man2/process_madvise.2

.\" Copyright (C) 2021 Suren Baghdasaryan <surenb@google.com>
.\" and Copyright (C) 2021 Minchan Kim <minchan@kernel.org>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.\" Commit ecb8ac8b1f146915aa6b96449b66dd48984caacc
.\"
.TH PROCESS_MADVISE 2 2021-01-12 "Linux" "Linux Programmer's Manual"
.SH NAME
process_madvise \- give advice about use of memory to a process
.SH SYNOPSIS
.nf
.B #include <sys/uio.h>
.PP
.BI "ssize_t process_madvise(int " pidfd ", const struct iovec *" iovec ,
.BI "                        size_t " vlen ", int " advice ,
.BI "                        unsigned int " flags ");"
.fi
.PP
.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.\" FIXME: See <https://sourceware.org/bugzilla/show_bug.cgi?id=27380>
.SH DESCRIPTION
The
.BR 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
.I iovec
and
.IR vlen .
The goal of such advice is to improve system or application performance.
.PP
The
.I pidfd
argument is a PID file descriptor (see
.BR pidfd_open (2))
that specifies the process to which the advice is to be applied.
.PP
The pointer
.I iovec
points to an array of
.I iovec
structures, defined in
.IR <sys/uio.h>
as:
.PP
.in +4n
.EX
struct iovec {
    void  *iov_base;    /* Starting address */
    size_t iov_len;     /* Length of region */
};
.EE
.in
.PP
The
.I iovec
structure describes address ranges beginning at
.I iov_base
address and with the size of
.I iov_len
bytes.
.PP
The
.I vlen
specifies the number of elements in the
.I iovec
structure.
This value must be less than or equal to
.BR IOV_MAX
(defined in
.I <limits.h>
or accessible via the call
.IR sysconf(_SC_IOV_MAX) ).
.PP
The
.I advice
argument is one of the following values:
.TP
.BR MADV_COLD
See
.BR madvise (2).
.TP
.BR MADV_PAGEOUT
See
.BR madvise (2).
.PP
The
.I flags
argument is reserved for future use; currently, this argument must be
specified as 0.
.PP
The
.I vlen
and
.I iovec
arguments are checked before applying any advice.
If
.I vlen
is too big, or
.I iovec
is invalid,
then an error will be returned immediately and no advice will be applied.
.PP
The advice might be applied to only a part of
.I 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.)
.PP
Permission to apply advice to another process is governed by a
ptrace access mode
.B PTRACE_MODE_READ_REALCREDS
check (see
.BR ptrace (2));
in addition,
because of the performance implications of applying the advice,
the caller must have the
.B CAP_SYS_ADMIN
capability.
.SH RETURN VALUE
On success,
.BR 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
.I iovec
elements were already processed.
The caller should check the return value to determine whether a partial
advice occurred.
.PP
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I pidfd
is not a valid PID file descriptor.
.TP
.B EFAULT
The memory described by
.I iovec
is outside the accessible address space of the process referred to by
.IR pidfd .
.TP
.B EINVAL
.I flags
is not 0.
.TP
.B EINVAL
The sum of the
.I iov_len
values of
.I iovec
overflows a
.I ssize_t
value.
.TP
.B EINVAL
.I vlen
is too large.
.TP
.B ENOMEM
Could not allocate memory for internal copies of the
.I iovec
structures.
.TP
.B EPERM
The caller does not have permission to access the address space of the process
.IR pidfd .
.TP
.B ESRCH
The target process does not exist (i.e., it has terminated and been waited on).
.SH VERSIONS
This system call first appeared in Linux 5.10.
.\" commit ecb8ac8b1f146915aa6b96449b66dd48984caacc
Support for this system call is optional,
depending on the setting of the
.B CONFIG_ADVISE_SYSCALLS
configuration option.
.SH CONFORMING TO
The
.BR process_madvise ()
system call is Linux-specific.
.SH NOTES
Glibc does not provide a wrapper for this system call; call it using
.BR syscall (2).
.SH SEE ALSO
.BR madvise (2),
.BR pidfd_open (2),
.BR process_vm_readv (2),
.BR process_vm_write (2)