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SubjectRe: [RFC PATCH] userfaultfd: Add feature to request for a signal delivery

On 6/28/17 6:18 AM, Mike Rapoport wrote:
> On Tue, Jun 27, 2017 at 09:01:20AM -0700, Prakash Sangappa wrote:
>> On 6/27/17 8:35 AM, Mike Rapoport wrote:
>>> On Tue, Jun 27, 2017 at 09:06:43AM +0200, Michal Hocko wrote:
>>>> This is an user visible API so let's CC linux-api mailing list.
>>>> On Mon 26-06-17 12:46:13, Prakash Sangappa wrote:
>>>>> Any access to mapped address over holes in the file, which can occur due
>>>>> to bugs in the application, is considered invalid and expect the process
>>>>> to simply receive a SIGBUS. However, currently when a hole in the file is
>>>>> accessed via the mapped address, kernel/mm attempts to automatically
>>>>> allocate a page at page fault time, resulting in implicitly filling the
>>>>> hole in the file. This may not be the desired behavior for applications
>>>>> like the database that want to explicitly manage page allocations of
>>>>> hugetlbfs files.
>>>> So you register UFFD_FEATURE_SIGBUS on each region tha you are unmapping
>>>> and than just let those offenders die?
>>> If I understand correctly, the database will create the mapping, then it'll
>>> open userfaultfd and register those mappings with the userfault.
>>> Afterwards, when the application accesses a hole userfault will cause
>>> SIGBUS and the application will process it in whatever way it likes, e.g.
>>> just die.
>> Yes.
>>> What I don't understand is why won't you use userfault monitor process that
>>> will take care of the page fault events?
>>> It shouldn't be much overhead running it and it can keep track on all the
>>> userfault file descriptors for you and it will allow more versatile error
>>> handling that SIGBUS.
>> Co-ordination with the external monitor process by all the database
>> processes
>> to send their userfaultfd is still an overhead.
> You are planning to register in userfaultfd only the holes you punch to
> deallocate pages, am I right?

No, the entire mmap'ed region. The DB processes would mmap(MAP_NORESERVE)
hugetlbfs files, register this mapped address with userfaultfd ones
right after
the mmap() call.

> And the co-ordination of the userfault file descriptor with the monitor
> would have been added after calls to fallocate() and userfaultfd_register()?

Well, the database application does not need to deal with a monitor.

> I've just been thinking that maybe it would be possible to use
> UFFD_EVENT_REMOVE for this case. We anyway need to implement the generation
> of UFFD_EVENT_REMOVE for the case of hole punching in hugetlbfs for
> non-cooperative userfaultfd. It could be that it will solve your issue as
> well.

Will this result in a signal delivery?

In the use case described, the database application does not need any event
for hole punching. Basically, just a signal for any invalid access to
area over holes in the file.

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