[01/23] userfaultfd: linux/Documentation/vm/userfaultfd.txt

Add documentation.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 Documentation/vm/userfaultfd.txt | 140 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 140 insertions(+)
 create mode 100644 Documentation/vm/userfaultfd.txt

On 05/14/2015 07:30 PM, Andrea Arcangeli wrote:
> Add documentation.

Hi Andrea,

I do not recall... Did you write a man page also for this new system call?

Thanks,

Michael


> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>  Documentation/vm/userfaultfd.txt | 140 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 140 insertions(+)
>  create mode 100644 Documentation/vm/userfaultfd.txt
> 
> diff --git a/Documentation/vm/userfaultfd.txt b/Documentation/vm/userfaultfd.txt
> new file mode 100644
> index 0000000..c2f5145
> --- /dev/null
> +++ b/Documentation/vm/userfaultfd.txt
> @@ -0,0 +1,140 @@
> += Userfaultfd =
> +
> +== Objective ==
> +
> +Userfaults allow the implementation of on-demand paging from userland
> +and more generally they allow userland to take control 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. The UFFDIO_API ioctl if
> +successful (i.e. if the requested uffdio_api.api is spoken also by the
> +running kernel), will return into uffdio_api.features and
> +uffdio_api.ioctls two 64bit bitmasks of respectively the activated
> +feature 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.
> +
> +== 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.
> +
> +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).
> --
> To unsubscribe from this list: send the line "unsubscribe linux-api" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>

Hi Andrea,

On 09/11/2015 10:47 AM, Michael Kerrisk (man-pages) wrote:
> On 05/14/2015 07:30 PM, Andrea Arcangeli wrote:
>> Add documentation.
> 
> Hi Andrea,
> 
> I do not recall... Did you write a man page also for this new system call?

No response to my last mail, so I'll try again... Did you 
write any man page for this interface?

Thanks,

Michael


>> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
>> ---
>>  Documentation/vm/userfaultfd.txt | 140 +++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 140 insertions(+)
>>  create mode 100644 Documentation/vm/userfaultfd.txt
>>
>> diff --git a/Documentation/vm/userfaultfd.txt b/Documentation/vm/userfaultfd.txt
>> new file mode 100644
>> index 0000000..c2f5145
>> --- /dev/null
>> +++ b/Documentation/vm/userfaultfd.txt
>> @@ -0,0 +1,140 @@
>> += Userfaultfd =
>> +
>> +== Objective ==
>> +
>> +Userfaults allow the implementation of on-demand paging from userland
>> +and more generally they allow userland to take control 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. The UFFDIO_API ioctl if
>> +successful (i.e. if the requested uffdio_api.api is spoken also by the
>> +running kernel), will return into uffdio_api.features and
>> +uffdio_api.ioctls two 64bit bitmasks of respectively the activated
>> +feature 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.
>> +
>> +== 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.
>> +
>> +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).
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-api" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
> 
>

Hello Michael,

On Fri, Dec 04, 2015 at 04:50:03PM +0100, Michael Kerrisk (man-pages) wrote:
> Hi Andrea,
> 
> On 09/11/2015 10:47 AM, Michael Kerrisk (man-pages) wrote:
> > On 05/14/2015 07:30 PM, Andrea Arcangeli wrote:
> >> Add documentation.
> > 
> > Hi Andrea,
> > 
> > I do not recall... Did you write a man page also for this new system call?
> 
> No response to my last mail, so I'll try again... Did you 
> write any man page for this interface?

I wished I would answer with the manpage itself to give a more
satisfactory answer, but answer is still no at this time. Right now
there's the write protection tracking feature posted to linux-mm and
I'm currently reviewing that. It's worth documenting that part too in
the manpage as it's going to happen sooner than later.

Lack of manpage so far didn't prevent userland to use it (qemu
postcopy is already in upstream qemu and it depends on userfaultfd),
nor review of the code nor other kernel contributors to extend the
syscall API. Other users started testing the syscall too. This is just
to explain why unfortunately the manpage didn't get the top priority
yet, but nevertheless the manpage should happen too and it's
important. Advice on how to proceed is welcome.

Thanks,
Andrea