Re: Warning on memory offline (and possible in usual migration?)

[Christoph Lameter <clameter@sgi.com>]

On Tue, 15 Apr 2008, KAMEZAWA Hiroyuki wrote:

> > An !uptodate page is not migratable. Maybe we need to add better checking?
> > 
> adding check is good but...
> 
> I found I can reproduce this. I'd like to chase this a bit more.
> 
> following is just a report.
> ==
> a page which caused trouble was
> - Dirty, Private, Locked (locked because of migration)
> - a file cache of ext3. (maybe)
> 
> When I added following check,
> ==
> @@ -648,6 +649,10 @@ static int unmap_and_move(new_page_t get
>                         goto move_newpage;
>                 lock_page(page);
>         }
> +       /* All caches should be Uptodate before migration.*/
> +       if (page_mapping(page) && !PageUptodate(page))
> +               goto unlock;
> +
> 
>         if (PageWriteback(page)) {
>                 if (!force)
> ==
> A page offlining never ends until I run "echo 3 > /proc/sys/vm/drop_caches".

Page migration should stop after 10 retries though? You need to set the RC 
to another value than -EAGAIN in order to avoid the retries.

if (page_mapping(page) && !PageUptodate(page)) {
	rc = -EBUSY;
	goto unlock;
}

will stop retries.

> Once I use drop_caches, memory offlining works very well.
> (even under some workload.) If the code I added is bad, plz blame me.

The retries during page migration may hold off the completion of bringing 
up a page up to date since the PageLock is repeatedly acquired. So either 
pass more pages in one go to migrate_pages() or pause once in awhile?

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