[PATCH v3 1/1] process_madvise.2: Add process_madvise man page

Michael Kerrisk (man-pages) mtk.manpages at gmail.com
Thu Feb 18 07:55:31 UTC 2021

Hello Suren,

>> Thanks. I added a few words to clarify this.>
> Any link where I can see the final version?


Also rendered below.



       process_madvise - give advice about use of memory to a process

       #include <sys/uio.h>

       ssize_t process_madvise(int pidfd, const struct iovec *iovec,
                               size_t vlen, int advice,
                               unsigned int flags);

       Note: There is no glibc wrapper for this system call; see NOTES.

       The process_madvise() system call is used to give advice or direc‐
       tions 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 <lim‐
       its.h> or accessible via the call sysconf(_SC_IOV_MAX)).

       The advice argument is one of the following values:

              See madvise(2).

              See madvise(2).

       The flags argument is reserved for future use; currently, this ar‐
       gument must be specified as 0.

       The  vlen  and iovec arguments are checked before applying any ad‐
       vice.  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 ca‐

       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 ad‐
              dress 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

       EINVAL vlen is too large.

       ENOMEM Could not allocate memory for internal copies of the  iovec

       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  CON‐
       FIG_ADVISE_SYSCALLS configuration option.

       The process_madvise() system call is Linux-specific.

       Glibc does not provide a wrapper for this system call; call it us‐
       ing syscall(2).

       madvise(2),          pidfd_open(2),           process_vm_readv(2),

Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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