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

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Message-ID: <2807E5FD2F6FDA4886F6618EAC48510E79CA51BA@CRSMSX101.amr.corp.intel.com>
Date:   Tue, 9 Apr 2019 14:55:31 +0000
From:   "Weiny, Ira" <ira.weiny@...el.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Huang Shijie <sjhuang@...vatar.ai>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>,
        "palmer@...ive.com" <palmer@...ive.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/2] mm/gup.c: fix the wrong comments

> On Tue, Apr 09, 2019 at 11:04:18AM +0800, Huang Shijie wrote:
> > 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.
> 
> gup_fast() and gup_unlocked() should return the pages in the same order.
> If they do not, then it is a bug.

Is there a reproducer for this?  Or do you have some debug output which shows this problem?

Ira