linux-toradex.git/fs/userfaultfd.c, branch v4.4.93

userfaultfd: don't block on the last VM updates at exit time

2016-03-16T15:43:01+00:00

commit 39680f50ae54cbbb6e72ac38b8329dd3eb9105f4 upstream.

The exit path will do some final updates to the VM of an exiting process
to inform others of the fact that the process is going away.

That happens, for example, for robust futex state cleanup, but also if
the parent has asked for a TID update when the process exits (we clear
the child tid field in user space).

However, at the time we do those final VM accesses, we've already
stopped accepting signals, so the usual "stop waiting for userfaults on
signal" code in fs/userfaultfd.c no longer works, and the process can
become an unkillable zombie waiting for something that will never
happen.

To solve this, just make handle_userfault() abort any user fault
handling if we're already in the exit path past the signal handling
state being dead (marked by PF_EXITING).

This VM special case is pretty ugly, and it is possible that we should
look at finalizing signals later (or move the VM final accesses
earlier).  But in the meantime this is a fairly minimally intrusive fix.

Reported-and-tested-by: Dmitry Vyukov 
Acked-by: Andrea Arcangeli 
Signed-off-by: Linus Torvalds 
Cc: Sedat Dilek 
Signed-off-by: Greg Kroah-Hartman

userfaultfd: revert "userfaultfd: waitqueue: add nr wake parameter to __wake_up_locked_key"

2015-09-22T22:09:53+00:00

This reverts commit 51360155eccb907ff8635bd10fc7de876408c2e0 and adapts
fs/userfaultfd.c to use the old version of that function.

It didn't look robust to call __wake_up_common with "nr == 1" when we
absolutely require wakeall semantics, but we've full control of what we
insert in the two waitqueue heads of the blocked userfaults.  No
exclusive waitqueue risks to be inserted into those two waitqueue heads
so we can as well stick to "nr == 1" of the old code and we can rely
purely on the fact no waitqueue inserted in one of the two waitqueue
heads we must enforce as wakeall, has wait->flags WQ_FLAG_EXCLUSIVE set.

Signed-off-by: Andrea Arcangeli 
Cc: Dr. David Alan Gilbert 
Cc: Michael Ellerman 
Cc: Shuah Khan 
Cc: Thierry Reding 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

userfaultfd: add missing mmput() in error path

2015-09-18T04:16:07+00:00

This fixes a memleak if anon_inode_getfile() fails in userfaultfd().

Signed-off-by: Eric Biggers 
Signed-off-by: Andrea Arcangeli 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

userfaultfd: avoid missing wakeups during refile in userfaultfd_read

2015-09-04T23:54:41+00:00

During the refile in userfaultfd_read both waitqueues could look empty to
the lockless wake_userfault().  Use a seqcount to prevent this false
negative that could leave an userfault blocked.

Signed-off-by: Andrea Arcangeli 
Cc: Pavel Emelyanov 
Cc: Dave Hansen 
Cc: Linus Torvalds 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

userfaultfd: allow signals to interrupt a userfault

2015-09-04T23:54:41+00:00

This is only simple to achieve if the userfault is going to return to
userland (not to the kernel) because we can avoid returning VM_FAULT_RETRY
despite we temporarily released the mmap_sem.  The fault would just be
retried by userland then.  This is safe at least on x86 and powerpc (the
two archs with the syscall implemented so far).

Hint to verify for which archs this is safe: after handle_mm_fault
returns, no access to data structures protected by the mmap_sem must be
done by the fault code in arch/*/mm/fault.c until up_read(&mm->mmap_sem)
is called.

This has two main benefits: signals can run with lower latency in
production (signals aren't blocked by userfaults and userfaults are
immediately repeated after signal processing) and gdb can then trivially
debug the threads blocked in this kind of userfaults coming directly from
userland.

On a side note: while gdb has a need to get signal processed, coredumps
always worked perfectly with userfaults, no matter if the userfault is
triggered by GUP a kernel copy_user or directly from userland.

Signed-off-by: Andrea Arcangeli 
Cc: Pavel Emelyanov 
Cc: Dave Hansen 
Cc: Linus Torvalds 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

userfaultfd: require UFFDIO_API before other ioctls

2015-09-04T23:54:41+00:00

UFFDIO_API was already forced before read/poll could work.  This makes the
code more strict to force it also for all other ioctls.

All users would already have been required to call UFFDIO_API before
invoking other ioctls but this makes it more explicit.

This will ensure we can change all ioctls (all but UFFDIO_API/struct
uffdio_api) with a bump of uffdio_api.api.

There's no actual plan or need to change the API or the ioctl, the current
API already should cover fine even the non cooperative usage, but this is
just for the longer term future just in case.

Signed-off-by: Andrea Arcangeli 
Cc: Pavel Emelyanov 
Cc: Dave Hansen 
Cc: Linus Torvalds 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

userfaultfd: UFFDIO_COPY and UFFDIO_ZEROPAGE

2015-09-04T23:54:41+00:00

These two ioctl allows to either atomically copy or to map zeropages
into the virtual address space. This is used by the thread that opened
the userfaultfd to resolve the userfaults.

