On Tue, Jul 25, 2017 at 12:47:41AM -0400, 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. > > 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. > > 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 UFFD_FEATURE_SIGBUS feature to userfaultfd mechnism to > request for a SIGBUS signal. > > 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 > > Signed-off-by: Prakash Sangappa <prakash.sangappa@xxxxxxxxxx> Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> > --- > fs/userfaultfd.c | 3 +++ > include/uapi/linux/userfaultfd.h | 10 +++++++++- > 2 files changed, 12 insertions(+), 1 deletions(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index 1d622f2..0bbe7df 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -371,6 +371,9 @@ 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; > -- > 1.7.1 > -- Sincerely yours, Mike. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
