process_madvise
Section: System Calls (2)
Updated: 2022-11-01
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NAME
process_madvise - give advice about use of memory to a process
LIBRARY
Standard C library
(libc, -lc)
SYNOPSIS
#include <sys/mman.h> /* Definition of MADV_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <sys/uio.h> /* Definition of struct iovec type */
#include <unistd.h>
ssize_t syscall(SYS_process_madvise, int pidfd,
const struct iovec *iovec, size_t vlen, int advice,
unsigned int flags);
Note:
glibc provides no wrapper for
process_madvise(),
necessitating the use of
syscall(2).
DESCRIPTION
The
process_madvise()
system call is used to give advice or directions to the kernel about the
address ranges of another process or of the calling process.
It provides the advice for the address ranges described by
iovec
and
vlen.
The goal of such advice is to improve system or application performance.
The
pidfd
argument is a PID file descriptor (see
pidfd_open(2))
that specifies the process to which the advice is to be applied.
The pointer
iovec
points to an array of
iovec
structures, described in
iovec(3type).
vlen
specifies the number of elements in the array of
iovec
structures.
This value must be less than or equal to
IOV_MAX
(defined in
<limits.h>
or accessible via the call
sysconf(_SC_IOV_MAX)).
The
advice
argument is one of the following values:
- MADV_COLD
-
See
madvise(2).
- MADV_COLLAPSE
-
See
madvise(2).
- MADV_PAGEOUT
-
See
madvise(2).
- MADV_WILLNEED
-
See
madvise(2).
The
flags
argument is reserved for future use; currently, this argument must be
specified as 0.
The
vlen
and
iovec
arguments are checked before applying any advice.
If
vlen
is too big, or
iovec
is invalid,
then an error will be returned immediately and no advice will be applied.
The advice might be applied to only a part of
iovec
if one of its elements points to an invalid memory region in the
remote process.
No further elements will be processed beyond that point.
(See the discussion regarding partial advice in RETURN VALUE.)
Starting in Linux 5.12,
permission to apply advice to another process is governed by
ptrace access mode
PTRACE_MODE_READ_FSCREDS
check (see
ptrace(2));
in addition,
because of the performance implications of applying the advice,
the caller must have the
CAP_SYS_NICE
capability
(see
capabilities(7)).
RETURN VALUE
On success,
process_madvise()
returns the number of bytes advised.
This return value may be less than the total number of requested bytes,
if an error occurred after some
iovec
elements were already processed.
The caller should check the return value to determine whether a partial
advice occurred.
On error, -1 is returned and
errno
is set to indicate the error.
ERRORS
- EBADF
-
pidfd
is not a valid PID file descriptor.
- EFAULT
-
The memory described by
iovec
is outside the accessible address space of the process referred to by
pidfd.
- EINVAL
-
flags
is not 0.
- EINVAL
-
The sum of the
iov_len
values of
iovec
overflows a
ssize_t
value.
- EINVAL
-
vlen
is too large.
- ENOMEM
-
Could not allocate memory for internal copies of the
iovec
structures.
- EPERM
-
The caller does not have permission to access the address space of the process
pidfd.
- ESRCH
-
The target process does not exist (i.e., it has terminated and been waited on).
See
madvise(2)
for
advice-specific
errors.
VERSIONS
This system call first appeared in Linux 5.10.
Support for this system call is optional,
depending on the setting of the
CONFIG_ADVISE_SYSCALLS
configuration option.
STANDARDS
The
process_madvise()
system call is Linux-specific.
NOTES
When this system call first appeared in Linux 5.10,
permission to apply advice to another process was entirely governed by
ptrace access mode
PTRACE_MODE_ATTACH_FSCREDS
check (see
ptrace(2)).
This requirement was relaxed in Linux 5.12 so that the caller didn't require
full control over the target process.
SEE ALSO
madvise(2),
pidfd_open(2),
process_vm_readv(2),
process_vm_write(2)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- STANDARDS
-
- NOTES
-
- SEE ALSO
-
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Time: 08:22:34 GMT, May 03, 2024