shm: Document shm_create_largepage()
While here, move notes about FreeBSD-specific functionality to the COMPATIBILITY section, and document the ECAPMODE error for shm_open(). Reviewed by: pauamma, kib MFC after: 2 weeks Sponsored by: Klara, Inc. Sponsored by: Juniper Networks, Inc. Differential Revision: https://reviews.freebsd.org/D38282
This commit is contained in:
Mark Johnston
parent
a2286a1f46
commit
5f03f96fbe
@ -482,6 +482,7 @@ MLINKS+=setuid.2 setegid.2 \
|
||||
setuid.2 setgid.2
|
||||
MLINKS+=shmat.2 shmdt.2
|
||||
MLINKS+=shm_open.2 memfd_create.3 \
|
||||
shm_open.2 shm_create_largepage.3 \
|
||||
shm_open.2 shm_unlink.2 \
|
||||
shm_open.2 shm_rename.2
|
||||
MLINKS+=sigwaitinfo.2 sigtimedwait.2
|
||||
|
||||
@ -28,11 +28,11 @@
|
||||
.\"
|
||||
.\" $FreeBSD$
|
||||
.\"
|
||||
.Dd June 25, 2021
|
||||
.Dd January 30, 2023
|
||||
.Dt SHM_OPEN 2
|
||||
.Os
|
||||
.Sh NAME
|
||||
.Nm memfd_create , shm_open , shm_rename, shm_unlink
|
||||
.Nm memfd_create , shm_create_largepage , shm_open , shm_rename, shm_unlink
|
||||
.Nd "shared memory object operations"
|
||||
.Sh LIBRARY
|
||||
.Lb libc
|
||||
@ -43,6 +43,14 @@
|
||||
.Ft int
|
||||
.Fn memfd_create "const char *name" "unsigned int flags"
|
||||
.Ft int
|
||||
.Fo shm_create_largepage
|
||||
.Fa "const char *path"
|
||||
.Fa "int flags"
|
||||
.Fa "int psind"
|
||||
.Fa "int alloc_policy"
|
||||
.Fa "mode_t mode"
|
||||
.Fc
|
||||
.Ft int
|
||||
.Fn shm_open "const char *path" "int flags" "mode_t mode"
|
||||
.Ft int
|
||||
.Fn shm_rename "const char *path_from" "const char *path_to" "int flags"
|
||||
@ -51,7 +59,7 @@
|
||||
.Sh DESCRIPTION
|
||||
The
|
||||
.Fn shm_open
|
||||
system call opens (or optionally creates) a
|
||||
function opens (or optionally creates) a
|
||||
POSIX
|
||||
shared memory object named
|
||||
.Fa path .
|
||||
@ -114,9 +122,7 @@ see
|
||||
and
|
||||
.Xr fcntl 2 .
|
||||
.Pp
|
||||
As a
|
||||
.Fx
|
||||
extension, the constant
|
||||
The constant
|
||||
.Dv SHM_ANON
|
||||
may be used for the
|
||||
.Fa path
|
||||
@ -143,6 +149,131 @@ will fail with
|
||||
All other flags are ignored.
|
||||
.Pp
|
||||
The
|
||||
.Fn shm_create_largepage
|
||||
function behaves similarly to
|
||||
.Fn shm_open ,
|
||||
except that the
|
||||
.Dv O_CREAT
|
||||
flag is implicitly specified, and the returned
|
||||
.Dq largepage
|
||||
object is always backed by aligned, physically contiguous chunks of memory.
|
||||
This ensures that the object can be mapped using so-called
|
||||
.Dq superpages ,
|
||||
which can improve application performance in some workloads by reducing the
|
||||
number of translation lookaside buffer (TLB) entries required to access a
|
||||
mapping of the object,
|
||||
and by reducing the number of page faults performed when accessing a mapping.
|
||||
This happens automatically for all largepage objects.
|
||||
.Pp
|
||||
An existing largepage object can be opened using the
|
||||
.Fn shm_open
|
||||
function.
|
||||
Largepage shared memory objects behave slightly differently from non-largepage
|
||||
objects:
|
||||
.Bl -bullet -offset indent
|
||||
.It
|
||||
Memory for a largepage object is allocated when the object is
|
||||
extended using the
|
||||
.Xr ftruncate 2
|
||||
system call, whereas memory for regular shared memory objects is allocated
|
||||
lazily and may be paged out to a swap device when not in use.
|
||||
.It
|
||||
The size of a mapping of a largepage object must be a multiple of the
|
||||
underlying large page size.
|
||||
Most attributes of such a mapping can only be modified at the granularity
|
||||
of the large page size.
|
||||
For example, when using
|
||||
.Xr munmap 2
|
||||
to unmap a portion of a largepage object mapping, or when using
|
||||
.Xr mprotect 2
|
||||
to adjust protections of a mapping of a largepage object, the starting address
|
||||
must be large page size-aligned, and the length of the operation must be a
|
||||
multiple of the large page size.
|
||||
If not, the corresponding system call will fail and set
|
||||
.Va errno
|
||||
to
|
||||
.Er EINVAL .
|
||||
.El
|
||||
.Pp
|
||||
The
|
||||
.Fa psind
|
||||
argument to
|
||||
.Fn shm_create_largepage
|
||||
specifies the size of large pages used to back the object.
|
||||
This argument is an index into the page sizes array returned by
|
||||
.Xr getpagesizes 3 .
