linux-ext4 - Re: [RFC 00/10] fs/dax: Fix FS DAX page reference counts

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Message-ID: <20240411173559.GX5383@nvidia.com>
Date: Thu, 11 Apr 2024 14:35:59 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
	david@...morbit.com, jhubbard@...dia.com, rcampbell@...dia.com,
	willy@...radead.org, linux-fsdevel@...r.kernel.org, jack@...e.cz,
	djwong@...nel.org, hch@....de, david@...hat.com,
	ruansy.fnst@...itsu.com, nvdimm@...ts.linux.dev,
	linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	jglisse@...hat.com
Subject: Re: [RFC 00/10] fs/dax: Fix FS DAX page reference counts

On Thu, Apr 11, 2024 at 10:28:56AM -0700, Dan Williams wrote:
> Alistair Popple wrote:
> > FS DAX pages have always maintained their own page reference counts
> > without following the normal rules for page reference counting. In
> > particular pages are considered free when the refcount hits one rather
> > than zero and refcounts are not added when mapping the page.
> 
> > Tracking this requires special PTE bits (PTE_DEVMAP) and a secondary
> > mechanism for allowing GUP to hold references on the page (see
> > get_dev_pagemap). However there doesn't seem to be any reason why FS
> > DAX pages need their own reference counting scheme.
> 
> This is fair. However, for anyone coming in fresh to this situation
> maybe some more "how we get here" history helps. That longer story is
> here:
> 
> http://lore.kernel.org/all/166579181584.2236710.17813547487183983273.stgit@dwillia2-xfh.jf.intel.com/

This never got merged? :(

Jason