On Mon, 11 Mar 2019, Dave Chinner wrote: > > Direct IO on a mmapped file backed page doesnt make any sense. > > People have used it for many, many years as zero-copy data movement > pattern. i.e. mmap the destination file, use direct IO to DMA direct > into the destination file page cache pages, fdatasync() to force > writeback of the destination file. Well we could make that more safe through a special API that designates a range of pages in a file in the same way as for RDMA. This is inherently not reliable as we found out. > Now we have copy_file_range() to optimise this sort of data > movement, the need for games with mmap+direct IO largely goes away. > However, we still can't just remove that functionality as it will > break lots of random userspace stuff... It is already broken and unreliable. Are there really "lots" of these things around? Can we test this by adding a warning in the kernel and see where it actually crops up?
