Re: [RFC][PATCH] OVERCOMMIT_ALWAYS extension

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

On Wed, 2005-10-19 at 16:47 -0400, Jeff Dike wrote:
> On Wed, Oct 19, 2005 at 11:50:55AM -0700, Badari Pulavarty wrote:
> > Darren Hart is working on patch to add madvise(DISCARD) to extend
> > the functionality of madvise(DONTNEED) to really drop those pages.
> > I was going to ask your opinion on that approach :) 
> > 
> > shmget(SHM_NORESERVE) + madvise(DISCARD) should do what I was
> > hoping for. (BTW, none of this has been tested with database stuff -
> > I am just concentrating on reasonable extensions.
> 
> madvise(DISCARD) has a promising name, but the implementation seems to be
> very differant from what the name says.
> 
> This would seem to throw out all pages in the file after offset, which
> makes the end parameter kind of pointless:
> 
> +               down(i_sem);
> +               truncate_inode_pages(vma->vm_file->f_mapping, offset);
> +               up(i_sem);
> 
> It will also fully truncate files which you have only partially
> mapped, which is somewhat counterintuitive.

Yes. I agree. We were just trying to re-use existing code to see if it
even works.

Initial plan was to use invalidate_inode_pages2_range(). But it didn't
really do what we wanted. So we ended up using truncate_inode_pages().
If it really works, then I plan to add truncate_inode_pages2_range()
to which works on a range of pages, instead of the whole file.
madvise(DONTNEED) followed by madvise(DISCARD) should be able to drop
all the pages in the given range.

Does this make sense ? Does this seem like right approach ?

Thanks,
Badari

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