Re: [RFC PATCH v2] userfaultfd: Add feature to request for a signal delivery

[prakash.sangappa]

On 07/04/2017 11:28 AM, Mike Rapoport wrote:
> On Tue, Jun 27, 2017 at 09:08:40AM -0700, Prakash Sangappa wrote:
> > Applications like the database use 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 mmap'ed and accessed by many
> > single threaded 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 mmap'ed 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. The requirement here is for a way to prevent the kernel
> > from implicitly allocating a page  to fill holes in hugetbfs file.
> >
> > This can be achieved using userfaultfd mechanism to intercept page-fault
> > events when mmap'ed address over holes in the file are accessed, and
> > prevent kernel from implicitly filling the hole. However, currently using
> > userfaultfd would require each of the database processes to use a monitor
> > thread and the setup cost associated with it,  is considered an overhead.
> >
> > It would be better if userfaultd mechanism could have a way to request
> > simply sending a signal,for the robustness use case described above.
> > This would not require the use of a monitor thread.
> >
> > This patch adds the feature to userfaultfd mechanism to request for a
> > SIGBUS signal delivery to the faulting process, instead of the
> > page-fault event.
> >
> > See following for previous discussion about a different solution
> > to the above database requirement, leading to this proposal to enhance
> > userfaultfd, as suggested by Andrea.
> >
> > http://www.spinics.net/lists/linux-mm/msg129224.html
> >
> > Signed-off-by: Prakash <prakash.sangappa@xxxxxxxxxx>
> > ---
> >   fs/userfaultfd.c                 |  5 +++++
> >   include/uapi/linux/userfaultfd.h | 10 +++++++++-
> >   2 files changed, 14 insertions(+), 1 deletion(-)
> Apparently your mail client clobbered the white space, can you please
> resend with proper formatting?
>   

Ok, Will resend the patch along with suggested changes.

> > 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;
> > +    }
> Please remove the curly braces.

Ok,

> > +
> >       /*
> >        * 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.
> I think that it maybe worth making UFFD_FEATURE_SIGBUS mutually exclusive
> with the non-cooperative events. There is no point of having monitor if the
> page fault handler will anyway just kill the faulting process.


Will this not be too restrictive?. The non-cooperative events could
still be useful if an application wants to track changes
to VA ranges that are registered even though it expects
a signal on page fault.