linux-kernel - Re: [PATCH 4/6] dm,dax,pmem: prepare dax_copy_to/from_iter() APIs with DAXDEV_F

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Date:   Fri, 22 Oct 2021 00:49:15 +0000
From:   Jane Chu <jane.chu@...cle.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     "david@...morbit.com" <david@...morbit.com>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "agk@...hat.com" <agk@...hat.com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "vgoyal@...hat.com" <vgoyal@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH 4/6] dm,dax,pmem: prepare dax_copy_to/from_iter() APIs
 with DAXDEV_F_RECOVERY

On 10/21/2021 4:27 AM, Christoph Hellwig wrote:
> On Wed, Oct 20, 2021 at 06:10:57PM -0600, Jane Chu wrote:
>> Prepare dax_copy_to/from_iter() APIs with DAXDEV_F_RECOVERY flag
>> such that when the flag is set, the underlying driver implementation
>> of the APIs may deal with potential poison in a given address
>> range and read partial data or write after clearing poison.
> 
> FYI, I've been wondering for a while if we could just kill off these
> methods entirely.  Basically the driver interaction consists of two
> parts:
> 
>   a) wether to use the flushcache/mcsafe variants of the generic helpers
>   b) actually doing remapping for device mapper
> 
> to me it seems like we should handle a) with flags in dax_operations,
> and only have a remap callback for device mapper.  That way we'd avoid
> the indirect calls for the native case, and also avoid tons of
> boilerplate code.  "futher decouple DAX from block devices" series
> already massages the device mapper into a form suitable for such
> callbacks.
> 

I've looked through your "futher decouple DAX from block devices" series 
and likes the use of xarray in place of the host hash list.
Which upstream version is the series based upon?
If it's based on your development repo, I'd be happy to take a clone
and rebase my patches on yours if you provide a link. Please let me
know the best way to cooperate.

That said, I'm unclear at what you're trying to suggest with respect
to the 'DAXDEV_F_RECOVERY' flag.  The flag came from upper dax-fs
call stack to the dm target layer, and the dm targets are equipped
with handling pmem driver specific task, so it appears that the flag 
would need to be passed down to the native pmem layer, right?
Am I totally missing your point?

thanks,
-jane