Re: [RFC] madvise(MADV_TRUNCATE)

[Badari Pulavarty <pbadari@us.ibm.com>]

On Thu, 2005-10-27 at 11:20 -0700, Andrew Morton wrote:
> err, guys.
> 
> Andrea Arcangeli <andrea@suse.de> wrote:
> >
> > ...
> >
> > tmpfs (the short term big need of this feature).
> > 
> > ...
> >
> > Freeing swap entries is the most important thing and at the same time
> > the most complex in the patch (that's why the previous MADV_DISCARD was
> > so simple ;).
> > 
> 
> I think there's something you're not telling us!
> 
> googling MADV_DISCARD comes up with basically nothing.  MADV_TRUNCATE comes
> up with precisely nothing.

I sent out a patch (linux-mm) for review madvise(MADV_DISCARD) to drop
the pagecache pages for shared memory segments. Andrea & Hugh commented
that - its not good enough, since:

(1) It doesn't work on shmfs, if the blocks are swapped out.
(2) it doesn't work on real filesystems and corrupts stuff (because
we are thrashing pagecache without filesystem knowledge).

> 
> Why does tmpfs need this feature?  What's the requirement here?  Please
> spill the beans ;)

I have 2 reasons (I don't know if Andrea has more uses/reasons):

(1) Our database folks want to drop parts of shared memory segments
when they see memory pressure or memory hotplug/virtualization stuff.
madvise(DONTNEED) is not really releasing the pagecache pages. So 
they want madvise(DISCARD).

(2) Jeff Dike wants to use this for UML.

> 
> 
> Comment on the patch: doing it via madvise sneakily gets around the
> problems with partial-page truncation (we don't currently have a way to
> release anything but the the tail-end of a page's blocks).
> 
> But if we start adding infrastructure of this sort people are, reasonably,
> going to want to add sys_holepunch(fd, start, len) and it's going to get
> complexer.

Please advise on what you would prefer. A small extension to madvise()
to solve few problems right now OR lets do real sys_holepunch() and
bite the bullet (even though we may not get any more users for it).

Thanks,
Badari

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>