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authorPrakash Sangappa <prakash.sangappa@oracle.com>2017-09-06 16:23:39 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-09-06 17:27:29 -0700
commit2d6d6f5a09a96cc1fec7ed992b825e05f64cb50e (patch)
tree0410e47be4df76c2e71da34aa32a92b7a39d4378 /fs/userfaultfd.c
parentc41f012ade0b95b0a6e25c7150673e0554736165 (diff)
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mm: userfaultfd: add feature to request for a signal delivery
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 Link: http://lkml.kernel.org/r/1501552446-748335-2-git-send-email-prakash.sangappa@oracle.com Signed-off-by: Prakash Sangappa <prakash.sangappa@oracle.com> Reviewed-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Shuah Khan <shuah@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs/userfaultfd.c')
-rw-r--r--fs/userfaultfd.c3
1 files changed, 3 insertions, 0 deletions
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 01a85e2660b8..5fd4d846691f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -370,6 +370,9 @@ 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