Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 01 May 2018 23:28:10 +0000 | Subject | Re: [PATCH 0/6] use memcpy_mcsafe() for copy_to_iter() |
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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.
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