Re: MADV_FREE functionality

[Andrew Morton]

On Mon, 30 Apr 2007 20:54:02 -0400 Rik van Riel <riel@xxxxxxxxxx> wrote:

> Andrew Morton wrote:
> 
> >  lazy-freeing-of-memory-through-madv_free.patch
> >  lazy-freeing-of-memory-through-madv_free-vs-mm-madvise-avoid-exclusive-mmap_sem.patch
> >  restore-madv_dontneed-to-its-original-linux-behaviour.patch
> > 
> > I think the MADV_FREE changes need more work:
> > 
> > We need crystal-clear statements regarding the present functionality, the new
> > functionality and how these relate to the spec and to implmentations in other
> > OS'es.  Once we have that info we are in a position to work out whether the
> > code can be merged as-is, or if additional changes are needed.
> 
> There are two MADV variants that free pages, both do the exact
> same thing with mapped file pages, but both do something slightly
> different with anonymous pages.
> 
> MADV_DONTNEED will unmap file pages and free anonymous pages.
> When a process accesses anonymous memory at an address that
> was zapped with MADV_DONTNEED, it will return fresh zero filled
> pages.
> 
> MADV_FREE will unmap file pages.  MADV_FREE on anonymous pages
> is interpreted as a signal that the application no longer needs
> the data in the pages, and they can be thrown away if the kernel
> needs the memory for something else.  However, if the process
> accesses the memory again before the kernel needs it, the process
> will simply get the original pages back.  If the kernel needed
> the memory first, the process will get a fresh zero filled page
> like with MADV_DONTNEED.
> 
> In short:
> - both MADV_FREE and MADV_DONTNEED only unmap file pages
> - after MADV_DONTNEED the application will always get back
>    fresh zero filled anonymous pages when accessing the
>    memory
> - after MADV_FREE the application can either get back the
>    original data (without a page fault) or zero filled
>    anonymous memory
> 
> The Linux MADV_DONTNEED behavior is not POSIX compliant.
> POSIX says that with MADV_DONTNEED the application's data
> will be preserved.
> 
> Currently glibc simply ignores POSIX_MADV_DONTNEED requests
> from applications on Linux.  Changing the behaviour which
> some Linux applications may rely on might not be the best
> idea.

OK, thanks.  I stuck that in the changelog.

Michael, do you think that's enough to finalise a manpage?

> If you need any additional information, please let me know.

The patch doesn't update the various comments in madvise.c at all, which is
a surprise.  Could you please check that they are all accurate and complete?

Also, where did we end up with the Solaris compatibility?

The patch I have at present retains MADV_FREE=0x05 for sparc and sparc64
which should be good.

Did we decide that the Solaris and Linux implementations of MADV_FREE are
compatible?

What about the Solaris and Linux MADV_DONTNEED implementations?
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