linux-kernel - Re: [PATCH v6 4/6] dax: add DAX_RECOVERY flag and .recovery_write dev_pgmap

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Message-ID: <Yjq0FspfsLrN/mrx@infradead.org>
Date:   Tue, 22 Mar 2022 22:45:58 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "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 v6 4/6] dax: add DAX_RECOVERY flag and .recovery_write
 dev_pgmap_ops

On Tue, Mar 22, 2022 at 11:05:09PM +0000, Jane Chu wrote:
> > This DAX_RECOVERY doesn't actually seem to be used anywhere here or
> > in the subsequent patches.  Did I miss something?
> 
> dax_iomap_iter() uses the flag in the same patch,
> +               if ((map_len == -EIO) && (iov_iter_rw(iter) == WRITE)) {
> +                       flags |= DAX_RECOVERY;
> +                       map_len = dax_direct_access(dax_dev, pgoff, nrpg,
> +                                               flags, &kaddr, NULL);

Yes, it passes it on to dax_direct_access, and dax_direct_access passes
it onto ->direct_access.  But nothing in this series actually checks
for it as far as I can tell.

> >> Also introduce a new dev_pagemap_ops .recovery_write function.
> >> The function is applicable to FSDAX device only. The device
> >> page backend driver provides .recovery_write function if the
> >> device has underlying mechanism to clear the uncorrectable
> >> errors on the fly.
> > 
> > Why is this not in struct dax_operations?
> 
> Per Dan's comments to the v5 series, adding .recovery_write to
> dax_operations causes a number of trivial dm targets changes.
> Dan suggested that adding .recovery_write to pagemap_ops could
> cut short the logistics of figuring out whether the driver backing
> up a page is indeed capable of clearing poison. Please see
> https://lkml.org/lkml/2022/2/4/31

But at least in this series there is  1:1 association between the
pgmap and the dax_device so that scheme won't work.   It would
have to lookup the pgmap based on the return physical address from
dax_direct_access.  Which sounds more complicated than just adding
the (annoying) boilerplate code to DM.

> include/linux/memremap.h doesn't know struct iov_iter which is defined 
> in include/linux/uio.h,  would you prefer to adding include/linux/uio.h 
> to include/linux/memremap.h ?

As it is not derefences just adding a

struct iov_iter;

line to memremap.h below the includes should be all that is needed.