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

Dan Williams
2016-05-02T18:10:29Z

On Mon, May 2, 2016 at 10:44 AM, Boaz Harrosh <boaz@plexistor.com> wrote:
> On 05/02/2016 07:49 PM, Dan Williams wrote:
>> On Mon, May 2, 2016 at 9:22 AM, Boaz Harrosh <boaz@plexistor.com> wrote:
>>> On 05/02/2016 07:01 PM, Dan Williams wrote:
>>>> On Mon, May 2, 2016 at 8:41 AM, Boaz Harrosh <boaz@plexistor.com> 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.
>>>
>>> We have tons of measurements. Is not hard to imagine the results though.
>>> Specially the 1000 threads case
>>>
>>>> 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.
>>>>
>>>
>>> I fail to see how in writes the device error semantics regarding remapping of
>>> blocks is any different between buffered and direct IO. As far as the block
>>> device it is the same exact code path. All The big difference is higher in the
>>> VFS.
>>>
>>> And ... So you are willing to sacrifice the 99% hotpath for the sake of the
>>> 1% error path? and piggybacking on poor O_DIRECT.
>>>
>>> Again there are tons of O_DIRECT apps out there, why are you forcing them to
>>> change if they want true pmem performance?
>>
>> This isn't forcing them to change. This is the path of least surprise
>> as error semantics are identical to a typical block device. Yes, an
>> application can go faster by switching to the "buffered" / dax_do_io()
>> path it can go even faster to switch to mmap() I/O and use DAX
>> directly. If we can later optimize the O_DIRECT path to bring it's
>> performance more in line with dax_do_io(), great, but the
>> implementation should be correct first and optimized later.
>>
>
> Why does it need to be either or. Why not both?
> And also I disagree if you are correct and dax_do_io is bad and needs fixing
> than you have broken applications. Because in current model:
>
> read => -EIO, write-bufferd, sync()
> gives you the same error semantics as: read => -EIO, write-direct-io
> In fact this is what the delete, restore from backup model does today.
> Who said it uses / must direct IO. Actually I think it does not.

The semantic I am talking about preserving is:

buffered / unaligned write of a bad sector => -EIO on reading into the
page cache

...and that the only guaranteed way to clear an error (assuming the
block device supports it) is an O_DIRECT write.

>
> Two things I can think of which are better:
> [1]
> Why not go deeper into the dax io loops, and for any WRITE
> failed page call bdev_rw_page() to let the pmem.c clear / relocate
> the error page.

Where do you get the rest of the data to complete a full page write?

> So reads return -EIO - is what you wanted no?

That's well understood. What we are debating is the method to clear
errors / ask the storage device to remap bad blocks.

> writes get a memory error and retry with bdev_rw_page() to let the bdev
> relocate / clear the error - is what you wanted no?
>
> In the partial page WRITE case on bad sectors. we can carefully read-modify-write
> sector-by-sector and zero-out the bad-sectors that could not be read, what else?
> (Or enhance the bdev_rw_page() API)

See all the previous discussions on why the fallback path is
problematic to implement.

>
> [2]
> Only switch to slow O_DIRECT, on presence of errors like you wanted. But I still
> hate that you overload error semantics with O_DIRECT which does not exist today
> see above

I still think we're talking past each other on this point. 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.