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

From: Dan Williams
Date: Mon May 02 2016 - 14:48:22 EST

On Mon, May 2, 2016 at 11:32 AM, Boaz Harrosh <boaz@xxxxxxxxxxxxx> 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.