linux-kernel - Re: [PATCH v2 1/2] dax: Introduce normal and recovery dax operation modes

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Date:   Mon, 8 Nov 2021 22:04:23 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Jane Chu <jane.chu@...cle.com>, david <david@...morbit.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.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>,
        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 1/2] dax: Introduce normal and recovery dax operation modes

On Mon, Nov 8, 2021 at 9:26 PM Ira Weiny <ira.weiny@...el.com> wrote:
>
> On Mon, Nov 08, 2021 at 09:02:29PM +0000, Jane Chu wrote:
> > On 11/6/2021 9:48 AM, Dan Williams wrote:
> > > On Fri, Nov 5, 2021 at 6:17 PM Jane Chu <jane.chu@...cle.com> wrote:
> > >>
> > >> Introduce DAX_OP_NORMAL and DAX_OP_RECOVERY operation modes to
> > >> {dax_direct_access, dax_copy_from_iter, dax_copy_to_iter}.
> > >> DAX_OP_NORMAL is the default or the existing mode, and
> > >> DAX_OP_RECOVERY is a new mode for data recovery purpose.
> > >>
> > >> When dax-FS suspects dax media error might be encountered
> > >> on a read or write, it can enact the recovery mode read or write
> > >> by setting DAX_OP_RECOVERY in the aforementioned APIs. A read
> > >> in recovery mode attempts to fetch as much data as possible
> > >> until the first poisoned page is encountered. A write in recovery
> > >> mode attempts to clear poison(s) in a page-aligned range and
> > >> then write the user provided data over.
> > >>
> > >> DAX_OP_NORMAL should be used for all non-recovery code path.
> > >>
> > >> Signed-off-by: Jane Chu <jane.chu@...cle.com>
> > > [..]
> > >> diff --git a/include/linux/dax.h b/include/linux/dax.h
> > >> index 324363b798ec..931586df2905 100644
> > >> --- a/include/linux/dax.h
> > >> +++ b/include/linux/dax.h
> > >> @@ -9,6 +9,10 @@
> > >>   /* Flag for synchronous flush */
> > >>   #define DAXDEV_F_SYNC (1UL << 0)
> > >>
> > >> +/* dax operation mode dynamically set by caller */
> > >> +#define        DAX_OP_NORMAL           0
> > >
> > > Perhaps this should be called DAX_OP_FAILFAST?
> >
> > Sure.
> >
> > >
> > >> +#define        DAX_OP_RECOVERY         1
> > >> +
> > >>   typedef unsigned long dax_entry_t;
> > >>
> > >>   struct dax_device;
> > >> @@ -22,8 +26,8 @@ struct dax_operations {
> > >>           * logical-page-offset into an absolute physical pfn. Return the
> > >>           * number of pages available for DAX at that pfn.
> > >>           */
> > >> -       long (*direct_access)(struct dax_device *, pgoff_t, long,
> > >> -                       void **, pfn_t *);
> > >> +       long (*direct_access)(struct dax_device *, pgoff_t, long, int,
> > >
> > > Would be nice if that 'int' was an enum, but I'm not sure a new
> > > parameter is needed at all, see below...
> >
> > Let's do your suggestion below. :)
> >
> > >
> > >> +                               void **, pfn_t *);
> > >>          /*
> > >>           * Validate whether this device is usable as an fsdax backing
> > >>           * device.
> > >> @@ -32,10 +36,10 @@ struct dax_operations {
> > >>                          sector_t, sector_t);
> > >>          /* copy_from_iter: required operation for fs-dax direct-i/o */
> > >>          size_t (*copy_from_iter)(struct dax_device *, pgoff_t, void *, size_t,
> > >> -                       struct iov_iter *);
> > >> +                       struct iov_iter *, int);
> > >
> > > I'm not sure the flag is needed here as the "void *" could carry a
> > > flag in the pointer to indicate that is a recovery kaddr.
> >
> > Agreed.
>
> Not sure if this is implied but I would like some macros or other helper
> functions to check these flags hidden in the addresses.
>
> For me I'm a bit scared about having flags hidden in the address like this
> because I can't lead to some confusions IMO.
>
> But if we have some macros or other calls which can make this more obvious of
> what is going on I think that would help.

You could go further and mark it as an 'unsigned long __bitwise' type
to get the compiler to help with enforcing accessors to strip off the
flag bits.