Manpages - process_madvise.2

process_madvise - give advice about use of memory to a process

  #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).

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.

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.

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).

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.

The *process_madvise*() system call is Linux-specific.

*madvise*(2), *pidfd_open*(2), *process_vm_readv*(2), *process_vm_write*(2)

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