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

[Suren Baghdasaryan <surenb@google.com>]

Hi Michael,

On Tue, Feb 2, 2021 at 2:45 AM Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
>
> Hello Suren (and Minchan and Michal)
>
> Thank you for the revisions!
>
> I've applied this patch, and done a few light edits.

Thanks!

>
> However, I have a questions about undocumented pieces in *madvise(2)*,
> as well as one other question. See below.
>
> On 2/2/21 6:30 AM, Suren Baghdasaryan wrote:
> > 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_read(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>
> > ---
> > changes in v2:
> > - Changed description of MADV_COLD per Michal Hocko's suggestion
> > - Applied fixes suggested by Michael Kerrisk
> > changes in v3:
> > - Added Michal's Reviewed-by
> > - Applied additional fixes suggested by Michael Kerrisk
> >
> > NAME
> >     process_madvise - give advice about use of memory to a process
> >
> > SYNOPSIS
> >     #include <sys/uio.h>
> >
> >     ssize_t process_madvise(int pidfd,
> >                            const struct iovec *iovec,
> >                            unsigned long vlen,
> >                            int advice,
> >                            unsigned int flags);
> >
> > DESCRIPTION
> >     The process_madvise() system call is used to give advice or directions
> >     to the kernel about the address ranges of another process or the calling
> >     process. It provides the advice to 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;     /* Number of bytes to transfer */
> >     };
> >
> >     The iovec structure describes address ranges beginning at iov_base address
> >     and with the size of iov_len bytes.
> >
> >     The vlen represents the number of elements in the iovec structure.
> >
> >     The advice argument is one of the values listed below.
> >
> >   Linux-specific advice values
> >     The following Linux-specific advice values have no counterparts in the
> >     POSIX-specified posix_madvise(3), and may or may not have counterparts
> >     in the madvise(2) interface available on other implementations.
> >
> >     MADV_COLD (since Linux 5.4.1)
>
> I just noticed these version numbers now, and thought: they can't be
> right (because the system call appeared only in v5.11). So I removed
> them. But, of course in another sense the version numbers are (nearly)
> right, since these advice values were added for madvise(2) in Linux 5.4.
> However, they are not documented in the madvise(2) manual page. Is it
> correct to assume that MADV_COLD and MADV_PAGEOUT have exactly the same
> meaning in madvise(2) (but just for the calling process, of course)?

Correct. They should be added in the madvise(2) man page as well IMHO.

>
> >         Deactive a given range of pages which will make them a more probable
>
> I changed: s/Deactive/Deactivate/

thanks!

>
> >         reclaim target should there be a memory pressure. This is a
> >         nondestructive operation. The advice might be ignored for some pages
> >         in the range when it is not applicable.
> >
> >     MADV_PAGEOUT (since Linux 5.4.1)
> >         Reclaim a given range of pages. This is done to free up memory occupied
> >         by these pages. If a page is anonymous it will be swapped out. If a
> >         page is file-backed and dirty it will be written back to the backing
> >         storage. The advice might be ignored for some pages in the range when
> >         it is not applicable.
>
> [...]
>
> >     The hint might be applied to 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.
>
> Is the above scenario the one that leads to the partial advice case described in
> RETURN VALUE? If yes, perhaps I should add some words to make that clearer.

Correct. This describes the case when partial advice happens.

>
> You can see the light edits that I made in
> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=e3ce016472a1b3ec5dffdeb23c98b9fef618a97b
> and following that I restructured DESCRIPTION a little in
> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=3aac0708a9acee5283e091461de6a8410bc921a6

The edits LGTM.
Thanks,
Suren.

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