Re: [PATCH] mm: memory migration: bug in touch_unmapped_address

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

Good catch.

That was the reason for the migration cache problems you were seeing?

On Mon, Feb 28, 2005 at 11:36:43AM -0600, Ray Bryant wrote:
> Hirokazu,
> 
> The length field in the call to get_user_pages() from touch_unmapped_pages()
> is incorrectly specified in bytes, not pages.
> 
> As a result of this, if you use the migration code to migrate a page, then
> subsequent pages (that are not necessarily currently allocated or mapped)
> can be allocated and mapped as a result of the migration call.
> 
> [touch_unmapped_pages() is added by the memory migration code from the 
> memory
> hotplug patch so this is not currently part of the mainline kernel]
> 
> See attached patch for the fix.
> -- 
> Best Regards,
> Ray
> -----------------------------------------------
>                   Ray Bryant
> 512-453-9679 (work)         512-507-7807 (cell)
> raybry@sgi.com             raybry@austin.rr.com
> The box said: "Requires Windows 98 or better",
>            so I installed Linux.
> -----------------------------------------------

> The "len" parameter (4th arg) of get_user_pages() is in pages, not
> bytes.  The effect of this bug is that if you migrate a page, and
> if this page is followed by valid virtual addresses, but these 
> pages have not yet been touched and allocated, then the migration
> call will cause those pages to be touched and allocated.  The number
> of pages so effected is the min of (16384, the remaining number of
> pages in the vma, the number of pages required to fill out the
> current pmd).
> 
> Signed-off-by: Ray Bryant <raybry@sgi.com>
> 
> Index: linux/mm/rmap.c
> ===================================================================
> --- linux.orig/mm/rmap.c	2005-01-30 10:34:03.000000000 -0800
> +++ linux/mm/rmap.c	2005-02-28 08:53:30.000000000 -0800
> @@ -554,8 +554,7 @@ touch_unmapped_address(struct list_head 
>  		vma = find_vma(v1->mm, v1->addr);
>  		if (vma == NULL)
>  			goto out;
> -		error = get_user_pages(current, v1->mm, v1->addr, PAGE_SIZE,
> -					0, 0, NULL, NULL);
> +		error = get_user_pages(current, v1->mm, v1->addr, 1, 0, 0, NULL, NULL);
>  		if (error < 0)
>  			ret = error;
>  	out:

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