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 >
