linux-kernel - Re: [PATCH v2 2/2] dax,pmem: Implement pmem based dax data recovery

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Date:   Tue, 9 Nov 2021 10:48:51 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jane Chu <jane.chu@...cle.com>, 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 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);
                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,
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.