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

[Huang Shijie <sjhuang@iluvatar.ai>]

On Mon, Apr 08, 2019 at 07:49:29PM -0700, Matthew Wilcox wrote:
> On Tue, Apr 09, 2019 at 09:08:33AM +0800, Huang Shijie wrote:
> > 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:
> > > > 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.
> 
> Right ... but if it's not filled entirely, it will be filled part-way,
> and then we stop.
> 
> > 			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.
> 
> Yes, to the point where gup_pgd_range() stopped.
> 
> > 			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
> 
> Right.
> 
> > 				/*
> > 				 * 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);
> 
> Yes.  But they'll be in the same order.
> 
> > BTW, I do not know why we mess up the page order. It maybe used in some special case.
> 
> I'm not discounting the possibility that you've found a bug.
> But documenting that a bug exists is not the solution; the solution is
> fixing the bug.
I do not think it is a bug :)

If we use the get_user_pages_unlocked(), DMA is okay, such as:
                     ....
		     get_user_pages_unlocked()
		     sg_alloc_table_from_pages()
	             .....

I think the comment is not accurate enough. So just add more comments, and tell the driver
users how to use the GUPs.

Thanks
Huang Shijie