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

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Date:	Mon, 2 May 2016 11:48:15 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Boaz Harrosh <boaz@...xistor.com>
Cc:	Vishal Verma <vishal.l.verma@...el.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	linux-block@...r.kernel.org, Jan Kara <jack@...e.cz>,
	Matthew Wilcox <matthew@....cx>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	XFS Developers <xfs@....sgi.com>, Jens Axboe <axboe@...com>,
	Linux MM <linux-mm@...ck.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

On Mon, May 2, 2016 at 11:32 AM, Boaz Harrosh <boaz@...xistor.com> wrote:
> On 05/02/2016 09:10 PM, Dan Williams wrote:
> <>
>>
>> The semantic I am talking about preserving is:
>>
>> buffered / unaligned write of a bad sector => -EIO on reading into the
>> page cache
>>
>
> What about aligned buffered write? like write 0-to-eof
> This still broken? (and is what restore apps do)
>
>> ...and that the only guaranteed way to clear an error (assuming the
>> block device supports it) is an O_DIRECT write.
>>
>
> Sure fixing dax_do_io will guaranty that.
>
> <>
>> I still think we're talking past each other on this point.
>
> Yes we are!
>
>> This patch
>> set is not overloading error semantics, it's fixing the error handling
>> problem that was introduced in this commit:
>>
>>    d475c6346a38 dax,ext2: replace XIP read and write with DAX I/O
>>
>> ...where we started overloading O_DIRECT and dax_do_io() semantics.
>>
>
> But above does not fix them does it? it just completely NULLs DAX for
> O_DIRECT which is a great pity, why did we do all this work in the first
> place.

This is hyperbole.  We don't impact "all the work" we did for the mmap
I/O case and the acceleration of the non-direct-I/O case.

> And then it keeps broken the aligned buffered writes, which are still
> broken after this set.

...identical to the current situation with a traditional disk.

> I have by now read the v2 patches. And I think you guys did not yet try
> the proper fix for dax_do_io. I think you need to go deeper into the loops
> and selectively call bdev_* when error on a specific page copy. No need to
> go through direct_IO path at all.

We still reach a point where the minimum granularity of
bdev_direct_access() is larger than a sector, so you end up still
needing to have the application understand how to send a properly
aligned I/O.  The semantics of how to send a properly aligned
direct-I/O are already well understood, so we simply reuse that path.

> Do you need that I send you a patch to demonstrate what I mean?

I remain skeptical of what you are proposing, but yes, a patch has a
better chance to move the discussion forward.
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