On Sun, Jan 10, 2021 at 04:19:15PM -0500, Mikulas Patocka wrote: > I put counters into vfs_read and vfs_readv. > > After a fresh boot of the virtual machine, the counters show "13385 4". > After a kernel compilation they show "4475220 8". > > So, the readv path is almost unused. > > My reasoning was that we should optimize for the "read" path and glue the > "readv" path on the top of that. Currently, the kernel is doing the > opposite - optimizing for "readv" and glueing "read" on the top of it.
But it's not about optimising for read vs readv. read_iter handles a host of other cases, such as pread(), preadv(), AIO reads, splice, and reads to in-kernel buffers. Some device drivers abused read() vs readv() to actually return different information, depending which you called. That's why there's now a prohibition against both. So let's figure out how to make iter_read() perform well for sys_read().
