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dd0db88d80
Patch series "userfaultfd non-cooperative further update for 4.11 merge
window".
Unfortunately I noticed one relevant bug in userfaultfd_exit while doing
more testing. I've been doing testing before and this was also tested
by kbuild bot and exercised by the selftest, but this bug never
reproduced before.
I dropped userfaultfd_exit as result. I dropped it because of
implementation difficulty in receiving signals in __mmput and because I
think -ENOSPC as result from the background UFFDIO_COPY should be enough
already.
Before I decided to remove userfaultfd_exit, I noticed userfaultfd_exit
wasn't exercised by the selftest and when I tried to exercise it, after
moving it to a more correct place in __mmput where it would make more
sense and where the vma list is stable, it resulted in the
event_wait_completion in D state. So then I added the second patch to
be sure even if we call userfaultfd_event_wait_completion too late
during task exit(), we won't risk to generate tasks in D state. The
same check exists in handle_userfault() for the same reason, except it
makes a difference there, while here is just a robustness check and it's
run under WARN_ON_ONCE.
While looking at the userfaultfd_event_wait_completion() function I
looked back at its callers too while at it and I think it's not ok to
stop executing dup_fctx on the fcs list because we relay on
userfaultfd_event_wait_completion to execute
userfaultfd_ctx_put(fctx->orig) which is paired against
userfaultfd_ctx_get(fctx->orig) in dup_userfault just before
list_add(fcs). This change only takes care of fctx->orig but this area
also needs further review looking for similar problems in fctx->new.
The only patch that is urgent is the first because it's an use after
free during a SMP race condition that affects all processes if
CONFIG_USERFAULTFD=y. Very hard to reproduce though and probably
impossible without SLUB poisoning enabled.
This patch (of 3):
I once reproduced this oops with the userfaultfd selftest, it's not
easily reproducible and it requires SLUB poisoning to reproduce.
general protection fault: 0000 [#1] SMP
Modules linked in:
CPU: 2 PID: 18421 Comm: userfaultfd Tainted: G ------------ T 3.10.0+ #15
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.1-0-g8891697-prebuilt.qemu-project.org 04/01/2014
task: ffff8801f83b9440 ti: ffff8801f833c000 task.ti: ffff8801f833c000
RIP: 0010:[<ffffffff81451299>] [<ffffffff81451299>] userfaultfd_exit+0x29/0xa0
RSP: 0018:ffff8801f833fe80 EFLAGS: 00010202
RAX: ffff8801f833ffd8 RBX: 6b6b6b6b6b6b6b6b RCX: ffff8801f83b9440
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8800baf18600
RBP: ffff8801f833fee8 R08: 0000000000000000 R09: 0000000000000001
R10: 0000000000000000 R11: ffffffff8127ceb3 R12: 0000000000000000
R13: ffff8800baf186b0 R14: ffff8801f83b99f8 R15: 00007faed746c700
FS: 0000000000000000(0000) GS:ffff88023fc80000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007faf0966f028 CR3: 0000000001bc6000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Call Trace:
do_exit+0x297/0xd10
SyS_exit+0x17/0x20
tracesys+0xdd/0xe2
Code: 00 00 66 66 66 66 90 55 48 89 e5 41 54 53 48 83 ec 58 48 8b 1f 48 85 db 75 11 eb 73 66 0f 1f 44 00 00 48 8b 5b 10 48 85 db 74 64 <4c> 8b a3 b8 00 00 00 4d 85 e4 74 eb 41 f6 84 24 2c 01 00 00 80
RIP [<ffffffff81451299>] userfaultfd_exit+0x29/0xa0
RSP <ffff8801f833fe80>
---[ end trace 9fecd6dcb442846a ]---
In the debugger I located the "mm" pointer in the stack and walking
mm->mmap->vm_next through the end shows the vma->vm_next list is fully
consistent and it is null terminated list as expected. So this has to
be an SMP race condition where userfaultfd_exit was running while the
vma list was being modified by another CPU.
