Re: [RFC] memory defragmentation to satisfy high order allocations

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

On Fri, Oct 08, 2004 at 04:00:28PM +0900, Hirokazu Takahashi wrote:
> Hi, Marcelo.
> 
> > It seems there is typo in the current version of the patch:
> > 
> > int try_to_migrate_pages(struct list_head *page_list)
> > {
> > ...
> >         current->flags |= PF_KSWAPD;    /*  It's fake */
> >         list_for_each_entry_safe(page, page2, page_list, lru) {
> >                 /*
> >                  * Start writeback I/O if it's a dirty page with buffers
> >                  * and it doesn't have migrate_page method.
> >                  */
> >                 if (PageDirty(page) && PagePrivate(page)) {
> >                         if (!TestSetPageLocked(page)) {
> >                                 mapping = page_mapping(page);
> >                                 if (!mapping || mapping->a_ops->migrate_page ||
> a> 						^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >                                     pageout(page, mapping) != PAGE_SUCCESS) {
> >                                         unlock_page(page);
> >                                 }
> >                         }
> >                 }
> > 
> > 
> > Shouldnt that be "!mapping->a_ops->migrate_page"?
> 
> "mapping->a_ops->migrate_page" is correct.
> 
> This code is just for optimization. If mapping->a_ops->migrate_page
> isn't implemented, migrate_page_common() is used to migrate the page.
> migrate_page_common() will try to write it back if it's dirty, so that
> it would be better to start writeback I/O for target pages in advance
> without waiting the I/O completions.

Right. But if migrate_page _is_ implemented, we also start writeback! 

Shouldnt that be "if we dont have migrate_page(), start writeback, since 
in this case well use migrate_page_common() anyway."

It seems the logic is inverted, or maybe I'm wrong.

> As you may know the migration code will work fine without this code.

OK!

> > That is, if we can't migrate the page, try to write it out?

I just didnt understand the logic very well, maybe I should just 
go reread the code.

Thanks!
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