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

[Ira Weiny]

On Wed, Apr 10, 2019 at 09:18:50AM +0800, Huang Shijie wrote:
> On Tue, Apr 09, 2019 at 01:23:16PM -0700, Ira Weiny 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:
> > > > > 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
> > >
> > 
> > Neither this nor the get_user_pages_unlocked is filling the pages a second
> The get_user_pages_unlocked() will call the handle_mm_fault which will allocate a
> new page for the empty PTE, and save the new page into the @pages array.

But shouldn't this happen if get_user_pages_unlocked() is called directly?

> 
> 
> > time.  It is adding to the page array having moved start and the page array
> > forward.
> 
> Yes. This will mess up the page order.
> 
> I will read the code again to check if I am wrong :)
> 
> > 
> > Are you doing a FOLL_LONGTERM GUP?  Or are you in the else clause below when
> > you get this bug?
> I do not use FOLL_LONGTERM, I just use the FOLL_WRITE.
> 
> So it seems it runs into the else clause below.

Ok thanks,
Ira

> 
> Thanks
> Huang Shijie
> 
> > 
> > Ira
> > 
> > > 							    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);
> > > 			}
> > > 		}
> > > 
> > > 
>