Re: [PATCH v4] ioctl_userfaultfd.2, userfaultfd.2: add minor fault mode

[Axel Rasmussen]

Whoops, I ought to have removed this part of the commit message, since
I updated the commit to mention shmem as well:

"This patch covers just the hugetlbfs implementation (in 5.13). Support
for shmem is forthcoming, but as it has not yet made it into a kernel
release candidate, it will be added in a future commit."

On Tue, Apr 12, 2022 at 12:56 PM Axel Rasmussen
<axelrasmussen@xxxxxxxxxx> wrote:
>
> Userfaultfd minor fault mode is supported starting from Linux 5.13.
>
> This commit adds a description of the new mode, as well as the new ioctl
> used to resolve such faults. The two go hand-in-hand: one can't resolve
> a minor fault without continue, and continue can't be used to resolve
> any other kind of fault.
>
> This patch covers just the hugetlbfs implementation (in 5.13). Support
> for shmem is forthcoming, but as it has not yet made it into a kernel
> release candidate, it will be added in a future commit.
>
> v1->v2:
> - Some spelling / phrasing improvements
> v2->v3:
> - Improved line wrapping in man2/ioctl_userfaultfd.2
> v3->v4:
> - Rebased onto http://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/ main
> - Mentioned minor fault shmem support
> - Improved line wrapping in man2/userfaultfd.2
>
> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> ---
>  man2/ioctl_userfaultfd.2 | 142 ++++++++++++++++++++++++++++++++++++---
>  man2/userfaultfd.2       |  95 ++++++++++++++++++++++----
>  2 files changed, 215 insertions(+), 22 deletions(-)
>
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index 15a681164c..d0cb0c9c8e 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -197,6 +197,16 @@ memory accesses to the regions registered with userfaultfd.
>  If this feature bit is set,
>  .I uffd_msg.pagefault.feat.ptid
>  will be set to the faulted thread ID for each page-fault message.
> +.TP
> +.BR UFFD_FEATURE_MINOR_HUGETLBFS " (since Linux 5.13)"
> +If this feature bit is set,
> +the kernel supports registering userfaultfd ranges
> +in minor mode on hugetlbfs-backed memory areas.
> +.TP
> +.BR UFFD_FEATURE_MINOR_SHMEM " (since Linux 5.14)"
> +If this feature bit is set,
> +the kernel supports registering userfaultfd ranges
> +in minor mode on shmem-backed memory areas.
>  .PP
>  The returned
>  .I ioctls
> @@ -256,14 +266,8 @@ by the current kernel version.
>  (Since Linux 4.3.)
>  Register a memory address range with the userfaultfd object.
>  The pages in the range must be "compatible".
> -.PP
> -Up to Linux kernel 4.11,
> -only private anonymous ranges are compatible for registering with
> -.BR UFFDIO_REGISTER .
> -.PP
> -Since Linux 4.11,
> -hugetlbfs and shared memory ranges are also compatible with
> -.BR UFFDIO_REGISTER .
> +Please refer to the list of register modes below
> +for the compatible memory backends for each mode.
>  .PP
>  The
>  .I argp
> @@ -302,9 +306,22 @@ the specified range:
>  .TP
>  .B UFFDIO_REGISTER_MODE_MISSING
>  Track page faults on missing pages.
> +Since Linux 4.3,
> +only private anonymous ranges are compatible.
> +Since Linux 4.11,
> +hugetlbfs and shared memory ranges are also compatible.
>  .TP
>  .B UFFDIO_REGISTER_MODE_WP
>  Track page faults on write-protected pages.
> +Since Linux 5.7,
> +only private anonymous ranges are compatible.
> +.TP
> +.B UFFDIO_REGISTER_MODE_MINOR
> +Track minor page faults.
> +Since Linux 5.13,
> +only hugetlbfs ranges are compatible.
> +Since Linux 5.14,
> +compatiblity with shmem ranges was added.
>  .PP
>  If the operation is successful, the kernel modifies the
>  .I ioctls
> @@ -331,6 +348,11 @@ The
>  The
>  .B UFFDIO_ZEROPAGE
>  operation is supported.
> +.TP
> +.B 1 << _UFFDIO_CONTINUE
> +The
> +.B UFFDIO_CONTINUE
> +operation is supported.
