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
