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

[Mike Rapoport]

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:
> >>>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.

You are planning to register in userfaultfd only the holes you punch to
deallocate pages, am I right?

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

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.

> >>>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@xxxxxxxxxx>
> >>>---
> >>>  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.
> >
>