>  .PP
>  This
>  .BR ioctl (2)
> @@ -731,6 +753,110 @@ or not registered with userfaultfd write-protect mode.
>  .TP
>  .B EFAULT
>  Encountered a generic fault during processing.
> +.\"
> +.SS UFFDIO_CONTINUE
> +(Since Linux 5.13.)
> +Resolve a minor page fault
> +by installing page table entries
> +for existing pages in the page cache.
> +.PP
> +The
> +.I argp
> +argument is a pointer to a
> +.I uffdio_continue
> +structure as shown below:
> +.PP
> +.in +4n
> +.EX
> +struct uffdio_continue {
> +    struct uffdio_range range; /* Range to install PTEs for and continue */
> +    __u64 mode;                /* Flags controlling the behavior of continue */
> +    __s64 mapped;              /* Number of bytes mapped, or negated error */
> +};
> +.EE
> +.in
> +.PP
> +The following value may be bitwise ORed in
> +.IR mode
> +to change the behavior of the
> +.B UFFDIO_CONTINUE
> +operation:
> +.TP
> +.B UFFDIO_CONTINUE_MODE_DONTWAKE
> +Do not wake up the thread that waits for page-fault resolution.
> +.PP
> +The
> +.I mapped
> +field is used by the kernel
> +to return the number of bytes that were actually mapped,
> +or an error in the same manner as
> +.BR UFFDIO_COPY .
> +If the value returned in the
> +.I mapped
> +field doesn't match the value that was specified in
> +.IR range.len ,
> +the operation fails with the error
> +.BR EAGAIN .
> +The
> +.I mapped
> +field is output-only;
> +it is not read by the
> +.B UFFDIO_CONTINUE
> +operation.
> +.PP
> +This
> +.BR ioctl (2)
> +operation returns 0 on success.
> +In this case,
> +the entire area was mapped.
> +On error, \-1 is returned and
> +.I errno
> +is set to indicate the error.
> +Possible errors include:
> +.TP
> +.B EAGAIN
> +The number of bytes mapped
> +(i.e., the value returned in the
> +.I mapped
> +field)
> +does not equal the value that was specified in the
> +.I range.len
> +field.
> +.TP
> +.B EINVAL
> +Either
> +.I range.start
> +or
> +.I range.len
> +was not a multiple of the system page size; or
> +.I range.len
> +was zero; or the range specified was invalid.
> +.TP
> +.B EINVAL
> +An invalid bit was specified in the
> +.IR mode
> +field.
> +.TP
> +.B EEXIST
> +One or more pages were already mapped in the given range.
> +.TP
> +.B ENOENT
> +The faulting process has changed its virtual memory layout simultaneously with
> +an outstanding
> +.B UFFDIO_CONTINUE
> +operation.
> +.TP
> +.B ENOMEM
> +Allocating memory needed to setup the page table mappings failed.
> +.TP
> +.B EFAULT
> +No existing page could be found in the page cache for the given range.
> +.TP
> +.BR ESRCH
> +The faulting process has exited at the time of a
> +.B UFFDIO_CONTINUE
> +operation.
> +.\"
>  .SH RETURN VALUE
>  See descriptions of the individual operations, above.
>  .SH ERRORS
> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> index 41741b4d88..f8dc4766b1 100644
> --- a/man2/userfaultfd.2
> +++ b/man2/userfaultfd.2
> @@ -65,7 +65,7 @@ all memory ranges that were registered with the object are unregistered
>  and unread events are flushed.
>  .\"
>  .PP
> -Userfaultfd supports two modes of registration:
> +Userfaultfd supports three modes of registration:
>  .TP
>  .BR UFFDIO_REGISTER_MODE_MISSING " (since 4.10)"
>  When registered with
> @@ -79,6 +79,18 @@ or an
>  .B UFFDIO_ZEROPAGE
>  ioctl.
>  .TP
> +.BR UFFDIO_REGISTER_MODE_MINOR " (since 5.13)"
> +When registered with
> +.B UFFDIO_REGISTER_MODE_MINOR
> +mode, user-space will receive a page-fault notification
> +when a minor page fault occurs.
> +That is, when a backing page is in the page cache, but
> +page table entries don't yet exist.
> +The faulted thread will be stopped from execution
> +until the page fault is resolved from user-space by an
> +.B UFFDIO_CONTINUE
> +ioctl.
