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

Vishal Verma
2016-05-02T15:51:37Z

On Mon, 2016-05-02 at 18:41 +0300, Boaz Harrosh 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.

We overloaded O_DIRECT a long time ago when we made DAX piggyback on
the same path:

static inline bool io_is_direct(struct file *filp)
{
return (filp->f_flags & O_DIRECT) || IS_DAX(filp->f_mapping->host);
}

Yes O_DIRECT on a DAX mounted file system will now be slower, but -

>
> >
> > This allows us a recovery path in the form of opening the file with
> > O_DIRECT and writing to it with the usual O_DIRECT semantics
> > (sector
> > alignment restrictions).
> >
> I understand that you want a sector aligned IO, right? for the
> clear of errors. But I hate it that you forced all O_DIRECT IO
> to be slow for this.
> Can you not make dax_do_io handle media errors? At least for the
> parts of the IO that are aligned.
> (And your recovery path application above can use only aligned
>  IO to make sure)
>
> Please look for another solution. Even a special
> IOCTL_DAX_CLEAR_ERROR

- see all the versions of this series prior to this one, where we try
to do a fallback...

>
> [*"less concurrent" because of the queuing done in bdev. Note how
>   pmem is not even multi-queue, and even if it was it will be much
>   slower then DAX because of the code depth and all the locks and
> task
>   switches done in the block layer. In DAX the final memcpy is done
> directly
>   on the user-mode thread]
>
> Thanks
> Boaz