On Thu, Apr 1, 2021 at 4:47 PM Peter Xu <pet...@redhat.com> wrote: > > Hi, Suren, > > On Thu, Apr 01, 2021 at 12:43:51PM -0700, Suren Baghdasaryan wrote: > > On Thu, Apr 1, 2021 at 11:59 AM Linus Torvalds > > <torva...@linux-foundation.org> wrote: > > > > > > On Thu, Apr 1, 2021 at 11:17 AM Suren Baghdasaryan <sur...@google.com> > > > wrote: > > > > > > > > We received a report that the copy-on-write issue repored by Jann Horn > > > > in > > > > https://bugs.chromium.org/p/project-zero/issues/detail?id=2045 is still > > > > reproducible on 4.14 and 4.19 kernels (the first issue with the > > > > reproducer > > > > coded in vmsplice.c). > > > > > > Gaah. > > > > > > > I confirmed this and also that the issue was not > > > > reproducible with 5.10 kernel. I tracked the fix to the following patch > > > > introduced in 5.9 which changes the do_wp_page() logic: > > > > > > > > 09854ba94c6a 'mm: do_wp_page() simplification' > > > > > > The problem here is that there's a _lot_ more patches than the few you > > > found that fixed various other cases (THP etc). > > > > > > > I backported this patch (#2 in the series) along with 2 prerequisite > > > > patches > > > > (#1 and #4) that keep the backports clean and two followup fixes to the > > > > main > > > > patch (#3 and #5). I had to skip the following fix: > > > > > > > > feb889fb40fa 'mm: don't put pinned pages into the swap cache' > > > > > > > > because it uses page_maybe_dma_pinned() which does not exists in earlier > > > > kernels. Because pin_user_pages() does not exist there as well, I > > > > *think* > > > > we can safely skip this fix on older kernels, but I would appreciate if > > > > someone could confirm that claim. > > > > > > Hmm. I think this means that swap activity can now break the > > > connection to a GUP page (the whole pre-pinning model), but it > > > probably isn't a new problem for 4.9/4.19. > > > > > > I suspect the test there should be something like > > > > > > /* Single mapper, more references than us and the map? */ > > > if (page_mapcount(page) == 1 && page_count(page) > 2) > > > goto keep_locked; > > > > > > in the pre-pinning days. > > > > > > But I really think that there are a number of other commits you're > > > missing too, because we had a whole series for THP fixes for the same > > > exact issue. > > > > > > Added Peter Xu to the cc, because he probably tracked those issues > > > better than I did. > > > > > > So NAK on this for now, I think this limited patch-set likely > > > introduces more problems than it fixes. > > > > Thanks for confirming my worries. I'll be happy to add additional > > backports if Peter can point me to them. > > If for a full-alignment with current upstream, I can at least think of below > series: > > Early cow for general pages: > https://lore.kernel.org/lkml/20200925222600.6832-1-pet...@redhat.com/ > > A race fix for copy_page and gup-fast: > https://lore.kernel.org/linux-mm/0-v4-908497cf359a+4782-gup_fork_...@nvidia.com/ > > Early cow for hugetlbfs (which is very recently): > https://lore.kernel.org/lkml/20210217233547.93892-1-pet...@redhat.com/ > > But I believe they'll bring a number of dependencies too like the page pinned > work; so seems not easy.
Thanks Peter. Let me try backporting these and I'll see if it's doable. > > Btw, AFAICT you don't need patch 4/5 in this series for 4.14/4.19, since > those're only for uffd-wp and it doesn't exist until 5.7. Got it. Will drop it from the next series. Thanks, Suren. > > Thanks, > > -- > Peter Xu >
