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

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Message-ID: <20190408141313.GU22763@bombadil.infradead.org>
Date:   Mon, 8 Apr 2019 07:13:13 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Huang Shijie <sjhuang@...vatar.ai>
Cc:     akpm@...ux-foundation.org, william.kucharski@...cle.com,
        ira.weiny@...el.com, palmer@...ive.com, axboe@...nel.dk,
        keescook@...omium.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/gup.c: fix the wrong comments

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?