Re: [Lhms-devel] new memory hotremoval patch

[IWAMOTO Toshihiro <iwamoto@valinux.co.jp>]

At Wed, 30 Jun 2004 22:28:44 -0700,
Dave Hansen wrote:
> 
> On Wed, 2004-06-30 at 20:05, IWAMOTO Toshihiro wrote:
> > At Wed, 30 Jun 2004 17:11:11 -0700,
> > Dave Hansen wrote:
> > > 
> > > On Wed, 2004-06-30 at 04:17, IWAMOTO Toshihiro wrote:
> > > > Due to struct page changes, page->mapping == NULL predicate can no
> > > > longer be used for detecting cancellation of an anonymous page
> > > > remapping operation.  So the PG_again bit is being used again.
> > > > It may be still possible to kill the PG_again bit, but the priority is
> > > > rather low.
> > > 
> > > But, you reintroduced it everywhere, including file-backed pages, not
> > > just for anonymous pages?  Why was this necessary?
> > 
> > Which PG_again check are you talking about?
> > I think BUG_ON()s in file backed page codes should be kept for now.
> 
> I'm referring to all of the code segments like this:
>         
>         +       if (PageAgain(page)) {
>         +               unlock_page(page);
>         +               page_cache_release(page);
>         +               goto again;
>         +       }
>         +       BUG_ON(PageAgain(page));

Such code only appears only in try_to_unuse and do_swap_page.  These
functions aren't for page caches.

I'm confused.  Weren't you talking about page cache code?

> For any page that's in the page cache that you want to remove, simply
> take the page lock, overwrite page->mapping, and the page cache code
> will take care of the rest, no PG_again needed.  

What do you mean by "to remove"?


> > For swap pages, one possibility to reserve a special swap entry
> > constant (SWAP_AGAIN) and check page->private instead of PageAgain
> > check, but I'm not sure if this is a good idea.
> > 
> > #define	SWAP_AGAIN	~0UL
> 
> Do you mean pages in the swap cache?

Yes.

--
IWAMOTO Toshihiro
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