|
||||
In particular, all large pages backing a largepage object must be of the
|
||||
same size.
|
||||
For example, on a system with large page sizes of 2MB and 1GB, a 2GB largepage
|
||||
object will consist of either 1024 2MB pages, or 2 1GB pages, depending on
|
||||
the value specified for the
|
||||
.Fa psind
|
||||
argument.
|
||||
The
|
||||
.Fa alloc_policy
|
||||
parameter specifies what happens when an attempt to use
|
||||
.Xr ftruncate 2
|
||||
to allocate memory for the object fails.
|
||||
The following values are accepted:
|
||||
.Bl -tag -offset indent -width SHM_
|
||||
.It Dv SHM_LARGEPAGE_ALLOC_DEFAULT
|
||||
If the (non-blocking) memory allocation fails because there is insufficient free
|
||||
contiguous memory, the kernel will attempt to defragment physical memory and
|
||||
try another allocation.
|
||||
The subsequent allocation may or may not succeed.
|
||||
If this subsequent allocation also fails,
|
||||
.Xr ftruncate 2
|
||||
will fail and set
|
||||
.Va errno
|
||||
to
|
||||
.Er ENOMEM .
|
||||
.It Dv SHM_LARGEPAGE_ALLOC_NOWAIT
|
||||
If the memory allocation fails,
|
||||
.Xr ftruncate 2
|
||||
will fail and set
|
||||
.Va errno
|
||||
to
|
||||
.Er ENOMEM .
|
||||
.It Dv SHM_LARGEPAGE_ALLOC_HARD
|
||||
The kernel will attempt defragmentation until the allocation succeeds,
|
||||
or an unblocked signal is delivered to the thread.
|
||||
However, it is possible for physical memory to be fragmented such that the
|
||||
allocation will never succeed.
|
||||
.El
|
||||
.Pp
|
||||
The
|
||||
.Dv FIOSSHMLPGCNF
|
||||
and
|
||||
.Dv FIOGSHMLPGCNF
|
||||
.Xr ioctl 2
|
||||
commands can be used with a largepage shared memory object to get and set
|
||||
largepage object parameters.
|
||||
Both commands operate on the following structure:
|
||||
.Bd -literal
|
||||
struct shm_largepage_conf {
|
||||
int psind;
|
||||
int alloc_policy;
|
||||
};
|
||||
|
||||
.Ed
|
||||
The
|
||||
.Dv FIOGSHMLPGCNF
|
||||
command populates this structure with the current values of these parameters,
|
||||
while the
|
||||
.Dv FIOSSHMLPGCNF
|
||||
command modifies the largepage object.
|
||||
Currently only the
|
||||
.Va alloc_policy
|
||||
parameter may be modified.
|
||||
Internally,
|
||||
.Fn shm_create_largepage
|
||||
works by creating a regular shared memory object using
|
||||
.Fn shm_open ,
|
||||
and then converting it into a largepage object using the
|
||||
.Dv FIOSSHMLPGCNF
|
||||
ioctl command.
|
||||
.Pp
|
||||
The
|
||||
.Fn shm_rename
|
||||
system call atomically removes a shared memory object named
|
||||
.Fa path_from
|
||||
@ -162,10 +293,6 @@ Return an error if an shm exists at
|
||||
.Fa path_to ,
|
||||
rather than unlinking it.
|
||||
.El
|
||||
.Fn shm_rename
|
||||
is also a
|
||||
.Fx
|
||||
extension.
|
||||
.Pp
|
||||
The
|
||||
.Fn shm_unlink
|
||||
@ -235,6 +362,17 @@ All functions return -1 on failure, and set
|
||||
to indicate the error.
|
||||
.Sh COMPATIBILITY
|
||||
The
|
||||
.Fn shm_create_largepage
|
||||
and
|
||||
.Fn shm_rename
|
||||
functions are
|
||||
.Fx
|
||||
extensions, as is support for the
|
||||
.Dv SHM_ANON
|
||||
value in
|
||||
.Fn shm_open .
|
||||
.Pp
|
||||
The
|
||||
.Fa path ,
|
||||
.Fa path_from ,
|
||||
and
|
||||
@ -377,6 +515,18 @@ and
|
||||
are specified and the named shared memory object does exist.
|
||||
.It Bq Er EACCES
|
||||
The required permissions (for reading or reading and writing) are denied.
|
||||
.It Bq Er ECAPMODE
|
||||
The process is running in capability mode (see
|
||||
.Xr capsicum 4 )
|
||||
and attempted to create a named shared memory object.
|
||||
.El
|
||||
.Pp
|
||||
.Fn shm_create_largepage
|
||||
can fail for the reasons listed above.
|
||||
It also fails with these error codes for the following conditions:
|
||||
.Bl -tag -width Er
|
||||
.It Bq Er ENOTTY
|
||||
The kernel does not support large pages on the current platform.
|
||||
.El
|
||||
.Pp
|
||||
The following errors are defined for
|
||||
@ -425,6 +575,7 @@ requires write permission to the shared memory object.
|
||||
.Xr close 2 ,
|
||||
.Xr fstat 2 ,
|
||||
.Xr ftruncate 2 ,
|
||||
.Xr ioctl 2 ,
|
||||
.Xr mmap 2 ,
|
||||
.Xr munmap 2 ,
|
||||
.Xr sendfile 2
|
||||
|
||||
Loading…
Reference in New Issue
Block a user