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

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

On Fri, Oct 08, 2004 at 09:23:19PM +0900, Hirokazu Takahashi wrote:
> Hi, Marcelo.
> 
> > 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 ||
> > > > 						^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > >                                     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.
> 
> It's my understanding that the previous code is equivalent to
> the following code. Am I missing something?
> 
> if (PageDirty(page) && PagePrivate(page)) {
>         if (!TestSetPageLocked(page)) {
>                 mapping = page_mapping(page);
>                 if (!mapping) {
>                         unlock_page(page);
> 		} else if (mapping->a_ops->migrate_page) {
>                         unlock_page(page);
> 		} else if (pageout(page, mapping) != PAGE_SUCCESS) {
>                         unlock_page(page);
> 		}
> 	}
> }

OK 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!

I'm thinking about how to implement a nonblocking version of generic_migrate_page().

For this purpose its really bad to allocate swap space to anonymous pages, well
need to figure out someother way of blocking the users via pagetablefault.

Like a "virtual" swap space but without allocating swap map space. 

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