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

Dan Williams
2024-04-11T17:56:51Z

Jason Gunthorpe wrote:
> 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? :(

Yeah... I got hung up on the "allocation" API to take ZONE_DEVICE pages
from recfount 0 to 1 and then never circled back.