Re: [PATCH] mm/hmm: Fix bad subpage pointer in try_to_unmap_one

[Jerome Glisse]

On Mon, Jul 15, 2019 at 11:14:31PM -0700, John Hubbard wrote:
> On 7/15/19 5:38 PM, Ralph Campbell wrote:
> > On 7/15/19 4:34 PM, John Hubbard wrote:
> > > On 7/15/19 3:00 PM, Andrew Morton wrote:
> > > > On Tue, 9 Jul 2019 18:24:57 -0700 Ralph Campbell <rcampbell@xxxxxxxxxx> wrote:
> > > > 
> > > >   mm/rmap.c |    1 +
> > > >   1 file changed, 1 insertion(+)
> > > > 
> > > > --- a/mm/rmap.c~mm-hmm-fix-bad-subpage-pointer-in-try_to_unmap_one
> > > > +++ a/mm/rmap.c
> > > > @@ -1476,6 +1476,7 @@ static bool try_to_unmap_one(struct page
> > > >                * No need to invalidate here it will synchronize on
> > > >                * against the special swap migration pte.
> > > >                */
> > > > +            subpage = page;
> > > >               goto discard;
> > > >           }
> > > 
> > > Hi Ralph and everyone,
> > > 
> > > While the above prevents a crash, I'm concerned that it is still not
> > > an accurate fix. This fix leads to repeatedly removing the rmap, against the
> > > same struct page, which is odd, and also doesn't directly address the
> > > root cause, which I understand to be: this routine can't handle migrating
> > > the zero page properly--over and back, anyway. (We should also mention more
> > > about how this is triggered, in the commit description.)
> > > 
> > > I'll take a closer look at possible fixes (I have to step out for a bit) soon,
> > > but any more experienced help is also appreciated here.
> > > 
> > > thanks,
> > 
> > I'm not surprised at the confusion. It took me quite awhile to
> > understand how migrate_vma() works with ZONE_DEVICE private memory.
> > The big point to be aware of is that when migrating a page to
> > device private memory, the source page's page->mapping pointer
> > is copied to the ZONE_DEVICE struct page and the page_mapcount()
> > is increased. So, the kernel sees the page as being "mapped"
> > but the page table entry as being is_swap_pte() so the CPU will fault
> > if it tries to access the mapped address.
> 
> Thanks for humoring me here...
> 
> The part about the source page's page->mapping pointer being *copied*
> to the ZONE_DEVICE struct page is particularly interesting, and belongs
> maybe even in a comment (if not already there). Definitely at least in
> the commit description, for now.
> 
> > So yes, the source anon page is unmapped, DMA'ed to the device,
> > and then mapped again. Then on a CPU fault, the zone device page
> > is unmapped, DMA'ed to system memory, and mapped again.
> > The rmap_walk() is used to clear the temporary migration pte so
> > that is another important detail of how migrate_vma() works.
> > At the moment, only single anon private pages can migrate to
> > device private memory so there are no subpages and setting it to "page"
> > should be correct for now. I'm looking at supporting migration of
> > transparent huge pages but that is a work in progress.
> 
> Well here, I worry, because subpage != tail page, right? subpage is a
> strange variable name, and here it is used to record the page that
> corresponds to *each* mapping that is found during the reverse page
> mapping walk.
> 
> And that makes me suspect that if there were more than one of these
> found (which is unlikely, given the light testing that we have available
> so far, I realize), then there could possibly be a problem with the fix,
> yes?

No THP when migrating to device memory so no tail vs head page here.

Cheers,
Jérôme