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