linux-ext4 - Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax

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Message-ID: <1462204291.11211.20.camel@kernel.org>
Date:	Mon, 02 May 2016 09:51:31 -0600
From:	Vishal Verma <vishal@...nel.org>
To:	Boaz Harrosh <boaz@...xistor.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	linux-nvdimm@...ts.01.org
Cc:	linux-block@...r.kernel.org, Jan Kara <jack@...e.cz>,
	Matthew Wilcox <matthew@....cx>,
	Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	Jens Axboe <axboe@...com>, linux-mm@...ck.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

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
> 

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