Re: [PATCH V2 1/2] mm: hotplug: implement non-movable version ofget_user_pages() called get_user_pages_non_movable()

From: Tang Chen
Date: Tue May 14 2013 - 00:12:47 EST


Hi Mel,

On 05/13/2013 05:19 PM, Mel Gorman wrote:
For memory hot-remove case, the aio pages are pined in memory and making
the pages cannot be offlined, furthermore, the pages cannot be removed.

IIUC, you mean implement migrate_unpin() and migrate_pin() callbacks in aio
subsystem, and call them when hot-remove code tries to offline
pages, right ?

If so, I'm wondering where should we put this callback pointers ?
In struct page ?


No, I would expect the callbacks to be part the address space operations
which can be found via page->mapping.


Two more problems I don't quite understand.

1. For an anonymous page, it has no address_space, and no address space
operation. But the aio ring problem just happened when dealing with
anonymous pages. Please refer to:
(https://lkml.org/lkml/2012/11/29/69)

If we put the the callbacks in page->mapping->a_ops, the anonymous pages
won't be able to use them.

And we cannot give a default callback because the situation we are dealing
with is a special situation.

So where to put the callback for anonymous pages ?


2. How to find out the reason why page->count != 1 in migrate_page_move_mapping() ?

In the problem we are dealing with, get_user_pages() is called to pin the pages
in memory. And the pages are migratable. So we want to decrease the page->count.

But get_user_pages() is not the only reason leading to page->count increased.
How can I know when should decrease teh page->count or when should not ?

The way I can figure out is to assign the callback pointer in get_user_pages()
because it is get_user_pages() who pins the pages.


Thanks. :)








--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/