Date:   Wed, 10 Nov 2021 18:26:01 +0000
From:   Jane Chu <jane.chu@...cle.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Christoph Hellwig <hch@...radead.org>, david <david@...morbit.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        device-mapper development <dm-devel@...hat.com>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vivek Goyal <vgoyal@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] dax,pmem: Implement pmem based dax data recovery

On 11/9/2021 1:02 PM, Dan Williams wrote:
> On Tue, Nov 9, 2021 at 11:59 AM Jane Chu <jane.chu@...cle.com> wrote:
>>
>> On 11/9/2021 10:48 AM, Dan Williams wrote:
>>> On Mon, Nov 8, 2021 at 11:27 PM Christoph Hellwig <hch@...radead.org> wrote:
>>>>
>>>> On Fri, Nov 05, 2021 at 07:16:38PM -0600, Jane Chu wrote:
>>>>>    static size_t pmem_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff,
>>>>>                 void *addr, size_t bytes, struct iov_iter *i, int mode)
>>>>>    {
>>>>> +     phys_addr_t pmem_off;
>>>>> +     size_t len, lead_off;
>>>>> +     struct pmem_device *pmem = dax_get_private(dax_dev);
>>>>> +     struct device *dev = pmem->bb.dev;
>>>>> +
>>>>> +     if (unlikely(mode == DAX_OP_RECOVERY)) {
>>>>> +             lead_off = (unsigned long)addr & ~PAGE_MASK;
>>>>> +             len = PFN_PHYS(PFN_UP(lead_off + bytes));
>>>>> +             if (is_bad_pmem(&pmem->bb, PFN_PHYS(pgoff) / 512, len)) {
>>>>> +                     if (lead_off || !(PAGE_ALIGNED(bytes))) {
>>>>> +                             dev_warn(dev, "Found poison, but addr(%p) and/or bytes(%#lx) not page aligned\n",
>>>>> +                                     addr, bytes);
>>>>> +                             return (size_t) -EIO;
>>>>> +                     }
>>>>> +                     pmem_off = PFN_PHYS(pgoff) + pmem->data_offset;
>>>>> +                     if (pmem_clear_poison(pmem, pmem_off, bytes) !=
>>>>> +                                             BLK_STS_OK)
>>>>> +                             return (size_t) -EIO;
>>>>> +             }
>>>>> +     }
>>>>
>>>> This is in the wrong spot.  As seen in my WIP series individual drivers
>>>> really should not hook into copying to and from the iter, because it
>>>> really is just one way to write to a nvdimm.  How would dm-writecache
>>>> clear the errors with this scheme?
>>>>
>>>> So IMHO going back to the separate recovery method as in your previous
>>>> patch really is the way to go.  If/when the 64-bit store happens we
>>>> need to figure out a good way to clear the bad block list for that.
>>>
>>> I think we just make error management a first class citizen of a
>>> dax-device and stop abstracting it behind a driver callback. That way
>>> the driver that registers the dax-device can optionally register error
>>> management as well. Then fsdax path can do:
>>>
>>>           rc = dax_direct_access(..., &kaddr, ...);
>>>           if (unlikely(rc)) {
>>>                   kaddr = dax_mk_recovery(kaddr);
>>
>> Sorry, what does dax_mk_recovery(kaddr) do?
> 
> I was thinking this just does the hackery to set a flag bit in the
> pointer, something like:
> 
> return (void *) ((unsigned long) kaddr | DAX_RECOVERY)

Okay, how about call it dax_prep_recovery()?

> 
>>
>>>                   dax_direct_access(..., &kaddr, ...);
>>>                   return dax_recovery_{read,write}(..., kaddr, ...);
>>>           }
>>>           return copy_{mc_to_iter,from_iter_flushcache}(...);
>>>
>>> Where, the recovery version of dax_direct_access() has the opportunity
>>> to change the page permissions / use an alias mapping for the access,
>>
>> again, sorry, what 'page permissions'?  memory_failure_dev_pagemap()
>> changes the poisoned page mem_type from 'rw' to 'uc-' (should be NP?),
>> do you mean to reverse the change?
> 
> Right, the result of the conversation with Boris is that
> memory_failure() should mark the page as NP in call cases, so
> dax_direct_access() needs to create a UC mapping and
> dax_recover_{read,write}() would sink that operation and either return
> the page to NP after the access completes, or convert it to WB if the
> operation cleared the error.

Okay,  will add a patch to fix set_mce_nospec().

How about moving set_memory_uc() and set_memory_np() down to
dax_recovery_read(), so that we don't split the set_memory_X calls
over different APIs, because we can't enforce what follows
dax_direct_access()?

> 
>>> dax_recovery_read() allows reading the good cachelines out of a
>>> poisoned page, and dax_recovery_write() coordinates error list
>>> management and returning a poison page to full write-back caching
>>> operation when no more poisoned cacheline are detected in the page.
>>>
>>
>> How about to introduce 3 dax_recover_ APIs:
>>     dax_recover_direct_access(): similar to dax_direct_access except
>>        it ignores error list and return the kaddr, and hence is also
>>        optional, exported by device driver that has the ability to
>>        detect error;
>>     dax_recovery_read(): optional, supported by pmem driver only,
>>        reads as much data as possible up to the poisoned page;
> 
> It wouldn't be a property of the pmem driver, I expect it would be a
> flag on the dax device whether to attempt recovery or not. I.e. get
> away from this being a pmem callback and make this a native capability
> of a dax device.
> 
>>     dax_recovery_write(): optional, supported by pmem driver only,
>>        first clear-poison, then write.
>>
>> Should we worry about the dm targets?
> 
> The dm targets after Christoph's conversion should be able to do all
> the translation at direct access time and then dax_recovery_X can be
> done on the resulting already translated kaddr.

I'm thinking about the mixed device dm where some provides
dax_recovery_X, others don't, in which case we don't allow
dax recovery because that causes confusion? or should we still
allow recovery for part of the mixed devices?

> 
>> Both dax_recovery_read/write() are hooked up to dax_iomap_iter().
> 
> Yes.
> 

thanks!
-jane

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