linux-kernel - Re: [PATCH] mm,migration: Avoid race between shift_arg_pages() and rmap_walk() during migration by not migrating temporary stacks

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Message-Id: <20100513091930.9b42e3b8.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 13 May 2010 09:19:30 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] mm,migration: Avoid race between shift_arg_pages() and
 rmap_walk() during migration by not migrating temporary stacks

On Wed, 12 May 2010 16:33:12 -0500 (CDT)
Christoph Lameter <cl@...ux.com> wrote:

> 
> Still think this special casing is not that good.
> 
> One can also disable migration by providing a migration function that
> always fails. One such function exists in mm/migrate.c:
> 
> /* Always fail migration. Used for mappings that are not movable */
> int fail_migrate_page(struct address_space *mapping,
>                         struct page *newpage, struct page *page)
> {
>         return -EIO;
> }
> EXPORT_SYMBOL(fail_migrate_page);
> 
> 
> The migration function is specified in
> 
> vma->vm_ops->migrate
> 
> If that is set to fail_migrate_page() then the pages in the vma will never
> be migrated. XFS uses it f.e. to avoid page migration:
> 
> STATIC int
> xfs_mapping_buftarg(
>         xfs_buftarg_t           *btp,
>         struct block_device     *bdev)
> {
>         struct backing_dev_info *bdi;
>         struct inode            *inode;
>         struct address_space    *mapping;
>         static const struct address_space_operations mapping_aops = {
>                 .sync_page = block_sync_page,
>                 .migratepage = fail_migrate_page,
>         };
> 
> 
> 
> Would it not be possible to do something similar for the temporary stack?
> 

Problem here is unmap->remap. ->migratepage() function is used as

	unmap 
	   -> migratepage() 
	      -> failed 
		-> remap

Then, migratepage() itself is no help. We need some check-callback before unmap
or lock to wait for an event we can make remapping progress.

Thanks,
-Kame


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