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process_madvise.2: Document process_madvise(2)
Initial version of process_madvise(2) manual page. Initial text was extracted from [1], amended after fix [2] and more details added using man pages of madvise(2) and process_vm_readv(2) as examples. It also includes the changes to required permission proposed in [3]. [1] https://lore.kernel.org/patchwork/patch/1297933/ [2] https://lkml.org/lkml/2020/12/8/1282 [3] https://patchwork.kernel.org/project/selinux/patch/20210111170622.2613577-1-surenb@google.com/#23888311 Signed-off-by: Suren Baghdasaryan <surenb@google.com> Reviewed-by: Michal Hocko <mhocko@suse.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Suren Baghdasaryan
Michael Kerrisk
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.\" Copyright (C) 2021 Suren Baghdasaryan <surenb@google.com>
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.\" and Copyright (C) 2021 Minchan Kim <minchan@kernel.org>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Commit ecb8ac8b1f146915aa6b96449b66dd48984caacc
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.\"
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.TH PROCESS_MADVISE 2 2021-01-12 "Linux" "Linux Programmer's Manual"
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.SH NAME
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process_madvise \- give advice about use of memory to a process
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.SH SYNOPSIS
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.nf
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.B #include <sys/uio.h>
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.PP
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.BI "ssize_t process_madvise(int " pidfd ,
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.BI " const struct iovec *" iovec ,
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.BI " unsigned long " vlen ,
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.BI " int " advice ,
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.BI " unsigned int " flags ");"
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.fi
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.SH DESCRIPTION
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The
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.BR process_madvise()
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system call is used to give advice or directions to the kernel about the
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address ranges of another process or the calling process.
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It provides the advice to the address ranges described by
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.I iovec
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and
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.IR vlen .
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The goal of such advice is to improve system or application performance.
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.PP
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The
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.I pidfd
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argument is a PID file descriptor (see
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.BR pidfd_open (2))
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that specifies the process to which the advice is to be applied.
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.PP
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The pointer
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.I iovec
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points to an array of
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.I iovec
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structures, defined in
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.IR <sys/uio.h>
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as:
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.PP
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.in +4n
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.EX
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struct iovec {
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void *iov_base; /* Starting address */
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size_t iov_len; /* Number of bytes to transfer */
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};
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.EE
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.in
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.PP
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The
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.I iovec
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structure describes address ranges beginning at
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.I iov_base
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address and with the size of
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.I iov_len
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bytes.
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.PP
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The
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.I vlen
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represents the number of elements in the
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.I iovec
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structure.
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.PP
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The
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.I advice
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argument is one of the values listed below.
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.\"
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.\" ======================================================================
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.\"
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.SS Linux-specific advice values
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The following Linux-specific
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.I advice
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values have no counterparts in the POSIX-specified
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.BR posix_madvise (3),
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and may or may not have counterparts in the
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.BR madvise (2)
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interface available on other implementations.
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.TP
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.BR MADV_COLD " (since Linux 5.4.1)"
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.\" commit 9c276cc65a58faf98be8e56962745ec99ab87636
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Deactive a given range of pages which will make them a more probable
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reclaim target should there be a memory pressure.
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This is a nondestructive operation.
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The advice might be ignored for some pages in the range when it is not
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applicable.
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.TP
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.BR MADV_PAGEOUT " (since Linux 5.4.1)"
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.\" commit 1a4e58cce84ee88129d5d49c064bd2852b481357
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Reclaim a given range of pages.
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This is done to free up memory occupied by these pages.
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If a page is anonymous it will be swapped out.
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If a page is file-backed and dirty it will be written back to the backing
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storage.
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The advice might be ignored for some pages in the range when it is not
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applicable.
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.PP
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The
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.I flags
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argument is reserved for future use; currently, this argument must be
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specified as 0.
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.PP
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The value specified in the
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.I vlen
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argument must be less than or equal to
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.BR IOV_MAX
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(defined in
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.I <limits.h>
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or accessible via the call
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.IR sysconf(_SC_IOV_MAX) ).
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.PP
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The
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.I vlen
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and
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.I iovec
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arguments are checked before applying any hints.
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If the
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.I vlen
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is too big, or
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.I iovec
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is invalid,
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an error will be returned immediately and no advice will be applied.
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.PP
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The hint might be applied to a part of
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.I iovec
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if one of its elements points to an invalid memory region in the
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remote process.
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No further elements will be processed beyond that point.
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.PP
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Permission to provide a hint to another process is governed by a
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ptrace access mode
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.B PTRACE_MODE_READ_REALCREDS
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check (see
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.BR ptrace (2));
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in addition, the caller must have the
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.B CAP_SYS_ADMIN
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capability due to performance implications of applying the hint.
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.SH RETURN VALUE
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On success, process_madvise() returns the number of bytes advised.
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This return value may be less than the total number of requested bytes,
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if an error occurred after some iovec elements were already processed.
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The caller should check the return value to determine whether a partial
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advice occurred.
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.PP
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On error, \-1 is returned and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EBADF
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.I pidfd
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is not a valid PID file descriptor.
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.TP
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.B EFAULT
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The memory described by
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.I iovec
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is outside the accessible address space of the process referred to by
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.IR pidfd .
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.TP
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.B EINVAL
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.I flags
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is not 0.
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.TP
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.B EINVAL
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The sum of the
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.I iov_len
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values of
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.I iovec
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overflows a
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.I ssize_t
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value.
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.TP
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.B EINVAL
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.I vlen
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is too large.
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.TP
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.B ENOMEM
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Could not allocate memory for internal copies of the
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.I iovec
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structures.
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.TP
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.B EPERM
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The caller does not have permission to access the address space of the process
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.IR pidfd .
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.TP
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.B ESRCH
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The target process does not exist (i.e., it has terminated and been waited on).
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.SH VERSIONS
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This system call first appeared in Linux 5.10.
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.\" commit ecb8ac8b1f146915aa6b96449b66dd48984caacc
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Support for this system call is optional,
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depending on the setting of the
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.B CONFIG_ADVISE_SYSCALLS
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configuration option.
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.SH SEE ALSO
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.BR madvise (2),
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.BR pidfd_open(2),
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.BR process_vm_readv (2),
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.BR process_vm_write (2)
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