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* userfaultfd: don't block on the last VM updates at exit timeLinus Torvalds2016-03-021-0/+6
| | | | | | | | | | | | | | | | | | | | | | | | | | | 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 <dvyukov@google.com> Acked-by: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
* userfaultfd: revert "userfaultfd: waitqueue: add nr wake parameter to ↵Andrea Arcangeli2015-09-221-4/+4
| | | | | | | | | | | | | | | | | | | | | | | __wake_up_locked_key" 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 <aarcange@redhat.com> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Shuah Khan <shuahkh@osg.samsung.com> Cc: Thierry Reding <treding@nvidia.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
* userfaultfd: add missing mmput() in error pathEric Biggers2015-09-171-1/+3
| | | | | | | | | This fixes a memleak if anon_inode_getfile() fails in userfaultfd(). Signed-off-by: Eric Biggers <ebiggers3@gmail.com> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
* userfaultfd: avoid missing wakeups during refile in userfaultfd_readAndrea Arcangeli2015-09-041-2/+24
| | | | | | | | | | | | | 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 <aarcange@redhat.com> Cc: Pavel Emelyanov <xemul@parallels.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
* userfaultfd: allow signals to interrupt a userfaultAndrea Arcangeli2015-09-041-3/+32
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | 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 <aarcange@redhat.com> Cc: Pavel Emelyanov <xemul@parallels.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
* userfaultfd: require UFFDIO_API before other ioctlsAndrea Arcangeli2015-09-041-1/+3
| | | | | | | | | | | | | | | | | | | | | | 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 <aarcange@redhat.com> Cc: Pavel Emelyanov <xemul@parallels.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
* userfaultfd: UFFDIO_COPY and UFFDIO_ZEROPAGEAndrea Arcangeli2015-09-041-0/+96
| | | | | | | | | | | | | | | | | | | | | | | | | 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 <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>
* userfaultfd: solve the race between UFFDIO_COPY|ZEROPAGE and readAndrea Arcangeli2015-09-041-15/+66
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | 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 <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>
* userfaultfd: allocate the userfaultfd_ctx cacheline alignedAndrea Arcangeli2015-09-041-8/+31
| | | | | | | | | | | | | | | | | | | | | | | Use proper slab to guarantee alignment. 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>
* userfaultfd: optimize read() and poll() to be O(1)Andrea Arcangeli2015-09-041-74/+111
| | | | | | | | | | | | | | | | | | | | | | | This makes read O(1) and poll that was already O(1) becomes lockless. 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>
* userfaultfd: wake pending userfaultsAndrea Arcangeli2015-09-041-22/+43
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This is an optimization but it's a userland visible one and it affects the API. The downside of this optimization is that if you call poll() and you get POLLIN, read(ufd) may still return -EAGAIN. The blocked userfault may be waken by a different thread, before read(ufd) comes around. This in short means that poll() isn't really usable if the userfaultfd is opened in blocking mode. userfaults won't wait in "pending" state to be read anymore and any UFFDIO_WAKE or similar operations that has the objective of waking userfaults after their resolution, will wake all blocked userfaults for the resolved range, including those that haven't been read() by userland yet. The behavior of poll() becomes not standard, but this obviates the need of "spurious" UFFDIO_WAKE and it lets the userland threads to restart immediately without requiring an UFFDIO_WAKE. This is even more significant in case of repeated faults on the same address from multiple threads. This optimization is justified by the measurement that the number of spurious UFFDIO_WAKE accounts for 5% and 10% of the total userfaults for heavy workloads, so it's worth optimizing those away. 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>
* userfaultfd: change the read API to return a uffd_msgAndrea Arcangeli2015-09-041-33/+46
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | 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>
* userfaultfd: Rename uffd_api.bits into .featuresPavel Emelyanov2015-09-041-2/+2
| | | | | | | | | | | | | | | | | | | | | | | | | | This is (seems to be) the minimal thing that is required to unblock standard uffd usage from the non-cooperative one. Now more bits can be added to the features field indicating e.g. UFFD_FEATURE_FORK and others needed for the latter use-case. Signed-off-by: Pavel Emelyanov <xemul@parallels.com> Signed-off-by: Andrea Arcangeli <aarcange@redhat.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>
* userfaultfd: add new syscall to provide memory externalizationAndrea Arcangeli2015-09-041-0/+1036
Once an userfaultfd has been created and certain region of the process virtual address space have been registered into it, the thread responsible for doing the memory externalization can manage the page faults in userland by talking to the kernel using the userfaultfd protocol. poll() can be used to know when there are new pending userfaults to be read (POLLIN). 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> Cc: Dan Carpenter <dan.carpenter@oracle.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>