On Wed, Apr 21, 2021 at 3:59 PM Vlastimil Babka <vbabka@xxxxxxx> wrote: > > On 4/21/21 10:01 PM, Suren Baghdasaryan wrote: > > On Wed, Apr 7, 2021 at 2:53 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > >> > >> On Wed, Apr 7, 2021 at 12:23 PM Linus Torvalds > >> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > >> > > >> > On Wed, Apr 7, 2021 at 11:47 AM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > >> > > > >> > > So, we fixed it, but we don't know why. > >> > > > >> > > Peter Xu's patchset that fixed it is here: > >> > > https://lore.kernel.org/lkml/20200821234958.7896-1-peterx@xxxxxxxxxx/ > >> > > >> > Yeah, that's the part that ends up being really painful to backport > >> > (with all the subsequent fixes too), so the 4.14 people would prefer > >> > to avoid it. > >> > > >> > But I think that if it's a "requires dax pmem and ptrace on top", it > >> > may simply be a non-issue for those users. Although who knows - maybe > >> > that ends up being a real issue on Android.. > >> > >> A lot to digest, so I need to do some reading now. Thanks everyone! > > > > After a delay due to vacation I prepared backports of 17839856fd58 > > ("gup: document and work around "COW can break either way" issue") for > > 4.14 and 4.19 kernels. As Linus pointed out, uffd-wp was introduced > > later in 5.7, so is not an issue for 4.x kernels. The issue with THPs > > is still unresolved, so with or without this patch it's still there > > (Android is not affected by this since we do not use THPs with older > > kernels). > > Which THP issue do you mean here? The race that was part of the same Project > zero report and was solved by a different patch adding some locking? Or the > vmsplice info leak but applied to THP's? Because if it's the latter then I > believe 17839856fd58 did solve that too. It was the later switch of approach to > rely just on page_count() that left THP side unfixed. I meant the "vmsplice info leak applied to THP's" but now I realize that 17839856fd58 does not use elevated reference count, so indeed that should not be a problem. Thanks for the note! > > > Andrea pointed out that there are other issues and to properly fix > > them his COR approach is needed. However it has not been accepted yet, > > so I can't really backport it. I'll be happy to do that though if it > > is accepted in the future. > > > > Peter, you mentioned https://lkml.org/lkml/2020/8/10/439 patch to > > distinguish real writes vs enforced COW read requests, however I also > > see that you had a later version of this patch here: > > https://lore.kernel.org/patchwork/patch/1286506/. Which one should I > > backport? Or is it not needed in the absence of uffd-wp support in the > > earlier kernels? > > Thanks, > > Suren. > > > >> > >> > > >> > Linus > > >
