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

[Hirokazu Takahashi <taka@valinux.co.jp>]

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.

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

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

Thank you,
Hirokazu Takahashi.
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