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authorAndrea Arcangeli <aarcange@redhat.com>2015-09-04 15:46:37 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2015-09-04 16:54:41 -0700
commita9b85f9415fd9e529d03299e5335433f614ec1fb (patch)
treea4ba811f5c13dd5db253da3a6296771203b59c78 /Documentation/vm
parent3f602d2724b1f7d2d27ddcd7963a040a5890fd16 (diff)
downloadlwn-a9b85f9415fd9e529d03299e5335433f614ec1fb.tar.gz
lwn-a9b85f9415fd9e529d03299e5335433f614ec1fb.zip
userfaultfd: change the read API to return a uffd_msg
I had requests to return the full address (not the page aligned one) to userland. It's not entirely clear how the page offset could be relevant because userfaults aren't like SIGBUS that can sigjump to a different place and it actually skip resolving the fault depending on a page offset. There's currently no real way to skip the fault especially because after a UFFDIO_COPY|ZEROPAGE, the fault is optimized to be retried within the kernel without having to return to userland first (not even self modifying code replacing the .text that touched the faulting address would prevent the fault to be repeated). Userland cannot skip repeating the fault even more so if the fault was triggered by a KVM secondary page fault or any get_user_pages or any copy-user inside some syscall which will return to kernel code. The second time FAULT_FLAG_RETRY_NOWAIT won't be set leading to a SIGBUS being raised because the userfault can't wait if it cannot release the mmap_map first (and FAULT_FLAG_RETRY_NOWAIT is required for that). Still returning userland a proper structure during the read() on the uffd, can allow to use the current UFFD_API for the future non-cooperative extensions too and it looks cleaner as well. Once we get additional fields there's no point to return the fault address page aligned anymore to reuse the bits below PAGE_SHIFT. The only downside is that the read() syscall will read 32bytes instead of 8bytes but that's not going to be measurable overhead. The total number of new events that can be extended or of new future bits for already shipped events, is limited to 64 by the features field of the uffdio_api structure. If more will be needed a bump of UFFD_API will be required. [akpm@linux-foundation.org: use __packed] Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Acked-by: Pavel Emelyanov <xemul@parallels.com> Cc: Sanidhya Kashyap <sanidhya.gatech@gmail.com> Cc: zhang.zhanghailiang@huawei.com Cc: "Kirill A. Shutemov" <kirill@shutemov.name> Cc: Andres Lagar-Cavilla <andreslc@google.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Rik van Riel <riel@redhat.com> Cc: Mel Gorman <mgorman@suse.de> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Hugh Dickins <hughd@google.com> Cc: Peter Feiner <pfeiner@google.com> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: "Huangpeng (Peter)" <peter.huangpeng@huawei.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/vm')
-rw-r--r--Documentation/vm/userfaultfd.txt12
1 files changed, 7 insertions, 5 deletions
diff --git a/Documentation/vm/userfaultfd.txt b/Documentation/vm/userfaultfd.txt
index 90912925425e..70a3c94d1941 100644
--- a/Documentation/vm/userfaultfd.txt
+++ b/Documentation/vm/userfaultfd.txt
@@ -46,11 +46,13 @@ is a corner case that would currently return -EBUSY).
When first opened the userfaultfd must be enabled invoking the
UFFDIO_API ioctl specifying a uffdio_api.api value set to UFFD_API (or
a later API version) which will specify the read/POLLIN protocol
-userland intends to speak on the UFFD. The UFFDIO_API ioctl if
-successful (i.e. if the requested uffdio_api.api is spoken also by the
-running kernel), will return into uffdio_api.features and
-uffdio_api.ioctls two 64bit bitmasks of respectively the activated
-feature of the read(2) protocol and the generic ioctl available.
+userland intends to speak on the UFFD and the uffdio_api.features
+userland requires. The UFFDIO_API ioctl if successful (i.e. if the
+requested uffdio_api.api is spoken also by the running kernel and the
+requested features are going to be enabled) will return into
+uffdio_api.features and uffdio_api.ioctls two 64bit bitmasks of
+respectively all the available features of the read(2) protocol and
+the generic ioctl available.
Once the userfaultfd has been enabled the UFFDIO_REGISTER ioctl should
be invoked (if present in the returned uffdio_api.ioctls bitmask) to