On Fri, Oct 13, 2017 at 12:22 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > On Wed, Oct 11, 2017 at 03:11:21PM -0700, Dan Williams wrote: >> > + /* >> > + * We don't support synchronous mappings for non-DAX files. At least >> > + * until someone comes with a sensible use case. >> > + */ >> > + if (!IS_DAX(file_inode(file)) && (map_flags & MAP_SYNC)) >> > + return -EOPNOTSUPP; >> >> Perhaps EPERM instead to differentiate the unsupported flags case? >> There's precedent for mmap returning EPERM for reasons other than >> incompatible PROT flags. > > Why would we want to voluntarily use arcane errors for a entirely > new interface under our control? > > -EOPNOTSUPP is nice and explicit about what is going on. We have this general and perennial problem of parsing why the kernel is throwing an error. It saves a debug step if the error code is unique, and in this case would indicate that the filesystem supports MAP_SYNC but the administrator failed to arrange for the device to be in DAX mode.
