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Merge branch 'akpm' (patches from Andrew)
Merge patch-bomb from Andrew Morton: - a few misc things - Andy's "ambient capabilities" - fs/nofity updates - the ocfs2 queue - kernel/watchdog.c updates and feature work. - some of MM. Includes Andrea's userfaultfd feature. [ Hadn't noticed that userfaultfd was 'default y' when applying the patches, so that got fixed in this merge instead. We do _not_ mark new features that nobody uses yet 'default y' - Linus ] * emailed patches from Andrew Morton <[email protected]>: (118 commits) mm/hugetlb.c: make vma_has_reserves() return bool mm/madvise.c: make madvise_behaviour_valid() return bool mm/memory.c: make tlb_next_batch() return bool mm/dmapool.c: change is_page_busy() return from int to bool mm: remove struct node_active_region mremap: simplify the "overlap" check in mremap_to() mremap: don't do uneccesary checks if new_len == old_len mremap: don't do mm_populate(new_addr) on failure mm: move ->mremap() from file_operations to vm_operations_struct mremap: don't leak new_vma if f_op->mremap() fails mm/hugetlb.c: make vma_shareable() return bool mm: make GUP handle pfn mapping unless FOLL_GET is requested mm: fix status code which move_pages() returns for zero page mm: memcontrol: bring back the VM_BUG_ON() in mem_cgroup_swapout() genalloc: add support of multiple gen_pools per device genalloc: add name arg to gen_pool_get() and devm_gen_pool_create() mm/memblock: WARN_ON when nid differs from overlap region Documentation/features/vm: add feature description and arch support status for batched TLB flush after unmap mm: defer flush of writable TLB entries mm: send one IPI per CPU to TLB flush all entries after unmapping pages ...
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Original file line number | Diff line number | Diff line change |
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@@ -0,0 +1,40 @@ | ||
# | ||
# Feature name: batch-unmap-tlb-flush | ||
# Kconfig: ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH | ||
# description: arch supports deferral of TLB flush until multiple pages are unmapped | ||
# | ||
----------------------- | ||
| arch |status| | ||
----------------------- | ||
| alpha: | TODO | | ||
| arc: | TODO | | ||
| arm: | TODO | | ||
| arm64: | TODO | | ||
| avr32: | .. | | ||
| blackfin: | TODO | | ||
| c6x: | .. | | ||
| cris: | .. | | ||
| frv: | .. | | ||
| h8300: | .. | | ||
| hexagon: | TODO | | ||
| ia64: | TODO | | ||
| m32r: | TODO | | ||
| m68k: | .. | | ||
| metag: | TODO | | ||
| microblaze: | .. | | ||
| mips: | TODO | | ||
| mn10300: | TODO | | ||
| nios2: | .. | | ||
| openrisc: | .. | | ||
| parisc: | TODO | | ||
| powerpc: | TODO | | ||
| s390: | TODO | | ||
| score: | .. | | ||
| sh: | TODO | | ||
| sparc: | TODO | | ||
| tile: | TODO | | ||
| um: | .. | | ||
| unicore32: | .. | | ||
| x86: | ok | | ||
| xtensa: | TODO | | ||
----------------------- |
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|
@@ -303,6 +303,7 @@ Code Seq#(hex) Include File Comments | |
0xA3 80-8F Port ACL in development: | ||
<mailto:[email protected]> | ||
0xA3 90-9F linux/dtlk.h | ||
0xAA 00-3F linux/uapi/linux/userfaultfd.h | ||
0xAB 00-1F linux/nbd.h | ||
0xAC 00-1F linux/raw.h | ||
0xAD 00 Netfilter device in development: | ||
|
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= Userfaultfd = | ||
|
||
== Objective == | ||
|
||
Userfaults allow the implementation of on-demand paging from userland | ||
and more generally they allow userland to take control of various | ||
memory page faults, something otherwise only the kernel code could do. | ||
|
||
For example userfaults allows a proper and more optimal implementation | ||
of the PROT_NONE+SIGSEGV trick. | ||
|
||
== Design == | ||
|
||
Userfaults are delivered and resolved through the userfaultfd syscall. | ||
|
||
The userfaultfd (aside from registering and unregistering virtual | ||
memory ranges) provides two primary functionalities: | ||
|
||
1) read/POLLIN protocol to notify a userland thread of the faults | ||
happening | ||
|
||
2) various UFFDIO_* ioctls that can manage the virtual memory regions | ||
registered in the userfaultfd that allows userland to efficiently | ||
resolve the userfaults it receives via 1) or to manage the virtual | ||
memory in the background | ||
|
||
The real advantage of userfaults if compared to regular virtual memory | ||
management of mremap/mprotect is that the userfaults in all their | ||
operations never involve heavyweight structures like vmas (in fact the | ||
userfaultfd runtime load never takes the mmap_sem for writing). | ||
|
||
Vmas are not suitable for page- (or hugepage) granular fault tracking | ||
when dealing with virtual address spaces that could span | ||
Terabytes. Too many vmas would be needed for that. | ||
|
||
The userfaultfd once opened by invoking the syscall, can also be | ||
passed using unix domain sockets to a manager process, so the same | ||
manager process could handle the userfaults of a multitude of | ||
different processes without them being aware about what is going on | ||
(well of course unless they later try to use the userfaultfd | ||
themselves on the same region the manager is already tracking, which | ||
is a corner case that would currently return -EBUSY). | ||
|
||
== API == | ||
|
||
When first opened the userfaultfd must be enabled invoking the | ||
UFFDIO_API ioctl specifying a uffdio_api.api value set to UFFD_API (or | ||
a later API version) which will specify the read/POLLIN protocol | ||
userland intends to speak on the UFFD and the uffdio_api.features | ||
