Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints

[Minchan Kim <minchan-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Fri, Feb 06, 2015 at 04:57:50PM +0100, Michael Kerrisk (man-pages) wrote:
> Hi Michael
> 
> On 02/05/2015 04:41 PM, Michal Hocko wrote:
> > On Wed 04-02-15 20:24:27, Michael Kerrisk wrote:
> > [...]
> >> So, how about this text:
> >>
> >>               After a successful MADV_DONTNEED operation, the seman‐
> >>               tics  of  memory  access  in  the specified region are
> >>               changed: subsequent accesses of  pages  in  the  range
> >>               will  succeed,  but will result in either reloading of
> >>               the memory contents from the  underlying  mapped  file
> > 
> > "
> > result in either providing the up-to-date contents of the underlying
> > mapped file
> > "
> 
> Thanks! I did something like that. See below.
> 
> > Would be more precise IMO because reload might be interpreted as a major
> > fault which is not necessarily the case (see below).
> > 
> >>               (for  shared file mappings, shared anonymous mappings,
> >>               and shmem-based techniques such  as  System  V  shared
> >>               memory  segments)  or  zero-fill-on-demand  pages  for
> >>               anonymous private mappings.
> > 
> > Yes, this wording is better because many users are not aware of
> > MAP_ANON|MAP_SHARED being file backed in fact and mmap man page doesn't
> > mention that.
> 
> (Michal, would you have a text to propose to add to the mmap(2) page?
> Maybe it would be useful to add something there.)
> 
> > 
> > I am just wondering whether it makes sense to mention that MADV_DONTNEED
> > for shared mappings might be surprising and not freeing the backing
> > pages thus not really freeing memory until there is a memory
> > pressure. But maybe this is too implementation specific for a man
> > page. What about the following wording on top of yours?
> > "
> > Please note that the MADV_DONTNEED hint on shared mappings might not
> > lead to immediate freeing of pages in the range. The kernel is free to
> > delay this until an appropriate moment. RSS of the calling process will
> > be reduced however.
> > "
> 
> Thanks! I added this, but dropped in the word "immediately" in the last 
> sentence, since I assume that was implied. So now we have:
> 
>               After  a  successful MADV_DONTNEED operation, the seman‐
>               tics of  memory  access  in  the  specified  region  are
>               changed:  subsequent accesses of pages in the range will
>               succeed, but will result in either repopulating the mem‐
>               ory  contents from the up-to-date contents of the under‐
>               lying mapped file  (for  shared  file  mappings,  shared
>               anonymous  mappings,  and shmem-based techniques such as
>               System V shared memory segments) or  zero-fill-on-demand
>               pages for anonymous private mappings.
> 
>               Note  that,  when applied to shared mappings, MADV_DONT‐
>               NEED might not lead to immediate freeing of the pages in
>               the  range.   The  kernel  is  free to delay freeing the
>               pages until an appropriate  moment.   The  resident  set
>               size  (RSS)  of  the calling process will be immediately
>               reduced however.

Looks good. So, I can parse it that anonymous private mappings will lead
to immediate freeing of the pages in the range so it's clearly different
with MADV_FREE.

> 
> The current draft of the page can be found in a branch,
> http://git.kernel.org/cgit/docs/man-pages/man-pages.git/log/?h=draft_madvise
> 
> Thanks,
> 
> Michael
> 
> 
> 
> -- 
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/

-- 
Kind regards,
Minchan Kim
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