---
>>  man2/userfaultfd.2 | 135
>++++++++++++++++++++++++++++++++++++++++++++++++++---
>>  1 file changed, 128 insertions(+), 7 deletions(-)
>> 
>> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
>> index cfea5cb..44af3e4 100644
>> --- a/man2/userfaultfd.2
>> +++ b/man2/userfaultfd.2
>> @@ -75,7 +75,7 @@ flag in
>>  .PP
>>  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.
>> +and unread events are flushed.
>>  .\"
>>  .SS Usage
>>  The userfaultfd mechanism is designed to allow a thread in a
>multithreaded
>> @@ -99,6 +99,20 @@ In such non-cooperative mode,
>>  the process that monitors userfaultfd and handles page faults
>>  needs to be aware of the changes in the virtual memory layout
>>  of the faulting process to avoid memory corruption.
>> +
>> +Starting from Linux 4.11,
>> +userfaultfd may notify the fault-handling threads about changes
>> +in the virtual memory layout of the faulting process.
>> +In addition, if the faulting process invokes
>> +.BR fork (2)
>> +system call,
>> +the userfaultfd objects associated with the parent may be duplicated
>> +into the child process and the userfaultfd monitor will be notified
>> +about the file descriptor associated with the userfault objects
>
>What does "notified about the file descriptor" mean?

Well, seems that I've made this one really awkward :)
When the monitored process forks, all the userfault objects associated​ with it are duplicated into the child process. For each duplicated object, userfault generates event of type UFFD_EVENT_FORK and the uffdio_msg for this event contains the file descriptor that should be used to manipulate the duplicated userfault object.
Hope this clarifies.

>> +created for the child process,
>> +which allows userfaultfd monitor to perform user-space paging
>> +for the child process.
>> +
>>  .\" FIXME elaborate about non-cooperating mode, describe its
>limitations
>>  .\" for kernels before 4.11, features added in 4.11
>>  .\" and limitations remaining in 4.11
>> @@ -144,6 +158,10 @@ Details of the various
>>  operations can be found in
>>  .BR ioctl_userfaultfd (2).
>>  
>> +Since Linux 4.11, events other than page-fault may enabled during
>> +.B UFFDIO_API
>> +operation.
>> +
>>  Up to Linux 4.11,
>>  userfaultfd can be used only with anonymous private memory mappings.
>>  
>> @@ -156,7 +174,8 @@ Each
>>  .BR read (2)
>>  from the userfaultfd file descriptor returns one or more
>>  .I uffd_msg
>> -structures, each of which describes a page-fault event:
>> +structures, each of which describes a page-fault event
>> +or an event required for the non-cooperative userfaultfd usage:
>>  
>>  .nf
>>  .in +4n
>> @@ -168,6 +187,23 @@ struct uffd_msg {
>>              __u64 flags;        /* Flags describing fault */
>>              __u64 address;      /* Faulting address */
>>          } pagefault;
>> +        struct {
>> +            __u32 ufd;          /* userfault file descriptor
>> +                                   of the child process */
>> +        } fork;                 /* since Linux 4.11 */
>> +        struct {
>> +            __u64 from;         /* old address of the
>> +                                   remapped area */
>> +            __u64 to;           /* new address of the
>> +                                   remapped area */
>> +            __u64 len;          /* original mapping length */
>> +        } remap;                /* since Linux 4.11 */
>> +        struct {
>> +            __u64 start;        /* start address of the
>> +                                   removed area */
>> +            __u64 end;          /* end address of the
>> +                                   removed area */
>> +        } remove;               /* since Linux 4.11 */
>>          ...
>>      } arg;
>>  
>> @@ -194,14 +230,73 @@ structure are as follows:
>>  .TP
>>  .I event
>>  The type of event.
>> -Currently, only one value can appear in this field:
>> -.BR UFFD_EVENT_PAGEFAULT ,
>> -which indicates a page-fault event.
>> +Depending of the event type,
>> +different fields of the
>> +.I arg
>> +union represent details required for the event processing.
>> +The non-page-fault events are generated only when appropriate
>feature
>> +is enabled during API handshake with
>> +.B UFFDIO_API
>> +.BR ioctl (2).
>> +
>> +The following values can appear in the
>> +.I event
>> +field:
>> +.RS
>> +.TP
>> +.B UFFD_EVENT_PAGEFAULT
>> +A page-fault event.
>> +The page-fault details are available in the
>> +.I pagefault
>> +field.
>>  .TP
>> -.I address
>> +.B UFFD_EVENT_FORK
>> +Generated when the faulting process invokes
>> +.BR fork (2)
>> +system call.
>> +The event details are available in the
>> +.I fork
>> +field.
>> +.\" FIXME descirbe duplication of userfault file descriptor during
>fork
>> +.TP
>> +.B UFFD_EVENT_REMAP
>> +Generated when the faulting process invokes
>> +.BR mremap (2)
>> +system call.
>> +The event details are available in the
>> +.I remap
>> +field.
>> +.TP
>> +.B UFFD_EVENT_REMOVE
>> +Generated when the faulting process invokes
>> +.BR madvise (2)
>> +system call with
>> +.BR MADV_DONTNEED
>> +or
>> +.BR MADV_REMOVE
>> +advice.
>> +The event details are available in the
>> +.I remove
>> +field.
>> +.TP
>> +.B UFFD_EVENT_UNMAP
>> +Generated when the faulting process unmaps a memory range,
>> +either explicitly using
>> +.BR munmap (2)
>> +system call or implicitly during
>> +.BR mmap (2)
>> +or
>> +.BR mremap (2)
>> +system calls.
>> +The event details are available in the
>> +.I remove
>> +field.
>> +.RE
>> +.TP
>> +.I pagefault.address
>>  The address that triggered the page fault.
>>  .TP
>> -.I flags
>> +.I pagefault.flags
>>  A bit mask of flags that describe the event.
>>  For
>>  .BR UFFD_EVENT_PAGEFAULT ,
>> @@ -218,6 +313,32 @@ otherwise it is a read fault.
>>  .\"
>>  .\" UFFD_PAGEFAULT_FLAG_WP is not yet supported.
>>  .RE
>> +.TP
>> +.I fork.ufd
>> +The file descriptor associated with the userfault object
>> +created for the child process
>> +.TP
>> +.I remap.from
>> +The original address of the memory range that was remapped using
>> +.BR mremap (2).
>> +.TP
>> +.I remap.to
>> +The new address of the memory range that was remapped using
>> +.BR mremap (2).
>> +.TP
>> +.I remap.len
>> +The original length of the the memory range that was remapped using
>> +.BR mremap (2).
>> +.TP
>> +.I remove.start
>> +The start address of the memory range that was freed using
>> +.BR madvise (2)
>> +or unmapped
>> +.TP
>> +.I remove.end
>> +The end address of the memory range that was freed using
>> +.BR madvise (2)
>> +or unmapped
>>  .PP
>>  A
>>  .BR read (2)
>
>Cheers,
>
>Michael

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