Re: [PATCH 1/2] mm/gup.c: fix the wrong comments

[Matthew Wilcox]

On Mon, Apr 08, 2019 at 10:37:45AM +0800, Huang Shijie wrote:
> When CONFIG_HAVE_GENERIC_GUP is defined, the kernel will use its own
> get_user_pages_fast().
> 
> In the following scenario, we will may meet the bug in the DMA case:
> 	    .....................
> 	    get_user_pages_fast(start,,, pages);
> 	        ......
> 	    sg_alloc_table_from_pages(, pages, ...);
> 	    .....................
> 
> The root cause is that sg_alloc_table_from_pages() requires the
> page order to keep the same as it used in the user space, but
> get_user_pages_fast() will mess it up.

I don't understand how get_user_pages_fast() can return the pages in a
different order in the array from the order they appear in userspace.
Can you explain?