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SubjectRe: [PATCH 1/2] mm/gup.c: fix the wrong comments
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);
> > > }
> > > }
> > >
> > >
>

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