> +.TP
>  .BR UFFDIO_REGISTER_MODE_WP " (since 5.7)"
>  When registered with
>  .B UFFDIO_REGISTER_MODE_WP
> @@ -199,9 +211,10 @@ a page fault occurring in the requested memory range, and satisfying
>  the mode defined at the registration time, will be forwarded by the kernel to
>  the user-space application.
>  The application can then use the
> -.B UFFDIO_COPY
> +.B UFFDIO_COPY ,
> +.B UFFDIO_ZEROPAGE ,
>  or
> -.B UFFDIO_ZEROPAGE
> +.B UFFDIO_CONTINUE
>  .BR ioctl (2)
>  operations to resolve the page fault.
>  .PP
> @@ -305,6 +318,59 @@ should have the flag
>  cleared upon the faulted page or range.
>  .PP
>  Write-protect mode supports only private anonymous memory.
> +.\"
> +.SS Userfaultfd minor fault mode (since 5.13)
> +Since Linux 5.13,
> +userfaultfd supports minor fault mode.
> +In this mode,
> +fault messages are produced not for major faults
> +(where the page was missing),
> +but rather for minor faults,
> +where a page exists in the page cache,
> +but the page table entries are not yet present.
> +The user needs to first check availability of this feature using the
> +.B UFFDIO_API
> +ioctl with the appropriate feature bits set before using this feature:
> +.B UFFD_FEATURE_MINOR_HUGETLBFS
> +since Linux 5.13,
> +or
> +.B UFFD_FEATURE_MINOR_SHMEM
> +since Linux 5.14.
> +.PP
> +To register with userfaultfd minor fault mode,
> +the user needs to initiate the
> +.B UFFDIO_REGISTER
> +ioctl with mode
> +.B UFFD_REGISTER_MODE_MINOR
> +set.
> +.PP
> +When a minor fault occurs,
> +user-space will receive a page-fault notification
> +whose
> +.I uffd_msg.pagefault.flags
> +will have the
> +.B UFFD_PAGEFAULT_FLAG_MINOR
> +flag set.
> +.PP
> +To resolve a minor page fault,
> +the handler should decide whether or not
> +the existing page contents need to be modified first.
> +If so,
> +this should be done in-place via a second,
> +non-userfaultfd-registered mapping
> +to the same backing page
> +(e.g., by mapping the shmem or hugetlbfs file twice).
> +Once the page is considered "up to date",
> +the fault can be resolved by initiating an
> +.B UFFDIO_CONTINUE
> +ioctl,
> +which installs the page table entries and
> +(by default)
> +wakes up the faulting thread(s).
> +.PP
> +Minor fault mode supports only hugetlbfs-backed (since Linux 5.13)
> +and shmem-backed (since Linux 5.14) memory.
> +.\"
>  .SS Reading from the userfaultfd structure
>  Each
>  .BR read (2)
> @@ -443,19 +509,20 @@ For
>  the following flag may appear:
>  .RS
>  .TP
> -.B UFFD_PAGEFAULT_FLAG_WRITE
> -If the address is in a range that was registered with the
> -.B UFFDIO_REGISTER_MODE_MISSING
> -flag (see
> -.BR ioctl_userfaultfd (2))
> -and this flag is set, this a write fault;
> -otherwise it is a read fault.
> +.B UFFD_PAGEFAULT_FLAG_WP
> +If this flag is set, then the fault was a write-protect fault.
> +.TP
> +.B UFFD_PAGEFAULT_FLAG_MINOR
> +If this flag is set, then the fault was a minor fault.
>  .TP
> +.B UFFD_PAGEFAULT_FLAG_WRITE
> +If this flag is set, then the fault was a write fault.
> +.PP
> +If neither
>  .B UFFD_PAGEFAULT_FLAG_WP
> -If the address is in a range that was registered with the
> -.B UFFDIO_REGISTER_MODE_WP
> -flag, when this bit is set, it means it is a write-protect fault.
> -Otherwise it is a page-missing fault.
> +nor
> +.B UFFD_PAGEFAULT_FLAG_MINOR
> +are set, then the fault was a missing fault.
>  .RE
>  .TP
>  .I pagefault.feat.pid
> --
> 2.35.1.1178.g4f1659d476-goog
>