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

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Message-ID: <632ba938754b5_349629486@dwillia2-xfh.jf.intel.com.notmuch>
Date:   Wed, 21 Sep 2022 17:15:52 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        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()

Jason Gunthorpe wrote:
> 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) ?

Yeah, makes sense.