linux-kernel - Re: [PATCH v5 2/7] dax: introduce dax device flag DAXDEV

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Date:   Sun, 6 Feb 2022 08:29:15 +0000
From:   Jane Chu <jane.chu@...cle.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@...radead.org>
CC:     "david@...morbit.com" <david@...morbit.com>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "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 v5 2/7] dax: introduce dax device flag DAXDEV_RECOVERY

On 2/3/2022 9:32 PM, Dan Williams wrote:
> On Thu, Feb 3, 2022 at 9:17 PM Dan Williams <dan.j.williams@...el.com> wrote:
>>
>> On Thu, Feb 3, 2022 at 5:43 AM Christoph Hellwig <hch@...radead.org> wrote:
>>>
>>> On Wed, Feb 02, 2022 at 09:27:42PM +0000, Jane Chu wrote:
>>>> Yeah, I see.  Would you suggest a way to pass the indication from
>>>> dax_iomap_iter to dax_direct_access that the caller intends the
>>>> callee to ignore poison in the range because the caller intends
>>>> to do recovery_write? We tried adding a flag to dax_direct_access, and
>>>> that wasn't liked if I recall.
>>>
>>> To me a flag seems cleaner than this magic, but let's wait for Dan to
>>> chime in.
>>
>> So back in November I suggested modifying the kaddr, mainly to avoid
>> touching all the dax_direct_access() call sites [1]. However, now
>> seeing the code and Chrisoph's comment I think this either wants type
>> safety (e.g. 'dax_addr_t *'), or just add a new flag. Given both of
>> those options involve touching all dax_direct_access() call sites and
>> a @flags operation is more extensible if any other scenarios arrive
>> lets go ahead and plumb a flag and skip the magic.
> 
> Just to be clear we are talking about a flow like:
> 
>          flags = 0;
>          rc = dax_direct_access(..., &kaddr, flags, ...);
>          if (unlikely(rc)) {
>                  flags |= DAX_RECOVERY;
>                  dax_direct_access(..., &kaddr, flags, ...);
>                  return dax_recovery_{read,write}(..., kaddr, ...);
>          }
>          return copy_{mc_to_iter,from_iter_flushcache}(...);

Okay, will go with a flag.

thanks!
-jane