linux-ext4 - Re: [PATCH v2 15/18] devdax: Use dax_insert_entry() + dax_delete_mapping

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Date:   Wed, 21 Sep 2022 19:23:13 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Matthew Wilcox <willy@...radead.org>,
        Jan Kara <jack@...e.cz>, "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 15/18] devdax: Use dax_insert_entry() +
 dax_delete_mapping_entry()

On Wed, Sep 21, 2022 at 08:48:22AM -0700, Dan Williams wrote:

> The fsdax core now manages pgmap references when servicing faults that
> install new mappings, and elevates the page reference until it is
> zapped. It coordinates with the VFS to make sure that all page
> references are dropped before the hosting inode goes out of scope
> (iput_final()).
>
> In order to delete the unnecessary pgmap reference taking in mm/gup.c
> devdax needs to move to the same model.

I think this patch is more about making devdax and fsdax use the same
set of functions and logic so that when it gets to patch 16/17 devdax
doesn't break. That understanding matches the first paragraph, at
least.

I would delete the remark about gup since it is really patch 17 that
allows gup to be fixed by making it so that refcount == 0 means not to
look at the pgmap (instead of refcount == 1 as is now) ?

Jason