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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(-) 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 |
