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a9b85f9415
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>
144 lines
6.9 KiB
Text
144 lines
6.9 KiB
Text
= Userfaultfd =
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== Objective ==
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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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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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Userfaults are delivered and resolved through the userfaultfd syscall.
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The userfaultfd (aside from registering and unregistering virtual
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memory ranges) provides two primary functionalities:
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1) read/POLLIN protocol to notify a userland thread of the faults
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happening
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2) various UFFDIO_* ioctls that can manage the virtual memory regions
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registered in the userfaultfd that allows userland to efficiently
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resolve the userfaults it receives via 1) or to manage the virtual
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memory in the background
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The real advantage of userfaults if compared to regular virtual memory
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management of mremap/mprotect is that the userfaults in all their
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operations never involve heavyweight structures like vmas (in fact the
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userfaultfd runtime load never takes the mmap_sem for writing).
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Vmas are not suitable for page- (or hugepage) granular fault tracking
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when dealing with virtual address spaces that could span
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Terabytes. Too many vmas would be needed for that.
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The userfaultfd once opened by invoking the syscall, can also be
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passed using unix domain sockets to a manager process, so the same
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manager process could handle the userfaults of a multitude of
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different processes without them being aware about what is going on
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(well of course unless they later try to use the userfaultfd
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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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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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a later API version) which will specify the read/POLLIN protocol
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userland intends to speak on the UFFD and the uffdio_api.features
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userland requires. The UFFDIO_API ioctl if successful (i.e. if the
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requested uffdio_api.api is spoken also by the running kernel and the
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requested features are going to be enabled) will return into
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uffdio_api.features and uffdio_api.ioctls two 64bit bitmasks of
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respectively all the available features of the read(2) protocol and
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the generic ioctl available.
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Once the userfaultfd has been enabled the UFFDIO_REGISTER ioctl should
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be invoked (if present in the returned uffdio_api.ioctls bitmask) to
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register a memory range in the userfaultfd by setting the
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uffdio_register structure accordingly. The uffdio_register.mode
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bitmask will specify to the kernel which kind of faults to track for
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the range (UFFDIO_REGISTER_MODE_MISSING would track missing
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pages). The UFFDIO_REGISTER ioctl will return the
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uffdio_register.ioctls bitmask of ioctls that are suitable to resolve
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userfaults on the range registered. Not all ioctls will necessarily be
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supported for all memory types depending on the underlying virtual
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memory backend (anonymous memory vs tmpfs vs real filebacked
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mappings).
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Userland can use the uffdio_register.ioctls to manage the virtual
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address space in the background (to add or potentially also remove
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memory from the userfaultfd registered range). This means a userfault
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could be triggering just before userland maps in the background the
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user-faulted page.
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The primary ioctl to resolve userfaults is UFFDIO_COPY. That
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atomically copies a page into the userfault registered range and wakes
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up the blocked userfaults (unless uffdio_copy.mode &
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UFFDIO_COPY_MODE_DONTWAKE is set). Other ioctl works similarly to
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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 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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externalization consisting of a virtual machine running with part or
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all of its memory residing on a different node in the cloud. The
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userfaultfd abstraction is generic enough that not a single line of
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KVM kernel code had to be modified in order to add postcopy live
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migration to QEMU.
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Guest async page faults, FOLL_NOWAIT and all other GUP features work
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just fine in combination with userfaults. Userfaults trigger async
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page faults in the guest scheduler so those guest processes that
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aren't waiting for userfaults (i.e. network bound) can keep running in
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the guest vcpus.
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It is generally beneficial to run one pass of precopy live migration
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just before starting postcopy live migration, in order to avoid
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generating userfaults for readonly guest regions.
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The implementation of postcopy live migration currently uses one
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single bidirectional socket but in the future two different sockets
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will be used (to reduce the latency of the userfaults to the minimum
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possible without having to decrease /proc/sys/net/ipv4/tcp_wmem).
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The QEMU in the source node writes all pages that it knows are missing
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in the destination node, into the socket, and the migration thread of
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the QEMU running in the destination node runs UFFDIO_COPY|ZEROPAGE
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ioctls on the userfaultfd in order to map the received pages into the
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guest (UFFDIO_ZEROCOPY is used if the source page was a zero page).
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A different postcopy thread in the destination node listens with
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poll() to the userfaultfd in parallel. When a POLLIN event is
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generated after a userfault triggers, the postcopy thread read() from
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the userfaultfd and receives the fault address (or -EAGAIN in case the
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userfault was already resolved and waken by a UFFDIO_COPY|ZEROPAGE run
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by the parallel QEMU migration thread).
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After the QEMU postcopy thread (running in the destination node) gets
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the userfault address it writes the information about the missing page
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into the socket. The QEMU source node receives the information and
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roughly "seeks" to that page address and continues sending all
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remaining missing pages from that new page offset. Soon after that
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(just the time to flush the tcp_wmem queue through the network) the
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migration thread in the QEMU running in the destination node will
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receive the page that triggered the userfault and it'll map it as
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usual with the UFFDIO_COPY|ZEROPAGE (without actually knowing if it
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was spontaneously sent by the source or if it was an urgent page
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requested through an userfault).
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By the time the userfaults start, the QEMU in the destination node
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doesn't need to keep any per-page state bitmap relative to the live
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migration around and a single per-page bitmap has to be maintained in
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the QEMU running in the source node to know which pages are still
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missing in the destination node. The bitmap in the source node is
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checked to find which missing pages to send in round robin and we seek
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over it when receiving incoming userfaults. After sending each page of
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course the bitmap is updated accordingly. It's also useful to avoid
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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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