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

[Michal Hocko]

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?

> 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
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxxx  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

-- 
Michal Hocko
SUSE Labs