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

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Message-ID: <CAPcyv4hCf0WpRyNx4vo0_+-w1ABX0cJTyLozPYEqiqR0i_H1_Q@mail.gmail.com>
Date:   Thu, 3 Feb 2022 21:32:27 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jane Chu <jane.chu@...cle.com>,
        "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 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}(...);