userland requires. The UFFDIO_API ioctl if successful (i.e. if the | ||
requested uffdio_api.api is spoken also by the running kernel and the | ||
requested features are going to be enabled) will return into | ||
uffdio_api.features and uffdio_api.ioctls two 64bit bitmasks of | ||
respectively all the available features of the read(2) protocol and | ||
the generic ioctl available. | ||
|
||
Once the userfaultfd has been enabled the UFFDIO_REGISTER ioctl should | ||
be invoked (if present in the returned uffdio_api.ioctls bitmask) to | ||
register a memory range in the userfaultfd by setting the | ||
uffdio_register structure accordingly. The uffdio_register.mode | ||
bitmask will specify to the kernel which kind of faults to track for | ||
the range (UFFDIO_REGISTER_MODE_MISSING would track missing | ||
pages). The UFFDIO_REGISTER ioctl will return the | ||
uffdio_register.ioctls bitmask of ioctls that are suitable to resolve | ||
userfaults on the range registered. Not all ioctls will necessarily be | ||
supported for all memory types depending on the underlying virtual | ||
memory backend (anonymous memory vs tmpfs vs real filebacked | ||
mappings). | ||
|
||
Userland can use the uffdio_register.ioctls to manage the virtual | ||
address space in the background (to add or potentially also remove | ||
memory from the userfaultfd registered range). This means a userfault | ||
could be triggering just before userland maps in the background the | ||
user-faulted page. | ||
|
||
The primary ioctl to resolve userfaults is UFFDIO_COPY. That | ||
atomically copies a page into the userfault registered range and wakes | ||
up the blocked userfaults (unless uffdio_copy.mode & | ||
UFFDIO_COPY_MODE_DONTWAKE is set). Other ioctl works similarly to | ||
UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an | ||
half copied page since it'll keep userfaulting until the copy has | ||
finished. | ||
|
||
== QEMU/KVM == | ||
|
||
QEMU/KVM is using the userfaultfd syscall to implement postcopy live | ||
migration. Postcopy live migration is one form of memory | ||
externalization consisting of a virtual machine running with part or | ||
all of its memory residing on a different node in the cloud. The | ||
userfaultfd abstraction is generic enough that not a single line of | ||
KVM kernel code had to be modified in order to add postcopy live | ||
migration to QEMU. | ||
|
||
Guest async page faults, FOLL_NOWAIT and all other GUP features work | ||
just fine in combination with userfaults. Userfaults trigger async | ||
page faults in the guest scheduler so those guest processes that | ||
aren't waiting for userfaults (i.e. network bound) can keep running in | ||
the guest vcpus. | ||
|
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It is generally beneficial to run one pass of precopy live migration | ||
just before starting postcopy live migration, in order to avoid | ||
generating userfaults for readonly guest regions. | ||
|
||
The implementation of postcopy live migration currently uses one | ||
single bidirectional socket but in the future two different sockets | ||
will be used (to reduce the latency of the userfaults to the minimum | ||
possible without having to decrease /proc/sys/net/ipv4/tcp_wmem). | ||
|
||
The QEMU in the source node writes all pages that it knows are missing | ||
in the destination node, into the socket, and the migration thread of | ||
the QEMU running in the destination node runs UFFDIO_COPY|ZEROPAGE | ||
ioctls on the userfaultfd in order to map the received pages into the | ||
guest (UFFDIO_ZEROCOPY is used if the source page was a zero page). | ||
|
||
A different postcopy thread in the destination node listens with | ||
poll() to the userfaultfd in parallel. When a POLLIN event is | ||
generated after a userfault triggers, the postcopy thread read() from | ||
the userfaultfd and receives the fault address (or -EAGAIN in case the | ||
userfault was already resolved and waken by a UFFDIO_COPY|ZEROPAGE run | ||
by the parallel QEMU migration thread). | ||
|
||
After the QEMU postcopy thread (running in the destination node) gets | ||
the userfault address it writes the information about the missing page | ||
into the socket. The QEMU source node receives the information and | ||
roughly "seeks" to that page address and continues sending all | ||
remaining missing pages from that new page offset. Soon after that | ||
(just the time to flush the tcp_wmem queue through the network) the | ||
migration thread in the QEMU running in the destination node will | ||
receive the page that triggered the userfault and it'll map it as | ||
usual with the UFFDIO_COPY|ZEROPAGE (without actually knowing if it | ||
was spontaneously sent by the source or if it was an urgent page | ||
requested through an userfault). | ||
|
||
By the time the userfaults start, the QEMU in the destination node | ||
doesn't need to keep any per-page state bitmap relative to the live | ||
migration around and a single per-page bitmap has to be maintained in | ||
the QEMU running in the source node to know which pages are still | ||
missing in the destination node. The bitmap in the source node is | ||
checked to find which missing pages to send in round robin and we seek | ||
over it when receiving incoming userfaults. After sending each page of | ||
course the bitmap is updated accordingly. It's also useful to avoid | ||
sending the same page twice (in case the userfault is read by the | ||
postcopy thread just before UFFDIO_COPY|ZEROPAGE runs in the migration | ||
thread). |
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