Re: [PATCH 4/5] iomap: implement direct I/O

[Jens Axboe <axboe@kernel.dk>]

On Fri, Nov 04 2016, Christoph Hellwig wrote:
> This adds a full fledget direct I/O implementation using the iomap
> interface. Full fledged in this case means all features are supported:
> AIO, vectored I/O, any iov_iter type including kernel pointers, bvecs
> and pipes, support for hole filling and async apending writes.  It does
> not mean supporting all the warts of the old generic code.  We expect
> i_rwsem to be held over the duration of the call, and we expect to
> maintain i_dio_count ourselves, and we pass on any kinds of mapping
> to the file system for now.
> 
> The algorithm used is very simple: We use iomap_apply to iterate over
> the range of the I/O, and then we use the new bio_iov_iter_get_pages
> helper to lock down the user range for the size of the extent.
> bio_iov_iter_get_pages can currently lock down twice as many pages as
> the old direct I/O code did, which means that we will have a better
> batch factor for everything but overwrites of badly fragmented files.

This looks pretty good, I'll give it a whirl on my fast(er) devices.

One question, since it isn't commented and isn't immediately obvious to
me - what's the purpose of using the cmpxchg() on dio->error?

-- 
Jens Axboe