Messages in this thread | | | From | Dan Williams <> | Date | Tue, 1 May 2018 16:31:44 -0700 | Subject | Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter() |
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On Tue, May 1, 2018 at 4:28 PM, Andy Lutomirski <luto@kernel.org> wrote: > On Tue, May 1, 2018 at 4:02 PM Dan Williams <dan.j.williams@intel.com> > wrote: > >> On Tue, May 1, 2018 at 2:05 PM, Linus Torvalds >> <torvalds@linux-foundation.org> wrote: >> > On Tue, May 1, 2018 at 1:55 PM Dan Williams <dan.j.williams@intel.com> >> > wrote: >> > >> >> The result of the bypass is that the kernel treats machine checks > during >> >> read as system fatal (reboot) when they could simply be flagged as an >> >> I/O error, similar to performing reads through the pmem driver. Prevent >> >> this fatal condition by deploying memcpy_mcsafe() in the fsdax read >> >> path. >> > >> > How about just changing the rules, and go the old "Don't do that then" > way? >> > >> > IOW, get rid of the whole idea that MCS errors should be fatal. It's > wrong >> > and pointless anyway. >> > >> > The while approach seems fundamentally buggered, if you ever want to > mmap >> > one of these things. And don't you want that? >> > >> > So why continue down a fundamentally broken path? > >> I'm confused. Are you talking about getting rid of the block-layer >> bypass or changing how MCS errors are handled? If it's the former I've >> gotten push back in the past trying to remove the bypass, but I feel >> better about my chances to slay that beast wielding the +5 Hammer of >> Linus. If it's the latter, MCS error handling, I don't see how get >> around something like copy_to_iter_mcsafe(). > >> You mention mmap. Yes, we want the predominant access model to be >> dax-mmap for Persistent Memory, but there's still the question about >> what to do with media errors. To date we are trying to mirror the >> error handling model for System Memory, i.e. SIGBUS to the process >> that consumed the error. Is that error handling model also problematic >> in your view? > > I'm not sure exactly what you mean here, but my understanding of the status > quo is that memory errors in user code are non-fatal but that memory errors > in kernel code are fatal unless there's an appropriate extable entry. The > old iov_iter code assumes that memcpy() on kernel addresses can't fail. > I'm not sure how else it could work.
Right, I'm trying to clarify the "IOW, get rid of the whole idea that MCS errors should be fatal" comment. Especially as I am about to go fix memory_failure() to understand that ZONE_DEVICE pages != typical "struct page", and do the right thing with respect to un-mapping userspace dax mapped pages.
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