Date:   Thu, 23 Sep 2021 13:55:40 -0700
From:   Jane Chu <jane.chu@...cle.com>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 0/3] dax: clear poison on the fly along pwrite

On 9/15/2021 9:15 AM, Darrick J. Wong wrote:
> On Wed, Sep 15, 2021 at 12:22:05AM -0700, Jane Chu wrote:
>> Hi, Dan,
>>
>> On 9/14/2021 9:44 PM, Dan Williams wrote:
>>> On Tue, Sep 14, 2021 at 4:32 PM Jane Chu <jane.chu@...cle.com> wrote:
>>>>
>>>> If pwrite(2) encounters poison in a pmem range, it fails with EIO.
>>>> This is unecessary if hardware is capable of clearing the poison.
>>>>
>>>> Though not all dax backend hardware has the capability of clearing
>>>> poison on the fly, but dax backed by Intel DCPMEM has such capability,
>>>> and it's desirable to, first, speed up repairing by means of it;
>>>> second, maintain backend continuity instead of fragmenting it in
>>>> search for clean blocks.
>>>>
>>>> Jane Chu (3):
>>>>     dax: introduce dax_operation dax_clear_poison
>>>
>>> The problem with new dax operations is that they need to be plumbed
>>> not only through fsdax and pmem, but also through device-mapper.
>>>
>>> In this case I think we're already covered by dax_zero_page_range().
>>> That will ultimately trigger pmem_clear_poison() and it is routed
>>> through device-mapper properly.
>>>
>>> Can you clarify why the existing dax_zero_page_range() is not sufficient?
>>
>> fallocate ZERO_RANGE is in itself a functionality that applied to dax
>> should lead to zero out the media range.  So one may argue it is part
>> of a block operations, and not something explicitly aimed at clearing
>> poison.
> 
> Yeah, Christoph suggested that we make the clearing operation explicit
> in a related thread a few weeks ago:
> https://lore.kernel.org/linux-fsdevel/YRtnlPERHfMZ23Tr@infradead.org/
> 
> I like Jane's patchset far better than the one that I sent, because it
> doesn't require a block device wrapper for the pmem, and it enables us
> to tell application writers that they can handle media errors by
> pwrite()ing the bad region, just like they do for nvme and spinners.
> 
>> I'm also thinking about the MOVEDIR64B instruction and how it
>> might be used to clear poison on the fly with a single 'store'.
>> Of course, that means we need to figure out how to narrow down the
>> error blast radius first.
> 
> That was one of the advantages of Shiyang Ruan's NAKed patchset to
> enable byte-granularity media errors to pass upwards through the stack
> back to the filesystem, which could then tell applications exactly what
> they lost.
> 
> I want to get back to that, though if Dan won't withdraw the NAK then I
> don't know how to move forward...
> 
>> With respect to plumbing through device-mapper, I thought about that,
>> and wasn't sure. I mean the clear-poison work will eventually fall on
>> the pmem driver, and thru the DM layers, how does that play out thru
>> DM?
> 
> Each of the dm drivers has to add their own ->clear_poison operation
> that remaps the incoming (sector, len) parameters as appropriate for
> that device and then calls the lower device's ->clear_poison with the
> translated parameters.
> 
> This (AFAICT) has already been done for dax_zero_page_range, so I sense
> that Dan is trying to save you a bunch of code plumbing work by nudging
> you towards doing s/dax_clear_poison/dax_zero_page_range/ to this series
> and then you only need patches 2-3.

Thanks Darrick for the explanation!
I don't mind to add DM layer support, it sounds straight forward.
I also like your latest patch and am wondering if the clear_poison API
is still of value.

thanks,
-jane

> 
>> BTW, our customer doesn't care about creating dax volume thru DM, so.
> 
> They might not care, but anything going upstream should work in the
> general case.
> 
> --D
> 
>> thanks!
>> -jane
>>
>>
>>>
>>>>     dax: introduce dax_clear_poison to dax pwrite operation
>>>>     libnvdimm/pmem: Provide pmem_dax_clear_poison for dax operation
>>>>
>>>>    drivers/dax/super.c   | 13 +++++++++++++
>>>>    drivers/nvdimm/pmem.c | 17 +++++++++++++++++
>>>>    fs/dax.c              |  9 +++++++++
>>>>    include/linux/dax.h   |  6 ++++++
>>>>    4 files changed, 45 insertions(+)
>>>>
>>>> --
>>>> 2.18.4
>>>>

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