Signed-off-by: Andrea Arcangeli 
Acked-by: Pavel Emelyanov 
Cc: Sanidhya Kashyap 
Cc: zhang.zhanghailiang@huawei.com
Cc: "Kirill A. Shutemov" 
Cc: Andres Lagar-Cavilla 
Cc: Dave Hansen 
Cc: Paolo Bonzini 
Cc: Rik van Riel 
Cc: Mel Gorman 
Cc: Andy Lutomirski 
Cc: Hugh Dickins 
Cc: Peter Feiner 
Cc: "Dr. David Alan Gilbert" 
Cc: Johannes Weiner 
Cc: "Huangpeng (Peter)" 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

userfaultfd: solve the race between UFFDIO_COPY|ZEROPAGE and read

2015-09-04T23:54:41+00:00

Solve in-kernel the race between UFFDIO_COPY|ZEROPAGE and
userfaultfd_read if they are run on different threads simultaneously.

Until now qemu solved the race in userland: the race was explicitly
and intentionally left for userland to solve. However we can also
solve it in kernel.

Requiring all users to solve this race if they use two threads (one
for the background transfer and one for the userfault reads) isn't
very attractive from an API prospective, furthermore this allows to
remove a whole bunch of mutex and bitmap code from qemu, making it
faster. The cost of __get_user_pages_fast should be insignificant
considering it scales perfectly and the pagetables are already hot in
the CPU cache, compared to the overhead in userland to maintain those
structures.

Applying this patch is backwards compatible with respect to the
userfaultfd userland API, however reverting this change wouldn't be
backwards compatible anymore.

Without this patch qemu in the background transfer thread, has to read
the old state, and do UFFDIO_WAKE if old_state is missing but it
become REQUESTED by the time it tries to set it to RECEIVED (signaling
the other side received an userfault).

    vcpu                background_thr userfault_thr
    -----               -----          -----
    vcpu0 handle_mm_fault()

                        postcopy_place_page
                        read old_state -> MISSING
                        UFFDIO_COPY 0x7fb76a139000 (no wakeup, still pending)

    vcpu0 fault at 0x7fb76a139000 enters handle_userfault
    poll() is kicked

                                        poll() -> POLLIN
                                        read() -> 0x7fb76a139000
                                        postcopy_pmi_change_state(MISSING, REQUESTED) -> REQUESTED

                        tmp_state = postcopy_pmi_change_state(old_state, RECEIVED) -> REQUESTED
                        /* check that no userfault raced with UFFDIO_COPY */
                        if (old_state == MISSING && tmp_state == REQUESTED)
                                UFFDIO_WAKE from background thread

And a second case where a UFFDIO_WAKE would be needed is in the userfault thread:

    vcpu                background_thr userfault_thr
    -----               -----          -----
    vcpu0 handle_mm_fault()

                        postcopy_place_page
                        read old_state -> MISSING
                        UFFDIO_COPY 0x7fb76a139000 (no wakeup, still pending)
                        tmp_state = postcopy_pmi_change_state(old_state, RECEIVED) -> RECEIVED

    vcpu0 fault at 0x7fb76a139000 enters handle_userfault
    poll() is kicked

                                        poll() -> POLLIN
                                        read() -> 0x7fb76a139000

                                        if (postcopy_pmi_change_state(MISSING, REQUESTED) == RECEIVED)
                                                UFFDIO_WAKE from userfault thread

This patch removes the need of both UFFDIO_WAKE and of the associated
per-page tristate as well.

Signed-off-by: Andrea Arcangeli 
Acked-by: Pavel Emelyanov 
Cc: Sanidhya Kashyap 
Cc: zhang.zhanghailiang@huawei.com
Cc: "Kirill A. Shutemov" 
Cc: Andres Lagar-Cavilla 
Cc: Dave Hansen 
Cc: Paolo Bonzini 
Cc: Rik van Riel 
Cc: Mel Gorman 
Cc: Andy Lutomirski 
Cc: Hugh Dickins 
Cc: Peter Feiner 
Cc: "Dr. David Alan Gilbert" 
Cc: Johannes Weiner 
Cc: "Huangpeng (Peter)" 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

userfaultfd: allocate the userfaultfd_ctx cacheline aligned

2015-09-04T23:54:41+00:00

Use proper slab to guarantee alignment.

Signed-off-by: Andrea Arcangeli 
Acked-by: Pavel Emelyanov 
Cc: Sanidhya Kashyap 
Cc: zhang.zhanghailiang@huawei.com
Cc: "Kirill A. Shutemov" 
Cc: Andres Lagar-Cavilla 
Cc: Dave Hansen 
Cc: Paolo Bonzini 
Cc: Rik van Riel 
Cc: Mel Gorman 
Cc: Andy Lutomirski 
Cc: Hugh Dickins 
Cc: Peter Feiner 
Cc: "Dr. David Alan Gilbert" 
Cc: Johannes Weiner 
Cc: "Huangpeng (Peter)" 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

userfaultfd: optimize read() and poll() to be O(1)

2015-09-04T23:54:41+00:00

This makes read O(1) and poll that was already O(1) becomes lockless.

Signed-off-by: Andrea Arcangeli 
Acked-by: Pavel Emelyanov 
Cc: Sanidhya Kashyap 
Cc: zhang.zhanghailiang@huawei.com
Cc: "Kirill A. Shutemov" 
Cc: Andres Lagar-Cavilla 
Cc: Dave Hansen 
Cc: Paolo Bonzini 
Cc: Rik van Riel 
Cc: Mel Gorman 
Cc: Andy Lutomirski 
Cc: Hugh Dickins 
Cc: Peter Feiner 
Cc: "Dr. David Alan Gilbert" 
Cc: Johannes Weiner 
Cc: "Huangpeng (Peter)" 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds