linux-kernel - Re: regression introduced by "block: Add support for DAX reads/writes to block devices"

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Date:	Thu, 13 Aug 2015 08:32:10 +0300
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
	"linda.knippers@...com" <linda.knippers@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: regression introduced by "block: Add support for DAX reads/writes
 to block devices"

On 08/13/2015 12:11 AM, Jeff Moyer wrote:
> Boaz Harrosh <boaz@...xistor.com> writes:
> 
>> On 08/07/2015 11:41 PM, Jeff Moyer wrote:
>> <>
>>>
>>>> We need to cope with the case where the end of a partition isn't on a
>>>> page boundary though.
>>>
>>> Well, that's usually done by falling back to buffered I/O.  I gave that
>>> a try and panicked the box.  :)  I'll keep looking into it, but probably
>>> won't have another patch until next week.
>>>
>>
>> lets slow down for a sec, please.
>>
>> We have all established that an unaligned partition start is BAD and not supported?
> 
> No.  Unaligned partitions on RAID arrays or 512e devices are bad because
> they result in suboptimal performance.  They are most certainly still
> supported, though.
> 

What ?

I meant for dax on pmem or brd. I meant that we *do not* support dax access
on an unaligned partition start. (None dax is perfectly supported)

We did it this way because of the direct_access API that returns a pfn
with is PAGE_SIZE. We could introduce a pfn+offset but we thought it is
not worth it, and that dax should be page aligned for code simplicity

Cheers
Boaz

> -Jeff
> 


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