Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

From: Dan Williams
Date: Mon May 02 2016 - 12:02:07 EST

On Mon, May 2, 2016 at 8:41 AM, Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote:
> On 04/29/2016 12:16 AM, Vishal Verma wrote:
>> All IO in a dax filesystem used to go through dax_do_io, which cannot
>> handle media errors, and thus cannot provide a recovery path that can
>> send a write through the driver to clear errors.
>> Add a new iocb flag for DAX, and set it only for DAX mounts. In the IO
>> path for DAX filesystems, use the same direct_IO path for both DAX and
>> direct_io iocbs, but use the flags to identify when we are in O_DIRECT
>> mode vs non O_DIRECT with DAX, and for O_DIRECT, use the conventional
>> direct_IO path instead of DAX.
> Really? What are your thinking here?
> What about all the current users of O_DIRECT, you have just made them
> 4 times slower and "less concurrent*" then "buffred io" users. Since
> direct_IO path will queue an IO request and all.
> (And if it is not so slow then why do we need dax_do_io at all? [Rhetorical])
> I hate it that you overload the semantics of a known and expected
> O_DIRECT flag, for special pmem quirks. This is an incompatible
> and unrelated overload of the semantics of O_DIRECT.

I think it is the opposite situation, it us undoing the premature
overloading of O_DIRECT that went in without performance numbers.
This implementation clarifies that dax_do_io() handles the lack of a
page cache for buffered I/O and O_DIRECT behaves as it nominally would
by sending an I/O to the driver. It has the benefit of matching the
error semantics of a typical block device where a buffered write could
hit an error filling the page cache, but an O_DIRECT write potentially
triggers the drive to remap the block.