When userfaultfd_exit() run one of the ->vm_next pointers pointed to
SLAB_POISON (RBX is the vma pointer and is 0x6b6b..).
The reason is that it's not running in __mmput but while there are still
other threads running and it's not holding the mmap_sem (it can't as it
has to wait the even to be received by the manager). So this is an use
after free that was happening for all processes.
One more implementation problem aside from the race condition:
userfaultfd_exit has really to check a flag in mm->flags before walking
the vma or it's going to slowdown the exit() path for regular tasks.
One more implementation problem: at that point signals can't be
delivered so it would also create a task in D state if the manager
doesn't read the event.
The major design issue: it overall looks superfluous as the manager can
check for -ENOSPC in the background transfer:
if (mmget_not_zero(ctx->mm)) {
[..]
} else {
return -ENOSPC;
}
It's safer to roll it back and re-introduce it later if at all.
[rppt@linux.vnet.ibm.com: documentation fixup after removal of UFFD_EVENT_EXIT]
Link: http://lkml.kernel.org/r/1488345437-4364-1-git-send-email-rppt@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/20170224181957.19736-2-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
230 lines
11 KiB
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230 lines
11 KiB
Plaintext
= 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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The uffdio_api.features bitmask returned by the UFFDIO_API ioctl
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defines what memory types are supported by the userfaultfd and what
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events, except page fault notifications, may be generated.
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If the kernel supports registering userfaultfd ranges on hugetlbfs
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virtual memory areas, UFFD_FEATURE_MISSING_HUGETLBFS will be set in
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uffdio_api.features. Similarly, UFFD_FEATURE_MISSING_SHMEM will be
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set if the kernel supports registering userfaultfd ranges on shared
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memory (covering all shmem APIs, i.e. tmpfs, IPCSHM, /dev/zero
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MAP_SHARED, memfd_create, etc).
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The userland application that wants to use userfaultfd with hugetlbfs
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or shared memory need to set the corresponding flag in
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uffdio_api.features to enable those features.
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If the userland desires to receive notifications for events other than
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page faults, it has to verify that uffdio_api.features has appropriate
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UFFD_FEATURE_EVENT_* bits set. These events are described in more
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detail below in "Non-cooperative userfaultfd" section.
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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 a 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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== Non-cooperative userfaultfd ==
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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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layout. Userfaultfd can notify the manager about such changes using
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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_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_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_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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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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userfaultfd manager. In the former case, the virtual memory is
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removed, but the area is not, the area remains monitored by the
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userfaultfd, and if a page fault occurs in that area it will be
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delivered to the manager. The proper resolution for such page fault is
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to zeromap the faulting address. However, in the latter case, when an
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area is unmapped, either explicitly (with munmap() system call), or
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implicitly (e.g. during mremap()), the area is removed and in turn the
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userfaultfd context for such area disappears too and the manager will
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not get further userland page faults from the removed area. Still, the
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notification is required in order to prevent manager from using
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UFFDIO_COPY on the unmapped area.
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Unlike userland page faults which have to be synchronous and require
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explicit or implicit wakeup, all the events are delivered
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asynchronously and the non-cooperative process resumes execution as
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soon as manager executes read(). The userfaultfd manager should
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carefully synchronize calls to UFFDIO_COPY with the events
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processing. To aid the synchronization, the UFFDIO_COPY ioctl will
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return -ENOSPC when the monitored process exits at the time of
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UFFDIO_COPY, and -ENOENT, when the non-cooperative process has changed
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its virtual memory layout simultaneously with outstanding UFFDIO_COPY
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operation.
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The current asynchronous model of the event delivery is optimal for
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single threaded non-cooperative userfaultfd manager implementations. A
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synchronous event delivery model can be added later as a new
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userfaultfd feature to facilitate multithreading enhancements of the
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non cooperative manager, for example to allow UFFDIO_COPY ioctls to
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run in parallel to the event reception. Single threaded
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implementations should continue to use the current async event
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delivery model instead.
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