process_madvise(2) — Linux manual pages

Linux manual pages	Section 2

Name

process_madvise — give advice about use of memory to a process

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

.BR process_madvise (),
necessitating the use of

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, defined in <sys/uio.h> as:

struct iovec {

void * iov_base;
/* Starting address */

size_t iov_len;
/* Length of region */

};

The iovec structure describes address ranges beginning at iov_base address and with the size of iov_len bytes.

The vlen specifies the number of elements in the iovec structure. 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_PAGEOUT: 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.)

Permission to apply advice to another process is governed by a ptrace access mode PTRACE_MODE_READ_REALCREDS check (see ptrace(2)); in addition, because of the performance implications of applying the advice, the caller must have the CAP_SYS_ADMIN capability.

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).

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.

CONFORMING TO

The process_madvise() system call is Linux-specific.

COLOPHON

This page is part of release 5.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.

  Copyright (C) 2021 Suren Baghdasaryan <surenbgoogle.com>
and Copyright (C) 2021 Minchan Kim <minchankernel.org>

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Commit ecb8ac8b1f146915aa6b96449b66dd48984caacc

struct	iovec {
	void	*	`iov_base`;	/* Starting address */
	size_t		`iov_len`;	/* Length of region */
};