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

[Prakash Sangappa <prakash.sangappa@oracle.com>]

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:
>>> In some cases, userfaultfd mechanism should just deliver a SIGBUS signal
>>> to the faulting process, instead of the page-fault event. Dealing with
>>> page-fault event using a monitor thread can be an overhead in these
>>> cases. For example applications like the database could use the signaling
>>> mechanism for robustness purpose.
>> this is rather confusing. What is the reason that the monitor would be
>> slower than signal delivery and handling?
>>
>>> Database uses hugetlbfs for performance reason. Files on hugetlbfs
>>> filesystem are created and huge pages allocated using fallocate() API.
>>> Pages are deallocated/freed using fallocate() hole punching support.
>>> These files are mmapped and accessed by many processes as shared memory.
>>> The database keeps track of which offsets in the hugetlbfs file have
>>> pages allocated.
>>>
>>> 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.


>>> Using userfaultfd mechanism, with this support to get a signal, database
>>> application can prevent pages from being allocated implicitly when
>>> processes access mapped address over holes in the file.
>>>
>>> This patch adds the feature to request for a SIGBUS signal to userfaultfd
>>> mechanism.
>>>
>>> See following for previous discussion about the database requirement
>>> leading to this proposal as suggested by Andrea.
>>>
>>> http://www.spinics.net/lists/linux-mm/msg129224.html
>> Please make those requirements part of the changelog.
>>
>>> Signed-off-by: Prakash <prakash.sangappa@oracle.com>
>>> ---
>>>   fs/userfaultfd.c                 |  5 +++++
>>>   include/uapi/linux/userfaultfd.h | 10 +++++++++-
>>>   2 files changed, 14 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
>>> index 1d622f2..5686d6d2 100644
>>> --- a/fs/userfaultfd.c
>>> +++ b/fs/userfaultfd.c
>>> @@ -371,6 +371,11 @@ int handle_userfault(struct vm_fault *vmf, unsigned
>>> long reason)
>>>       VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
>>>       VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
>>>
>>> +    if (ctx->features & UFFD_FEATURE_SIGBUS) {
>>> +        goto out;
>>> +    }
>>> +
>>>       /*
>>>        * If it's already released don't get it. This avoids to loop
>>>        * in __get_user_pages if userfaultfd_release waits on the
>>> diff --git a/include/uapi/linux/userfaultfd.h
>>> b/include/uapi/linux/userfaultfd.h
>>> index 3b05953..d39d5db 100644
>>> --- a/include/uapi/linux/userfaultfd.h
>>> +++ b/include/uapi/linux/userfaultfd.h
>>> @@ -23,7 +23,8 @@
>>>                  UFFD_FEATURE_EVENT_REMOVE |    \
>>>                  UFFD_FEATURE_EVENT_UNMAP |        \
>>>                  UFFD_FEATURE_MISSING_HUGETLBFS |    \
>>> -               UFFD_FEATURE_MISSING_SHMEM)
>>> +               UFFD_FEATURE_MISSING_SHMEM |        \
>>> +               UFFD_FEATURE_SIGBUS)
>>>   #define UFFD_API_IOCTLS                \
>>>       ((__u64)1 << _UFFDIO_REGISTER |        \
>>>        (__u64)1 << _UFFDIO_UNREGISTER |    \
>>> @@ -153,6 +154,12 @@ struct uffdio_api {
>>>        * UFFD_FEATURE_MISSING_SHMEM works the same as
>>>        * UFFD_FEATURE_MISSING_HUGETLBFS, but it applies to shmem
>>>        * (i.e. tmpfs and other shmem based APIs).
>>> +     *
>>> +     * UFFD_FEATURE_SIGBUS feature means no page-fault
>>> +     * (UFFD_EVENT_PAGEFAULT) event will be delivered, instead
>>> +     * a SIGBUS signal will be sent to the faulting process.
>>> +     * The application process can enable this behavior by adding
>>> +     * it to uffdio_api.features.
>>>        */
>>>   #define UFFD_FEATURE_PAGEFAULT_FLAG_WP        (1<<0)
>>>   #define UFFD_FEATURE_EVENT_FORK            (1<<1)
>>> @@ -161,6 +168,7 @@ struct uffdio_api {
>>>   #define UFFD_FEATURE_MISSING_HUGETLBFS        (1<<4)
>>>   #define UFFD_FEATURE_MISSING_SHMEM        (1<<5)
>>>   #define UFFD_FEATURE_EVENT_UNMAP        (1<<6)
>>> +#define UFFD_FEATURE_SIGBUS            (1<<7)
>>>       __u64 features;
>>>
>>>       __u64 ioctls;
>>> -- 
>>> 2.7.4
>>>
>> -- 
>> Michal Hocko
>> SUSE Labs
>>
> --
> Sincerely yours,
> Mike.
>

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