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

[Huang Shijie]

On Mon, Apr 08, 2019 at 07:13:13AM -0700, Matthew Wilcox wrote:
> 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?
Please see the code in gup.c:

	int get_user_pages_fast(unsigned long start, int nr_pages,
				unsigned int gup_flags, struct page **pages)
	{
		.......
		if (gup_fast_permitted(start, nr_pages)) {
			local_irq_disable();
			gup_pgd_range(addr, end, gup_flags, pages, &nr);               // The @pages array maybe filled at the first time.
			local_irq_enable();
			ret = nr;
		}
		.......
		if (nr < nr_pages) {
			/* Try to get the remaining pages with get_user_pages */
			start += nr << PAGE_SHIFT;
			pages += nr;                                                  // The @pages is moved forward.

			if (gup_flags & FOLL_LONGTERM) {
				down_read(&current->mm->mmap_sem);
				ret = __gup_longterm_locked(current, current->mm,      // The @pages maybe filled at the second time
							    start, nr_pages - nr,
							    pages, NULL, gup_flags);
				up_read(&current->mm->mmap_sem);
			} else {
				/*
				 * retain FAULT_FOLL_ALLOW_RETRY optimization if
				 * possible
				 */
				ret = get_user_pages_unlocked(start, nr_pages - nr,    // The @pages maybe filled at the second time.
							      pages, gup_flags);
			}
		}


		.....................

BTW, I do not know why we mess up the page order. It maybe used in some special case.

Thanks
Huang Shijie