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    Subject[PATCH 02/21] userfaultfd: linux/Documentation/vm/userfaultfd.txt
    Date
    Add documentation.

    Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
    ---
    Documentation/vm/userfaultfd.txt | 97 ++++++++++++++++++++++++++++++++++++++++
    1 file changed, 97 insertions(+)
    create mode 100644 Documentation/vm/userfaultfd.txt

    diff --git a/Documentation/vm/userfaultfd.txt b/Documentation/vm/userfaultfd.txt
    new file mode 100644
    index 0000000..2ec296c
    --- /dev/null
    +++ b/Documentation/vm/userfaultfd.txt
    @@ -0,0 +1,97 @@
    += Userfaultfd =
    +
    +== Objective ==
    +
    +Userfaults allow to implement on demand paging from userland and more
    +generally they allow userland to take control various memory page
    +faults, something otherwise only the kernel code could do.
    +
    +For example userfaults allows a proper and more optimal implementation
    +of the PROT_NONE+SIGSEGV trick.
    +
    +== Design ==
    +
    +Userfaults are delivered and resolved through the userfaultfd syscall.
    +
    +The userfaultfd (aside from registering and unregistering virtual
    +memory ranges) provides for two primary functionalities:
    +
    +1) read/POLLIN protocol to notify an userland thread of the faults
    + happening
    +
    +2) various UFFDIO_* ioctls that can mangle over the virtual memory
    + regions registered in the userfaultfd that allows userland to
    + efficiently resolve the userfaults it receives via 1) or to mangle
    + the virtual memory in the background
    +
    +The real advantage of userfaults if compared to regular virtual memory
    +management of mremap/mprotect is that the userfaults in all their
    +operations never involve heavyweight structures like vmas (in fact the
    +userfaultfd runtime load never takes the mmap_sem for writing).
    +
    +Vmas are not suitable for page(or hugepage)-granular fault tracking
    +when dealing with virtual address spaces that could span
    +Terabytes. Too many vmas would be needed for that.
    +
    +The userfaultfd once opened by invoking the syscall, can also be
    +passed using unix domain sockets to a manager process, so the same
    +manager process could handle the userfaults of a multitude of
    +different process without them being aware about what is going on
    +(well of course unless they later try to use the userfaultfd themself
    +on the same region the manager is already tracking, which is a corner
    +case that would currently return -EBUSY).
    +
    +== API ==
    +
    +When first opened the userfaultfd must be enabled invoking the
    +UFFDIO_API ioctl specifying an uffdio_api.api value set to UFFD_API
    +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.bits and uffdio_api.ioctls two 64bit bitmasks of
    +respectively the activated feature bits below PAGE_SHIFT in the
    +userfault addresses returned by read(2) 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
    +register a memory range in the userfaultfd by setting the
    +uffdio_register structure accordingly. The uffdio_register.mode
    +bitmask will specify to the kernel which kind of faults to track for
    +the range (UFFDIO_REGISTER_MODE_MISSING would track missing
    +pages). The UFFDIO_REGISTER ioctl will return the
    +uffdio_register.ioctls bitmask of ioctls that are suitable to resolve
    +userfaults on the range reigstered. Not all ioctls will necessarily be
    +supported for all memory types depending on the underlying virtual
    +memory backend (anonymous memory vs tmpfs vs real filebacked
    +mappings).
    +
    +Userland can use the uffdio_register.ioctls to mangle the virtual
    +address space in the background (to add or potentially also remove
    +memory from the userfaultfd registered range). This means an userfault
    +could be triggering just before userland maps in the background the
    +user-faulted page. To avoid POLLIN resulting in an unexpected blocking
    +read (if the UFFD is not opened in nonblocking mode in the first
    +place), we don't allow the background thread to wake userfaults that
    +haven't been read by userland yet. If we would do that likely the
    +UFFDIO_WAKE ioctl could be dropped. This may change in the future
    +(with a UFFD_API protocol bumb combined with the removal of the
    +UFFDIO_WAKE ioctl) if it'll be demonstrated that it's a valid
    +optimization and worthy to force userland to use the UFFD always in
    +nonblocking mode if combined with POLLIN.
    +
    +userfaultfd is also a generic enough feature, that it allows KVM to
    +implement postcopy live migration (one form of memory externalization
    +consisting of a virtual machine running with part or all of its memory
    +residing on a different node in the cloud) without having to modify a
    +single line of KVM kernel code. Guest async page faults, FOLL_NOWAIT
    +and all other GUP features works just fine in combination with
    +userfaults (userfaults trigger async page faults in the guest
    +scheduler so those guest processes that aren't waiting for userfaults
    +can keep running in the guest vcpus).
    +
    +The primary ioctl to resolve userfaults is UFFDIO_COPY. That
    +atomically copies a page into the userfault registered range and wakes
    +up the blocked userfaults (unless uffdio_copy.mode &
    +UFFDIO_COPY_MODE_DONTWAKE is set). Other ioctl works similarly to
    +UFFDIO_COPY.

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