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docs/vm: userfaultfd.txt: convert to ReST format
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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@ -1,6 +1,11 @@
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= Userfaultfd =
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.. _userfaultfd:
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== Objective ==
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===========
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Userfaultfd
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===========
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Objective
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=========
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Userfaults allow the implementation of on-demand paging from userland
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and more generally they allow userland to take control of various
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@ -9,7 +14,8 @@ memory page faults, something otherwise only the kernel code could do.
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For example userfaults allows a proper and more optimal implementation
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of the PROT_NONE+SIGSEGV trick.
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== Design ==
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Design
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======
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Userfaults are delivered and resolved through the userfaultfd syscall.
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@ -41,7 +47,8 @@ different processes without them being aware about what is going on
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themselves on the same region the manager is already tracking, which
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is a corner case that would currently return -EBUSY).
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== API ==
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API
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===
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When first opened the userfaultfd must be enabled invoking the
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UFFDIO_API ioctl specifying a uffdio_api.api value set to UFFD_API (or
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@ -101,7 +108,8 @@ UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an
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half copied page since it'll keep userfaulting until the copy has
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finished.
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== QEMU/KVM ==
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QEMU/KVM
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========
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QEMU/KVM is using the userfaultfd syscall to implement postcopy live
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migration. Postcopy live migration is one form of memory
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@ -163,7 +171,8 @@ sending the same page twice (in case the userfault is read by the
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postcopy thread just before UFFDIO_COPY|ZEROPAGE runs in the migration
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thread).
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== Non-cooperative userfaultfd ==
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Non-cooperative userfaultfd
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===========================
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When the userfaultfd is monitored by an external manager, the manager
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must be able to track changes in the process virtual memory
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@ -172,27 +181,30 @@ the same read(2) protocol as for the page fault notifications. The
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manager has to explicitly enable these events by setting appropriate
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bits in uffdio_api.features passed to UFFDIO_API ioctl:
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UFFD_FEATURE_EVENT_FORK - enable userfaultfd hooks for fork(). When
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this feature is enabled, the userfaultfd context of the parent process
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is duplicated into the newly created process. The manager receives
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UFFD_EVENT_FORK with file descriptor of the new userfaultfd context in
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the uffd_msg.fork.
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UFFD_FEATURE_EVENT_FORK
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enable userfaultfd hooks for fork(). When this feature is
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enabled, the userfaultfd context of the parent process is
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duplicated into the newly created process. The manager
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receives UFFD_EVENT_FORK with file descriptor of the new
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userfaultfd context in the uffd_msg.fork.
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UFFD_FEATURE_EVENT_REMAP - enable notifications about mremap()
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calls. When the non-cooperative process moves a virtual memory area to
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a different location, the manager will receive UFFD_EVENT_REMAP. The
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uffd_msg.remap will contain the old and new addresses of the area and
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its original length.
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UFFD_FEATURE_EVENT_REMAP
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enable notifications about mremap() calls. When the
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non-cooperative process moves a virtual memory area to a
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different location, the manager will receive
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UFFD_EVENT_REMAP. The uffd_msg.remap will contain the old and
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new addresses of the area and its original length.
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UFFD_FEATURE_EVENT_REMOVE - enable notifications about
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madvise(MADV_REMOVE) and madvise(MADV_DONTNEED) calls. The event
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UFFD_EVENT_REMOVE will be generated upon these calls to madvise. The
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uffd_msg.remove will contain start and end addresses of the removed
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area.
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UFFD_FEATURE_EVENT_REMOVE
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enable notifications about madvise(MADV_REMOVE) and
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madvise(MADV_DONTNEED) calls. The event UFFD_EVENT_REMOVE will
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be generated upon these calls to madvise. The uffd_msg.remove
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will contain start and end addresses of the removed area.
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UFFD_FEATURE_EVENT_UNMAP - enable notifications about memory
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unmapping. The manager will get UFFD_EVENT_UNMAP with uffd_msg.remove
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containing start and end addresses of the unmapped area.
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UFFD_FEATURE_EVENT_UNMAP
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enable notifications about memory unmapping. The manager will
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get UFFD_EVENT_UNMAP with uffd_msg.remove containing start and
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end addresses of the unmapped area.
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Although the UFFD_FEATURE_EVENT_REMOVE and UFFD_FEATURE_EVENT_UNMAP
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are pretty similar, they quite differ in the action expected from the
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