Messages in this thread | | | Date | Thu, 5 May 2016 08:22:30 -0700 | From | Christoph Hellwig <> | Subject | Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io |
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On Thu, May 05, 2016 at 08:15:32AM -0700, Dan Williams wrote: > > Agreed - makig O_DIRECT less direct than not having it is plain stupid, > > and I somehow missed this initially. > > Of course I disagree because like Dave argues in the msync case we > should do the correct thing first and make it fast later, but also > like Dave this arguing in circles is getting tiresome.
We should do the right thing first, and make it fast later. But this proposal is not getting it right - it still does not handle errors for the fast path, but magically makes it work for direct I/O by in general using a less optional path for O_DIRECT. It's getting the worst of all choices.
As far as I can tell the only sensible option is to:
- always try dax-like I/O first - have a custom get_user_pages + rw_bytes fallback handles bad blocks when hitting EIO
And then we need to sort out the concurrent write synchronization. Again there I think we absolutely have to obey Posix for the !O_DIRECT case and can avoid it for O_DIRECT, similar to the existing non-DAX semantics. If we want any special additional semantics we _will_ need a special O_DAX flag.
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