forked from luck/tmp_suning_uos_patched
8d2afd96c2
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>
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| .. | ||
| 9p | ||
| adfs | ||
| affs | ||
| afs | ||
| autofs4 | ||
| befs | ||
| bfs | ||
| btrfs | ||
| cachefiles | ||
| ceph | ||
| cifs | ||
| coda | ||
| configfs | ||
| cramfs | ||
| debugfs | ||
| devpts | ||
| dlm | ||
| ecryptfs | ||
| efivarfs | ||
| efs | ||
| exofs | ||
| exportfs | ||
| ext2 | ||
| ext4 | ||
| f2fs | ||
| fat | ||
| freevxfs | ||
| fscache | ||
| fuse | ||
| gfs2 | ||
| hfs | ||
| hfsplus | ||
| hostfs | ||
| hpfs | ||
| hugetlbfs | ||
| isofs | ||
| jbd2 | ||
| jffs2 | ||
| jfs | ||
| kernfs | ||
| lockd | ||
| logfs | ||
| minix | ||
| ncpfs | ||
| nfs | ||
| nfs_common | ||
| nfsd | ||
| nilfs2 | ||
| nls | ||
| notify | ||
| ntfs | ||
| ocfs2 | ||
| omfs | ||
| openpromfs | ||
| overlayfs | ||
| proc | ||
| pstore | ||
| qnx4 | ||
| qnx6 | ||
| quota | ||
| ramfs | ||
| reiserfs | ||
| romfs | ||
| squashfs | ||
| sysfs | ||
| sysv | ||
| tracefs | ||
| ubifs | ||
| udf | ||
| ufs | ||
| xfs | ||
| aio.c | ||
| anon_inodes.c | ||
| attr.c | ||
| bad_inode.c | ||
| binfmt_aout.c | ||
| binfmt_elf_fdpic.c | ||
| binfmt_elf.c | ||
| binfmt_em86.c | ||
| binfmt_flat.c | ||
| binfmt_misc.c | ||
| binfmt_script.c | ||
| block_dev.c | ||
| buffer.c | ||
| char_dev.c | ||
| compat_binfmt_elf.c | ||
| compat_ioctl.c | ||
| compat.c | ||
| coredump.c | ||
| dax.c | ||
| dcache.c | ||
| dcookies.c | ||
| direct-io.c | ||
| drop_caches.c | ||
| eventfd.c | ||
| eventpoll.c | ||
| exec.c | ||
| fcntl.c | ||
| fhandle.c | ||
| file_table.c | ||
| file.c | ||
| filesystems.c | ||
| fs_pin.c | ||
| fs_struct.c | ||
| fs-writeback.c | ||
| inode.c | ||
| internal.h | ||
| ioctl.c | ||
| Kconfig | ||
| Kconfig.binfmt | ||
| libfs.c | ||
| locks.c | ||
| Makefile | ||
| mbcache.c | ||
| mount.h | ||
| mpage.c | ||
| namei.c | ||
| namespace.c | ||
| no-block.c | ||
| nsfs.c | ||
| open.c | ||
| pipe.c | ||
| pnode.c | ||
| pnode.h | ||
| posix_acl.c | ||
| proc_namespace.c | ||
| read_write.c | ||
| readdir.c | ||
| select.c | ||
| seq_file.c | ||
| signalfd.c | ||
| splice.c | ||
| stack.c | ||
| stat.c | ||
| statfs.c | ||
| super.c | ||
| sync.c | ||
| timerfd.c | ||
| userfaultfd.c | ||
| utimes.c | ||
| xattr.c | ||